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Hi all I have been having difficulty with my speedometer. The unit is one of the pot- metal early tr3 units. The odometer reads erratic and will jump hundreds of miles at a time. I took it apart and discovered that the little gear teeth that fit on the brass worm gear looking thing are ground off about a third of the way. I figured that the trip dial and the odometer were perhaps binding up and causing drag in the unit and damaging the teeth. Does anyone know if those little gears are available somewhere. I left the trip odometer alone as a unit but took the main odometer apart. I found this to be quit an interesting challenge because the main odometer unit looks like it needs to stay on the main body right next to the hair wire that drives the needle. One question I have is how freely do odometers move? I can advance the system by moving the ring gear with my fingers, but it feels like it has too much drag on it to be driven by those nonmetal gears. And can I randomly re-assemble the unit and then advance the mileage by lifting up the twang thingies. Anyway my plan is to oil the units up and find some new, I guess bake light gears, and then see what happens.
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TR2/3/3A tr3 speedometer
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dklawson
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If you don't already have this, download and print out the PDF by Anthony Rhodes on repairing Smiths speedometers.
https://home.comcast.net/~rhodes/speedo.html
I suspect what you'll need to do at this point is source another TR3 speedo for parts. Perhaps a speedo from some other British car would be a suitable donor. Look for the turns-per-mile number on the face of your gauge. It's likely that another gauge from a different car but having the same TPM number could be a worm and pinion donor for your gauge.
When you get the gauge torn down to the level of replacing gears, cleaning and lightly re-lubing the worm and pinion will be easy and should adress the tightness you noticed.
https://home.comcast.net/~rhodes/speedo.html
I suspect what you'll need to do at this point is source another TR3 speedo for parts. Perhaps a speedo from some other British car would be a suitable donor. Look for the turns-per-mile number on the face of your gauge. It's likely that another gauge from a different car but having the same TPM number could be a worm and pinion donor for your gauge.
When you get the gauge torn down to the level of replacing gears, cleaning and lightly re-lubing the worm and pinion will be easy and should adress the tightness you noticed.
martx-5
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I've been through this same scenario with my TR3 speedo. The shaft that's attached to that gear runs directly in the aluminium casing. Over the years, the grease used to lubricate it turns to paste and locks the shaft up. The fiber gear then strips. If they used oil or a much lighter grease, this probably would never happen. What I wound up doing is getting some nylon gears from McMaster that had the same number of teeth and approximately the same dimensions. However, it took quite a bit of work to get it to function. I had to ream the hole to press fit on the shaft. The diameter of the gear was slighty larger, so I also had to dremel each tooth down closer to the middle to get the gear to mesh properly...big PITA. It does work however.
<span style="font-weight: bold">dklawson's</span> suggestion to use parts from another speedo is most likely the best alternative...if the gears on the donor aren't also stripped. I have another speedo, and it has stripped gears also.
<span style="font-weight: bold">dklawson's</span> suggestion to use parts from another speedo is most likely the best alternative...if the gears on the donor aren't also stripped. I have another speedo, and it has stripped gears also.
Thanks Art and dklarson for your notes. I guess finding a different speedo would be nice and I do have some. The one I looked at for a donor has the skinny metal tin can case (perhaps 1959 and later) and the gears in it were perfect, but they seem to be different; they have more teeth. The old style ones come apart harder because the trip set goes through the speedo body without giving it adequate clearance for disassembly, so what I did was pull the pointer and dial off from the front. I talked to local guy and he said that I can remove the cotter key from the light hole and not go that route, but I could not get it that way and I think he was just trying to get rid of me because he could see I just wanted free advice. Anyways so Art you actually re- cut the teeth and bored out the hole? wow
martx-5
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BTW, if your interested, here is the thread where I go into much more detail on fitting the gear that I bought from McMaster.
Yes thanks Art that is right where I am at. I have read the Rhodes article but it does not cover the depth of your or my repair, and it does not cover the early tr3, the one with pot metal can. Anyway I wish I would have given thought to what you wrote earlier. Because what happened was about 6 months ago when I was fixing trip counters, I grabbed my old speedo I had put away in the 70,s and popped that in after I fixed that trip counter. My theory now is that this thing set in box for many years and the grease on the gears deteriorated and locked up and when I ran it, the gears striped. I am sure that thing worked fine when I put it away 35 years ago with a broken counter. What is painful is that point between maintenance and fixing something that is not broken, YET. I guess one should lube the instruments after a long restoration or the same thing could happen. Anyways what kind of oil should I use? I used some marvel mystery oil spray can type when I took it apart. Do you think that will work for a lubricant?