Odometer gear swapping

[AlfaBMWGuy]

I am trying to fix a broken speedometer.  I found after removing the speedometer housing that my flywheel had come off the shaft--a not uncommon failure point from my investigation.  There is a nice YouTube video that goes over the repair here: https://www.youtube.com/watch?v=oh4C5a1pSbI

 

There is also a FAQ thread that mentions this issue, but the video is more comprehensive so I'll just link that in case it proves useful to someone.

 

I found that my flywheel had not only come off the shaft, but would not spin freely in the housing when reattached, which I suspect is the reason it worked its way off the shaft over time in the first place.  So I proceeded to remove the odometer gear that's pressed into the housing with a ball bearing to see if I could get the flywheel to spin freely.  This piece inside the housing has a nylon gear at the end and is one of the failure points for odometers--the other being the tenths wheel slipping on the shaft between all the number wheels.  I wanted to just clean up this existing gear inside the housing, lubricate it, and hope the flywheel would turn smoothly when reassembled.  However, I found that there was quite a bit of wear on the nylon gear and even though the odometer was working when I removed it, I don't think it's long for the world.

 

I happen to have a spare speedometer, but it's both metric units (my 2002 is from Canada, but the car is now in the U.S.) and poorly calibrated along with not the correct crosshair face for my '74 tii and it has a needle for the earliest pre-'71 models so someone had messed with it in years gone by, probably leading to the horrible calibration.  So lots of reasons to replace it.  But, the nylon gear on this metric one is in much better shape.  What I'd like to do is use it instead.  But, the shaft it's mounted on, the worm gear part on the opposite end from the nylon gear, has a different thread pitch than one I'm using (the one for miles and not kilometers). 

 

From previous experimentation I've found that those thread pitches are critical to a metric vs. miles gauge (and I assume that's part of how speedo/odo's are matched to different diff ratios as well).  Below is a picture so you can see what I'm talking about with the thread pitch differences--the top one with the finer threads is for the metric speedo/odo (also has the nicer nylon gear), while the bottom one has the coarser threads (and the more mangled nylon gear as you can see).

 

With all that, can I remove the nylon gear from the shaft and just swap them over so I've got the worm gear from the bottom one in the picture and the nylon gear from the top one?  Is it just a friction fit?  Maybe push it off with a washer from below since I don't want to put pressure on the nylon gear threads?  Would some superglue make sure it wouldn't slip if I did swap them?  Or is it not possible and I just need to run the more mangled nylon gear and hope for the best so I know I've got the needed thread pitch for a properly calibrated odometer?

 

Thanks,

Gary

 

[Son of Marty]

Try odometergears.com and just get a new one, slipping in the odo gear train is common and usually starts when someone resets the odo while the car is in motion so you windup with the odo  going forward while your winding it back and the gears are forced to slip.

[AlfaBMWGuy]

Thanks!  I was going to mention odometergears.com: https://www.odometergears.com/products/BMW/2002+Series+68-76/23

 

But, my post was already too long.  From the link above, they only sell the nylon top of the gear and not the whole thing with the shaft.  So that was part of my assumption that it must come off because they are having you put on the new one they provide and, therefore, also take off the old one first.  But, if you are replacing, you don't care about damage to the existing one and that's not the case for one of mine that I want to swap over of course.

 

I was hoping to get this project done quicker (and saving $25 is never bad) since I do have a decent one, just not on the shaft with the worm gear drive I need.

[AlfaBMWGuy]

I am going to answer my own question here because it's clear I'm diving into fiddly work where few have gone.  Turns out that it is a "friction fit" for the nylon gear on the shaft with the worm gear drive at the other end.  Below in pictures you'll see how I used a small socket (along with a couple pairs of vice grips) to help extract the nylon gear without damaging it and then both my source and destination nylon gears and shafts once pulled apart.  You can see that the rough surface under the nylon gear is designed to grip tight even without glue so it was easy to tap on the better gear to the shaft I needed to have the right odometer calibration.

 

Astute readers might spot two things:  1. My source/good nylon gear is white where in my picture from yesterday it was yellow.  I actually had a third non-functioning odometer with that white gear I decided to go with.  2. There is a slight bend in the shaft for the source one--I wasn't very careful in removing the nylon gear from this one since I'm not planning on reusing it and this was the result.  Just another reminder that this is fiddly work with delicate parts.  The final pic shows what I intend to run, which I feel much better about in terms of having an odometer that will continue to function.  Note it's the top shaft with the bottom nylon gear (not mangled like the top nylon gear) from the 2nd pic.

