Gauges

6cyl temp gauge calibration

Author : Copied from a website

tr6 temperature gauge calibrationbration

 

Gauge Repair

Author : Scott Suhring

Here is a tech tip that I recently wrote for our club newsletter (when you’re the editor, you tend to write a lot of articles) on this exact subject:

Anyone who has disassembled their dash gauges can appreciate the frustration of first trying to remove the chrome ring that holds the glass secure to the gauge and then finding the dried up remains of the rubber gasket (if that is what this material ever was). Well, certainly one of the parts distributors must have these, but when you look in their catalog or check the parts manual, low and behold they are not listed. The reason for this is that the gaskets came as part of the gauge, which was produced by Smiths/Jaeger. So what do you do?

I have tried various materials and have talked to others that have used everything from windshield glazing to using the gaskets that are available to seal the gauge to the dash, none of which provide the proper seal for the chrome rings. The solution I have devised is to make my own gaskets. To do this, you will need the following materials:

  1. Permatex “The Right Stuff” gasket maker (part #29208)
  2. Universal Pump Dispenser (this is a plastic syringe found in the glue section of your local hardware store)
  3. An Exacto knife or any arts and crafts sharp cutting blade
  4. Q-Tips
  5. Pam cooking spray
  6. Small screwdriver or dental pick

The Permatex gasket maker is a silicon-based material that cures to a flexible rubbery composition that is easy to work with and makes a great gasket. Here is what you do to make your gasket:

  1. Clean out the inside of the gauge cover ring of any of the remains of the old gasket.
  2. Cut off the tip of the syringe to the size opening that allows enough material to by pushed out to the thickness of the gasket you need to make.
  3. Depending on the number of gasket you will be making, fill the syringe with the Permatex.
  4. Spray the tip of the Q-tip with the Pam and wipe this on the inside of the ring, but do not saturate the surface.
  5. Take the syringe and evenly apply the Permatex in the grove for the gasket to the height just below the inner lip of the ring. You will get some “waves” in the material and uneven application, but this is okay. Just be sure that it is a continuous bead of material that fills in the grove.
  6. Let the Permatex cure for 24 hours.
  7. Using the small screwdriver or the dental pick, work this under the gasket and pry up the gasket until it is released from the ring. There will be some spots that the Permatex will remain stuck in small bits and these can be removed by just scraping them off to clean up the surface.
  8. Using the exacto blade, trim the gasket of any uneven areas or bulges. Don’t be shy since the material is very pliable and cuts easily.

You will not have a perfect looking gasket, but this will absolutely provide you with a good seal and one that will remain flexible for the next time you need to disassemble the gauge.

suhringtr36@comcast.net

 

Jaeger/Smiths Speedometers

Author : ARhodes@compuserve.com.

Draft: 5/28/99

//ourworld.compuserve.com/homepages/arhodes

This manual covers mechanical Jaeger/Smiths speedometers from the early 1960’s through (at least) the late 1970’s. I have worked mostly with Triumph parts, but also MG as well. I found that they have essentially the same works inside their different size cases. Therefore, servicing these two types uses similar procedures.

After reading this, PLEASE give me some feedback. I want this manual to be as explicit and accurate as possible. Your comments, both good and critical, will be very useful for my attempts to refine this monograph. If you find that your speedometer differs from my description, please let me know what you found, and the speedometer serial number and the make/year of your car. I want to hear everything! Please email me at ARhodes@compuserve.com.

Please refer to the photographs on a separate page of my website. These ought to assist greatly with understanding the workings of the speedometers.

THE MALFUNCTIONING SPEEDOMETER

Non-speedometer Issues

You have almost nothing to fear except fear itself. You CAN fix your own speedometer. Here is some information that may help you. The first issue is to decide whether the speedometer itself is actually the cause of the problem. Some faults that are thought to be due to a malfunctioning speedometer are actually due to a problem with the cable running to the speedometer, or with the drive gear in the transmission.

Frequently the cable itself is the cause of a wavering speedometer pointer. It is unfortunately somewhat difficult to service the cable. You must be prepared to get under the car and remove the cable from the transmission. However, it is sometimes possible to service the cable simply from the speedometer end. You need to remove the speedometer, then pull up some slack in the cable so the end of the cable is protruding slightly from the dash. Then you may pull out the wire cable from the outer sheath. Lubricate the cable with white lithium grease or gear oil and then slide the cable back into the sheath. As you get to the last couple of inches you need to slowly spin the cable as you insert it. This will allow the square end of the cable to seat in the square orifice in the transmission drive gear (hopefully). If, after multiple attempts, you can not get the cable to seat, then you will have to get under the car, and unscrew the cable from the transmission. Then press the cable fully in the sheath, and attach the speedometer. Then, back under the car, you must gently seat the cable into the drive on the transmission and screw it down securely. Test the speedometer with the newly lubricated cable.
Test this before fully re-installing the speedometer in the dash.

Other causes of a wavering speedometer needle lie inside the speedometer itself. I have seen binding of the input shaft cause wavering as it slows down, then breaks free and turns faster briefly. Binding can also occur between eh shaft and the retaining flange. I have also seen binding in the odometer wheels (particularly the “old” style) cause cyclic resistance against turning, resulting in wavering. There can also be a dirt or lack of lubrication in the needle bushing between the magnet wheel and the pointer spindle. Binding odometer wheels and needle bearings often will casue speedometer wavering that is proportional to road speed.

Speedometer Issues

A speedometer is a simple device. It has three separate functions. The speedometer function is to indicate your speed. There are two odometers which are variations on the same theme. The main odometer shows the cumulative mileage of the car, and the trip odometer measures the mileage since the last reset. The trip odometer is just the same as the main odometer except that it displays 1/10’s of miles and is able to be reset. The main odometer has a 1/10 wheel, but it has no lettering and is not shown in the window.

PARTS EXCHANGE GUIDE

Repairs of the speedometer and odometer sub-sections can be made by exchange with intact/functional parts from other Smiths or Jaeger speedometers. Many parts will be common across a broad range of models and years. There are four main variations (that I know of) that will influence the possibility of exchange. Within a specific type, parts seem to be completely interchangeable. “Old” models have all metal construction except for the worm gear and also have separately driven main and trip odometers. “Intermediate” have plastic odometer wheels, and the trip odometer wheels are more widely spaced. The “new” models have mostly plastic construction and the trip odometer is driven by a gear from the main odometer, so there is only one worm and pawl. The spindle bearing in the magnet wheel is more shallowly set in the “new” type of speedometers. The Triumph cars seem to have had a slightly different variant speedometer than the MG’s. The primary difference is that the spindle to which the pointer is attached is longer (.180″ vs. .150″) and has a somewhat narrower taper (.032 to .035 vs .030 to .035). This makes it less than optimal to move the works from an MG to a Triumph because the Triumph pointer fits slightly loosely. The move of a Triumph works to an MG is even less possible due to the MG pointer being too tight to fit on the TR works. The taper of the long and short spindles is approximately the same, so the longer one reaches a more narrrow tip. It is possible to shorten the MG spindle to the Triumph length and thereby have the diameter correct for the Triumph pointer. Use a file or rotary stone on a dremel tool to shorten the spindle by a 30 thousandths or so, and try refitting the pointer. If it will not slide on, there may be a burr on the tip, so use a fine file to chanfer the edge.

Depending on the calibration required, the worm on the input shaft may have 20, 25 or 32 teeth (there could be others but I hav not seen them). It appears that 32 teeth were very commonly used on the “old” and “intermediate” versions, with 20 and 25 also seen. 20 and 32 teeth were used on the “new” styles. There was a wide variety of gears used on the odometer wheels to provide the final calibration. The calibration of the odometer is the number of teeth on the worm gear multiplied by the number of teeth on the odometer wheel shaft. This gives the number of input shaft turns for each odometer shaft turn.

Of course parts are completely interchangeable between identical units, but many parts are carried across a broad range of speedometers, and will be completely interchangeable. For instance there are only two types of magnet wheels that I have identified. One type has a shallowly set spindle bearing, and the other is more deeply set, so the magnet wheel can be interchanged quite freely. The main speedometer frame is identical across all models as far as I can tell, and are completely interchangeable. In the “old” and “intermediate” units, the spindle/main odometer frames are interchangeable as long as the pointer fits properly. In the end, it is usually possible to obtain sufficient parts to repair your speedometer without great difficulty.