 

 

 

 

-Gary

[Dug Nichols]

This is an awesome investigation write-up! Thank you. I've done self-work on both of my cars' odometers, but I hadn't found that particular Youtube link. Awesome. (My issues were related to the shaft-slipping issue, well documented by Curt Ingraham) 

 

Sounds like you've got a good solution planned, and would love to know results.

 

 

[AlfaBMWGuy]

Thanks Dug.  I was able to make some more progress today.  I now have my speedo/odo reassembled with that swapped nylon gear.  I used a flywheel/shaft from my original metric speedo because that one had never come apart (making me more confident in it vs. a superglued failed one) so it was a matter of swapping the couple worm drive gears from my failed mph speedo/odo so that I had the right calibration.  As I mentioned, parts from a '71-'73 odo won't do it as the gears are just different ratios than what is needed for the internals of a '74-'76 odometer.

 

The only other "trick" I needed to do was that the speedo housing for that nylon gear with the worm drive gear on the other end (the one pictured above) was just too tight of a fit in my metric speedo housing (the one without a failed flywheel) for it to spin easily as needed for everything to work reliably.  So I took off the nylon gear again and put the shaft in a drill and went to town with sanding off just enough of the circumference that it went in and spun smoothly.

 

Speaking of drilling, I was able to use my old speedo cable (since I originally thought my whole issue was the speedo cable and it turned out to be the gauge just slowly failing) hooked up to the back of the repaired speedo/odo and connected to my old school corded drill to verify I had a working speedometer and odometer when I was done with the basic repair (see pic below).  I even did a quick calibration test on the odo to make sure I wasn't completely off on all these odo gears (e.g., that it would still roll over in kms and not miles despite my parts swapping).  I drove the speedo at around 30 mph for 60 seconds and it did turn over approximately 0.5 miles (within my precision for controlling the speed of the drill holding it at 30 mph) as hoped.  With that I simply mounted the speedo back in my instrument cluster.

 

Now I just need to get the cluster back in my '74 tii and check it in the final configuration, which should happen in the next couple days and I'll report back whether I can declare victory or not.

 

-Gary

 

Edited June 20, 2022 by AlfaBMWGuy

[AlfaBMWGuy]

Well, it was a mixed success.  I do now have a working speedometer and odometer so my basic repair was fine.  But, I've learned that the internals of the speedometer housing that the nylon gear with the worm drive fits in is directly part of the speedo calibration.  The one I am using that never had the failed flywheel issue is from the metric speedo from my Canadian market tii that I swapped out for other reasons including just wanting a mph gauge.  But, now I'm getting a metric speedometer reading on my U.S. mph face.  I need to disassemble the speedo yet again and this time repair the failed flywheel as documented in the Youtube video that I linked in my original post.  I hope to post another update later this week.

 

-Gary

Edited June 21, 2022 by AlfaBMWGuy

[AlfaBMWGuy]

Closing the loop on my speedometer/odometer repair,  I now have a functioning repaired speedo/odo!  It did require doing the flywheel to shaft connection repair on my original speedometer back housing to get a speedometer calibrated to miles where the housing from my original metric speedo couldn't just be swapped over.  Below is a pic of my repaired flywheel after applying superglue and letting it sit for a day before putting it all back together.  You can see the superglue I applied where the shaft meets the flywheel--keeping glue away from the hole in the center of the shaft for the speedometer pin is critical!  The repaired speedo also has 90% of the bounciness in the needle gone along with the subtle but annoying clicking sound it was making while it was doing the bouncy thing at lower speeds.  Odometer functioning great with the swapped nylon gear as well.

 

One other trick I had to do to get that nylon to worm gear piece to spin freely in the housing after I extracted it by pounding out the ball bearing was to sand down the length of the shaft.  My theory is that just by pounding it out of the housing it bends the shaft enough to no longer let it spin smoothly.  I took off the nylon gear again and put the shaft in my drill to do the sanding that was needed.  A bit of sanding, test it in the housing, a bit more sanding, test it, etc.

 

As I put it all back together doing the old speedo cable connected to a drill test multiple times was also reassuring to verify function.   Now the only issue is that the speedo continues to read about 10% fast at highway speeds (I'm running typical 185/70R13 tires) just like it did before the failure.  I'd like to get that down to the usual 5% fast, but for now I'm just happy to have a functioning speedometer again.

 

For those who are having speedo/odo issues where advice in this thread and others doesn't solve the problem, I did find you can actually still get new 2002 speedometers.  A good number are even stateside currently waiting for buyers so no overseas shipping.  BluntTech, my go to new parts supplier, got me this information so I'd recommend contacting them.  They aren't cheap at $375, but it's surprising to still even have that option to buy new.

 

-Gary

 

Edited June 24, 2022 by AlfaBMWGuy