KPH and MPH speedometers are essentially the same and parts exchange guidelines apply here as well. As far as I can tell, the actual speedometer function is exactly the same. Only the printing on the dial face is different. The odometers are also essentially the same. The KPH units have 62% fewer teeth on the gear mounted on with the odometer wheels so there will be more turns of the odometer for the same number of miles. It is easily possible to convert a KPH speedometer to an MPH unit. All you need to do is exchange the dial face and install the proper odometer gear to set the desired calibration. If you have a KPH speedometer and want to convert to MPH, you can calculate the desired calibration by multiplying the calibration printed on the dial of the KPH speedometer by 1.609. The reverse calculation may be made by dividing by the same number. This number will usually not correspond to an actual calibration. You need to round to the nearest 20,25,or 32 (depending on the number of teeth on your worm gear). For example, a common TR6 KPH speedometer has a calibration of 740. This corresponds to an MPH calibration of 1190.6. This is just about centered between the two possible calibrations of 1180 and 1200. To settle the issue of the what calibration you REALLY should have, you ought to calculate your ideal calibration as described later in this manual, then translate that a MPH/KPH calibration and then look for the best possible calibration available.

List of Smiths/Jaeger types:

Old Style: Separate main & trip odometer frames and drives (all 120 mph?) TR2 thru 3A
Type 1: 25 tooth worm gear
Type 2: 20 tooth worm gear
Intermediate style: Separate main & trip odometer frames and drives (all 120 mph?) All 32 tooth worm gear (?) TR4 and TR4A (probably TR3B)
Type 1: Narrow trip odometer wheels (early)
Type 2: Wide trip odometer wheels (late)
New Style: Single main & trip odometer frame and drive (TR5/250? and TR6)
Type 1: 32 tooth worm gear (all 100, 120 mph) (MG only?)
Type 2 20 tooth worm gear (all 140 mph?) (TR only?)

Note: This summarizes the extent of my experience with TR and MG speedometers Any additional information will be greatly appreciated and incorporated into the next edition of this manual.

THE SPEEDOMETER: Mechanical Description

The speedometer (speed indicator, not odometer) functions in just the same way as a tachometer. The cable spins a thin bar magnet. Just in front of the bar magnet is a disk mounted on a spindle. Also attached to this, on the same spindle, is the pointer that is visible over the dial face. When the bar magnet spins, it causes the disk (drag cup) just in front of it to try to spin as well. The amount of twisting force (torque) imparted by the magnet to the disk is proportional to the rotational speed of the magnet. If the magnet spins twice as fast, the torque is approximately twice as great. The spindle is attached to a flat coiled return spring to resist rotation. The amount the spring winds is proportional to the torque. In this manner, the pointer moves progressively farther as the magnet spins faster.

There is very little that can go wrong with the speedometer. The places where a problem can occur are the following.

    1. The magnet wheel may not spin. In this case all functions cease and the drive cable (or angle drive) will break.

 

    1. The spindle may not move freely due to the disk binding against an obstruction. In this case the pointer is stuck in one place or will not rise above a certain level. The spindle pivots may lack lubrication and this will cause the needle to jump from one speed to the next rather than move smoothly. Lack of lubrication between the spindle needle tip and the bearing in the center of the magnet wheel may cause chatter at certain speeds.

 

    1. The return spring may be broken. In this case the pointer will wind completely around until it hits the stop. A professional repair is required.

 

    1. If everything moves normally, but it indicates the wrong speed, it is probably out of calibration. This is unlikely unless the pointer has been touched directly or you are using tires that vary substantially in rolling diameter from the originals. Also, violent swinging of the pointer may cause it to shift if it hits the stop forcefully.

 

    1. If there is still error at other speeds after calibration, the return spring may have weakened. A weakened spring will move more for a given speed than expected by the markings on the face. While you can calibrate it for a certain speed, it will register low below that speed and high above the speed. It is possible to unsolder the return spring, pull it tighter, and re-solder, but it is probably best to leave this to a professional speedometer repair shop. Alternatively, you may be able to swap the spindle/main odometer section of another unit. I have heard that it is possible for the bar magnet to lose its magnetism over time. If this happens, it is as if the return spring is too strong, and you will see the opposite problem than the weak spring described above. Professional shops are supposedly able to re-magnetize the bar magnet.

 
SPEEDOMETER: Inspection

To remove the speedometer works, first remove the metal bezel and the glass. Remove the two screws on the back and, if necessary, the tiny screw holding the reset cable (TR2-4). Then press the reset shaft (TR2-4) into the case and then push the threaded end (where the speedometer cable attaches) inward. The works should slide forward out of the case. There may be some adhesion to the rubber gasket inside the case, If the works do not move freely check the reset shaft and be sure that it is not catching on the case. Use a screwdriver to press it free.

When the works are free of the case you can now inspect them. The disk to which the pointer is attached should move freely. Twist the entire works back and forth. The pointer should move. You can use your finger to gently move the silver disc (drag cup). That should make the pointer move as well. Turn the speedometer drive at the attachment point of the cable by hand. You should see the worm move and after 32 turns (some TR6 models have 20) the pawl(s) should have gone through one complete cycle. As the pawl cycles, it should push the 1/10’s wheel ahead by one tooth. The wheel ought to have a ratcheting action to prevent the wheel from rolling backward.

SPEEDOMETER: Dismantling

Make a scratch on the drag cup in line with some obvious landmark on the frame while the pointer is resting at zero. This will allow you to reinstall the pointer without any significant loss of calibration. Remove the pointer by gently turning the drag cup until the pointer is at 60 mph. Then gently hold the drag cup in place. Hold the pointer by the hub and pull and twist the pointer until it pops free of the spindle. Now remove the two small screws attaching the face to the frame. The works are now free to work on. Much of the service can be done with no further disassembly.

Removing the odometer wheel sets will allow access to the magnet wheel and the seat of the needle pivot. Depending on the vintage of the speedometer the two wheel sets may be separate (early) or connected (late). You can tell if they are separate by looking for two separate gears driven off the worm on the shaft of the magnet wheel. The wheel sets and their frames are held in place by 4 small screws oriented on the top, bottom, left and right sides. These may be removed. If it uses a separate frame, the trip odometer may be lifted free after removing the small spring retaining the pawl. Remove the spring holding the pawl on main odometer. Gently turn the works upside down while supporting the upper part of the frame and then lift the bottom half of the frame free. You will have to move the main odometer pawl to allow the parts to separate. You may rest the upper frame and the main odometer wheels out of the way canted to one side upside down. Just do not allow it to rest on the spindle where the pointer attaches. To do so may stretch out the flat coil return spring.

NOTE: IT IS ESSENTIAL TO AVOID DAMAGE TO THE RETURN SPRING. WHEN THE SPEEDOMETER IS DISMANTLED THE DRAG CUP AND THE RETURN SPRING ATTACHED TO IT ARE POORLY SUPPORTED. IT IS EASY TO DAMAGE THE SPRING UNLESS YOU ARE CAREFUL TO SUPPORT THE DRAG CUP AT ALL TIMES.

The older speedometers had two screws holding the retaining flange of the magnet wheel and the input shaft to the frame. The newer ones are riveted. If you strongly suspect a problem with the input shaft bushing lubrication, you could drill out the rivets and then maybe it is possible to tap threads into the frame to replace the rivets with screws. I have never done this, so I can not comment on its feasibility. You will need to be very selective about which brass (non-magnetic) screws to use to re-secure the flange as protruding heads will interfere with the magnet wheel. If you have screws, they may be removed and the magnet wheel and shaft may be withdrawn from the frame. Clean the highly polished input shaft and the bushing in the frame. You may use light grease to re-lubricate it. Apply ample grease in the narrow mid-sections to provide lubrication for the long term.

Once greased, shaft can be replaced in the frame. Try spinning the magnet wheel. It should move freely, but the close tolerance of the input shaft as well as the grease do not allow it to spin multiple revolutions without constant pushing. Nevertheless, the resistance to movement should be very small.

The speedometer pointer spindle rides in a small bushing in the center of the magnet wheel. I do not know what sort of lubrication was used originally. They may have used none. The bushing may be cleaned with a jet spray of electrical contact cleaner. I got some from Radio Shack that included mineral oil lubricant. I think that “Brakleen” could also be used, and then you may need to use a very light machine oil. Engine oil will be too thick. Since the magnet wheel is constantly spinning, you want to have as little transfer of torque to the pointer spindle as possible. Viscous lubricant would cause significant torque on the spindle. This is the reason that I suspect that the spindle originally had no lubricant between the steel needle-tipped spindle and the brass bushing.

Now you may reassemble the lower frame to the odometer frame(s). Be sure to avoid damage to the spindle as you refit the frames. Also you need to have the lower frame rotated to the proper position on the upper frame. If you use the wrong orientation, only the upper and lower screw holes will line up. Once the frames are properly aligned, they may be secured with one screw below the main odometer. If your odometer has a separate trip odometer, it may be installed now as well. Use the other three screws when it is in place. Now install the two returnsprings on the pawls. A fine forceps will make this job much easier!

Replace the face and pointer and reindex the pointer to the proper location based on the scratch made in the beginning. If necessary calibrate the speedometer as listed below.

SPEEDOMETER: Calibration

One very observant person pointed out to me that there is a small dot at about -5 mph on the dial (on tachometers too). He found that when he pulled out the stop post on which the pointer rests when reading “zero”, the pointer came to balance pointing at the dot. MG speedometers have a line in about the same position. If you are restoring a speedometer without appreciably changing its calibration, then you can probably use that dot to set the speedometer. Unless the hair spring (return spring) or the magnet have lost strength over the years, then that dot ought to provide a fairly accurate initial calibration. If you are recalibrating the speedometer, then that dot will not be accurate at all and you will have to use the calibration technique outlined above.

One note, if you are recalibrating the speedometer to a setting considerably different from the original calibration, there will probably be some error particularly at the low speeds. At zero the pointer will have significantly more or less tension on the needle against the post. This may result in considerably more or less tendency for the pointer to move at low speeds resulting in low speed error. You can see that this is already an issue for the speedometer by looking at thes noticeably smaller space between 0 and 10 than between 10 and 20. This is due to the rest pressure of the hair spring on the pointer. At zero mph, there is NOT zero force on the pointer, forcing the magnet to have to turn at some speed just to get the pointer off the rest post. (about 5mph). Due to the error you might experience, it is best to calibrate the pointer to the speed you are most concerned about, such as the national speed limit. When calibrated to give no error at a certain speed, that is the ONE speed you can be positive about. Speeds close to that will have minimal error as well.

Driving Calibration

Calibration of your speedometer is easy. If you have not serviced it, it ought to beclose to the correct speed unless you have installed tires of a significantly differentrolling diameter. To fine-tune your calibration, you need to drive a measured mileat exactly 60 mph indicated on the dial (you may use any other speed as well, but60 is a nice mid-range number to use). Use a stopwatch to time your mile trip. Your actual speed is 3600 divided by the number of seconds it took to drive themile. Now remove the speedometer from the dash, and remove the works fromthe case. Do not remove the pointer or face. Move the pointer to the indicatedspeed you used in the mile trip (60mph). This is done by gently holding the dragcup in the works. Double check that the pointer is still at the indicated speed. Now gently push the pointer to the calculated speed while being sure that the diskdoes not move. Now pull up some slack speedometer cable into the dash andreconnect the works of the speedometer without reinstalling the case. Carefullyperch the speedometer in the dash opening without allowing the dash to contactany moving parts. Time another measured mile at exactly 60. If the time is notvery close to 60 seconds readjust the speedometer. You should be getting veryclose to perfectly calibrated with one or two resettings of the pointer.

Bench Calibration

If your speedometer has been serviced, you can do a bench calibration quite easily. You need the speedometer, a tachometer, a speedometer or tachometer cable, anda drill that will turn about 1000 to 2000 rpm. First you need to find out exactlyhow fast your drill turns. Attach the drill to the drive end of the cable and attachthe tachometer to the other end. Turn on the drill to maximum RPM. Watch thepointer of the tachometer. It should be quite steady. If it wavers significantly,either the drill does not turn a constant speed or the cable is binding. You canlube the cable by removing the center wire and greasing it lightly with lithiumgrease. Once you achieve a fairly constant reading, you can determine your drillRPM by dividing the reading by two (the ratio is indicated on the tachometer dialface: “2-1”). For instance my drill showed exactly 2400 rpm, so it was turningexactly 1200. This happens to be the maximum speed listed on its label.

Now you need to calculate what the speedometer OUGHT to be reading at thatRPM. You need to know one of two things. One is the calibration number printeron the dial face of the speedometer just above the “MPH”. This is the cable RPMthat it takes to indicate 60 MPH. If you know that your tires are substantiallydifferent from those originally supplied with the car, you can not use that numberas your final setting. If you DO know the engine RPM that corresponds with 60MPH, then divide that by 2.5 (known correct for the TR2-6 series, othertransmissions may differ. I suspect that Spitfires used 3.5) to determine the cableRPM that corresponds to 60 MPH. If you are not sure of this data, then set thespeedometer to the printed calibration and then do the driving calibrationdiscussed above. As an example, you want to set your speedometer to 1152calibration. You need to calculate what MPH should be indicated when you runthe drill at its maximum speed. The MPH will be 60 * RPM / 1152. So, in thiscase the MPH = 60 * 1200 / 1152 = 62.5. Now turn on the drill and look at theindicated speed on the speedometer. Turn off the drill and hold the disk with thepointer at the observed speed. Gently push the pointer to the calculated speed(62.5) and retest. Adjust the pointer until it reads exactly the calculated speed. Now you can attach the speedometer to the cable in the car and drive a measuredmile and adjust as outlined above. These adjustments will set the speedometeronly. It will not set the odometer, which is gear driven as stated below.

THE ODOMETER: Mechanical Description

The odometer is gear driven. The cable turns a worm screw which turns a 32(sometimes 20 or 25) tooth gear. This gear is directly attached to a pawl via aneccentric pivot. Every turn of the gear will pull the pawl once. The pawl turns agear at the end of the odometer wheels. This gear has a different number of teethdepending on the calibration of the odometer. This calibration is written in smallletters on the dial face above the “MPH”. 1152 and 1184 are the most common onthe TR4 series, Other speedometers may use a 20 or 25 tooth gear and more teethon the odometer wheel gear. Fewer teeth on the worm gear makes the movementof the odometer wheels much smoother. The calibration is always a wholemultiple of the 32 (or 20,25) tooth gear. For instance, 1152 is 32*36.

There are a few places where problems can occur.

    1. The worm may not turn, and this is the same problem as #1 in the speedometer section.

 

    1. The plastic 32 tooth gear can be stripped, or the clip that holds the pawl to the eccentric pivot may have fallen off allowing the pawl to fall from the eccentric.

 

    1. The spring pulling the pawl to the gear on the wheels may be weak or missing. This may prevent the pawl from touching the gear and thereby prevent any motion of the wheels.

 

    1. There is the very unlikely possibility that the wheels themselves no longer index properly.

 
ODOMETER: Calibration

Calibration of the odometer is not as simple as making an adjustment in the workssomewhere. Being completely gear driven, you need to replace the gear on theodometer wheel axle that is moved by the pawl. To do this you need theappropriate gear from an otherwise identical speedometer with the propercalibration. The simplest method to do this is to determine what calibration youneed. This can be calculated by driving a measured distance of road. The longerthe better, ten miles minimum. Then compare the actual mileage to the indicatedmileage. The calibration you will need will be: Old Calibration * Indicated Miles/ Actual Miles. If you drive 20 miles and read your odometer to about 1/2 of atenth, then you will get your correction factor to better than a half a percent andprobably close to a quarter of a percent. This is certainly a better calibration thanthe car had originally.

You can calculate the THEORETICAL calibration you need by finding the “turns-per-mile” (TPM) specification of your tires. Your driveshaft RPM (at 60 mph) isTPM * Differential Ratio. The differential for the TR2-4 series is usually 3.7. Itwill vary in other cars. You also need to know the number of driveshaft turns percable turn. On the TR2 through early 6 series it is 2.5 drive shaft turns to 1 cableturn (possibly 3.5 for Spits). For a TR2-4, the speedometer calibration you willneed is calculated by TPM * 3.7/2.5. This is a very theoretical number and youare much better off by driving a distance on a marked highway (at any speed) andcomparing the indicated milage to the real mileage.

Unless you are very lucky, you will not be able to find a speedometer with exactlythe required calibration. You can calculate the closest POSSIBLE (but notnecessarily available) calibration by dividing the calibration you think you need by32 (or 20, or whatever number of teeth had by the worm gear) then rounding tothe nearest whole number. For instance, if you find that your speedometer reads11 miles when you drove 10, and the calibration is 1184, then you need a newcalibration of 1184*11/10 = 1302. When divided by 32, this is 40.7. This roundsto 41, which tells us that the closest possible calibration is 32*41= 1312. Youneed to look for a real speedometer with a calibration of 1312, or at least as closeas possible to the calculated 1302. Any Smiths or Jaeger speedometer of the samevintage ought to be very similar in the works and may be able to be swapped. Forexample, I needed a 1280 speedometer and found one from an MGB of the 70’svintage. It would have been a perfect match for a TR6 speedometer and was aserviceable match for a TR4 speedometer. One from the 1960’s probably wouldhave been a perfect match. I will keep looking. After I did a 30 mile odometercalibration trip, I found that I need a calibration of 1344. This just happens to be awhole multiple of 32 (32*42), and it might be possible to find an odometer gear togive the exact calibration. Between all the cars that used these basic styles ofworks, there is a wide variety of calibrations to be found at fleamarkets. Youshould keep looking, but if you find a unit that has an odometer gear that is withinone tooth of your “ideal” calibration (around 2.5% variance), it will probably bequite sufficient.

Another option you have is to have an adapter made. It will have two gears with acertain number of teeth to convert a certain input cable RPM into a certain outputRPM. For instance, if I need a 1312 and I have a 1184 speedometer. They wouldprobably make an adapter with 41 teeth on the input gear and 37 teeth on theoutput wheel. One manufacturer of these is APT Instruments in Bloomington,MN (612-881-7095). The cost for one is reported to be about $40. I spoke withthem about the construction of an adapter. It seems that there is difficulty due tothe difference between English drive cables and US types. They made it soundrather complicated, and I did not spend the time (I was paying long distancecharges) to resolve the details of the difficulty. They suggested that thespeedometer “head” be recalibrated. I am not sure that the guy I talked tounderstood the exact construction of a vintage Smiths/Jaeger speedometer. He didsay that if I were to give him the true miles and odometer miles reading theywould be calibrate it perfectly for $120, and for that price, throw in a cleaning too! They would also then check, calibrate, and if necessary repair speedometer as wellas to calibrate the odometer. With an adapter, you do not need to look for scarceor impossible speedometer calibrations and still keep your original equipmentcompletely original.

Of course, during the calibration tests you need to be using the tires you plan onkeeping on the car and they need to be properly inflated. As the tires wear, thespeedometer calibration will vary. It is not worth getting too picky about theexact accuracy of the calibration since tires of the same nominal size may differby many tenths of a percent in their “turns-per-mile”.

ODOMETER: inspection

Remove the works from the speedometer as described in the prior section. Turnthe input shaft by hand. You should be able to twist the magnet wheel easily. Asyou turn the wheel, it should drive one or two gears from a worm. As each gearturns it should move a pawl via an eccentric pivot. The pawl should be seen toadvance the 1/10’s wheel of the odometer one tooth for each pull.

To further test the odometer wheels, you will need to be able to spin the works ata reasonable speed. The only way to do this without crashing your car is to use aspare tachometer or speedometer cable and a drill set to reverse. Run the drillabout 1000 rpm. As the drill turns the cable, you should see the worm gearsturning and the pawls advancing the 1/10’s wheel every few seconds. As the1/10’s passes 9, you should see the miles wheel advance by one.

If the gears and pawls are moving the wheels correctly, but the wheels fail toadvance, then there is a problem with the wheels themselves. The best correctionis to swap the entire wheel sets with a different speedometer of the samecalibration. If that is not possible, then the wheels themselves may be changed bydismantling the wheel sets. You will need a wheel set from a similarspeedometer, though the calibration will not matter.

Odometer: Mechanism

The “old” style odometers work by friction trying to turn all the odometer wheelsand then a restraining clip underneath the wheel prevents motion except at certaintimes. The drive gear is keyed to the shaft and there are keyed washers betweeneach of the wheels. The wheels themselves are not keyed and can turn freely. Asthe drive gear turns, it turns the shaft. The shaft turns the washers between thewheels. By friction, the wheels try to turn, but the clips prevent turning. Therestraining clips underlie two adjacent wheels, so one wheel can disengage theclip under the next wheel to the left. The left and right edge of each wheel have athin metal edge with notches. These notches engage the clips. On the right sideof each wheel the edge has ten notches. The left side of each wheel has one notch. When looking at two wheels, as the right wheel turns one entire revolution, itdisengages the clip under the left wheel once. The right wheel moves ahead byone notch, then the clip re-engages and prevents further forward motion untilagain disengaged.

The “intermediate” and “new” main and trip odometers function similarly. The”new” type has a different layout for the drive gear, but the odometer wheelsfunction similarly. The drive gear spins the shaft, then the shaft turns the 1/10thswheel. The main odometer has a 1/10ths wheel that is fairly narrow and has nolettering. The odometer wheels turn freely on the shaft and are indexed by a cogbetween adjacent wheels. The main odometer has the cog between the wheels sothere is no gap between the wheels. The trip odometer has more widely spacedwheels and the cogs are external to the wheels. These odometers turn much morefreely than the “old” types as they do not rely on friction to make any movement.

Odometer: Dismantling

It is usually only necessary to remove the pointer and face in order to access theodometer wheels. Further disassembly usually does not improve access to theodometer wheels, and exposes the pointer spindle and the coiled return spring topotential damage.

The worm-driven gears are held in place by a spring clip (see the explodeddiagrams). Remove this clip and then the gear may be withdrawn from the frame. Clean the bushing in the frame and the gear. Lubricate it with light grease andreinstall it. The spring clip is difficult to install, but with patience it will snap inplace. Damage to the odometer works is most likely to occur to this gear as it isthe only plastic part in the pre-1968 (or so) units. In later units, the entire works isplastic and damage can occur anywhere.

The odometer wheels turn on an axle. This axle is held in place by a spring clipon the left side of the wheels. In the older units this clip is a metal snap that has afinger which locks into a recess in the yoke holding the axle. The top edge islifted away from the yoke to unlock the snap, then it is pulled upward to remove. Then the axle may be slid out to the left or right. Depending on your needs onedirection may be more useful than the other. For instance, if you simply need toreplace the gear to the left of the wheels, then pressing the axle to the right slightlywill allow the gear to be removed without disturbing the odometer wheelsthemselves.

In newer units, the axle is retained by a split nylon washer. This washer is moredifficult to unlock from the axle. You need to use a fine screwdriver to displacethe leading edge of the washer out of its groove in the axle and then continue thedisplacement around the perimeter of the washer. Once it is out of the groove, theaxle may be slid out through the washer. Again, depending on the direction youneed to move the axle, you will need to displace the washer one way or the otheron the axle.

When the axle is free, you will see that the gear is pressed against a washer by aspring. This washer has indentations which match similar ridges on the gear. This causes the gear to “ratchet” forward and inhibits motion of the gear withoutthe pull of the pawl. When reinstalling the gear, washer, and spring, the springtends to fly across the room if given the opportunity. Be very careful thencompressing the spring and attempting to re-insert the axle through the spring. The wire of the spring will tend to snap into the groove for the lockingsnap/washer. A little fiddling with the axle and spring will allow the axle to movepast the spring.

It is possible to change the odometer reading by rotating the wheels. The wheelshave a locking mechanism that needs to be addressed in order to make theadjustment. Early speedometers have brass fingers under the wheels between theframe and the wheels which prevent forward movement, or backward movementpast zero. You can rotate the wheels backward by simply turning the wheel, butyou can not back up past zero. To rotate forward or backward at will, you need topress the brass finger underneath the wheel to disengage the lock and then spin thewheel

Newer odometers have cogs between the wheels. It is necessary to completelyremove the wheels from the axle, then turn each wheel individually to the properorientation, then re-stack the wheels and remount them on the axle.

The trip odometers are essentially the same except that only the gear on the end(in this case the gear is to the right of the 1/10ths wheel) will ever need to bechanged unless there is a damaged wheel. Newer odometers do not have a drivegear for the trip odometer. Instead there in are gears in the works between the twoodometer mechanisms to drive the trip 1/10ths wheel off the unlabeled 1/10thswheel of the main odometer.

This completes the article about speedometer calibration, maintenance, and repair. If you have any further questions, please feel free to Email me at ARhodes@compuserve.com.

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