Muncie M-20 Speedo question

[Jack P.]

Hi ,

Just when you think you done, you're not. As I try to finish all mechanical work on my 66 , prior to paint, I have decided to tackle the speedometer problem again. Rebuilt my dash speedo, and put in new driven gear (22 teeth) and fitting for 3.70 rear. Tranny is M-20 wide ratio, original to car along with motor.

I had speedometer working for first time in years..... for 90 miles !!!!

Dash speedo works........Driven gear chewed up at transmission fitting.

L79 M-20 and original rear was 3.36 Changed rear 35 years ago to 3.70

Question

Does the speedo drive gear on the tranny shaft have to be changed to be right for the 3.70 rear end or is the one on it OK for 3.36 and 3.70

Also, the spot where the driven gear (plastic) rests on the transmission housing is excessively worn. The plastic gear has vertical motion in fitting more than I think you should.

Can this be part of problem?

I was on my back all day to take out tranny,, Boy, a lift would really make that job easier. I only want to put this car back together once , before my trip this summer.

Comments on possible fixes please........

Jack

[Edward J.]

Re: Muncie M-20 Speedo question

Jack, as a rule the drive and driven gears are most always different colors and teeth numbers depending on the gears and tire sizes,

when the improper gears are not correct there maybe a gear meshing problem, also as you said the speedo housing should be tighter fit for the driven gear, also check for the rear tail shaft housing bushing to be worn, to much play in the rear tailshaft housing will cause a wearing problem.

Joe l. should be able to give you the correct gear Part no.s. for the ratio.

[Wayne M.]

Re: Muncie M-20 Speedo question

..... L79 M-20 and original rear was 3.36 Changed rear 35 years ago to 3.70
Question -- Does the speedo drive gear on the tranny shaft have to be changed to be right for the 3.70 rear end or is the one on it OK for 3.36 and 3.70 ...

Jack --- you could be in luck. Currently, 3.70 is the only ratio that has 22-tooth plastic driven gears available for both the large diameter mainshaft steel drive gear (factory for the small plastic gear family [3.08, 3.36, 3.55]) and the small diameter steel gear (factory for the large plastic gear family [3.70 and 4.11]). First pic shows the two gears, both 22-teeth. The gray gear, Gr 4.337 # 3987922, was not available until the '70s (Joe L. could verify). Your car would have originally had the large(r) dia. steel gear for the blue, small dia., 20-tooth plastic gear.

But need more info -- did you change the steel gear when you went to 3.70's a long time ago ? Did you leave the blue gear in place or go with the red 21-tooth to get a slightly better speedo reading ? Which color 22-tooth driven gear is in the trans now ?

Sounds like a small dia. shaft gear combined with a small dia. plastic driven gear that is not engaging enough and eventually wears under cable load, then slips.

[Jack P.]

Re: Muncie M-20 Speedo question

Jack --- you could be in luck. Currently, 3.70 is the only ratio that has 22-tooth plastic driven gears available for both the large diameter mainshaft steel drive gear (factory for the small plastic gear family [3.08, 3.36, 3.55]) and the small diameter steel gear (factory for the large plastic gear family [3.70 and 4.11]). First pic shows the two gears, both 22-teeth. The gray gear, Gr 4.337 # 3987922, was not available until the '70s (Joe L. could verify). Your car would have originally had the large(r) dia. steel gear for the blue, small dia., 20-tooth plastic gear.

But need more info -- did you change the steel gear when you went to 3.70's a long time ago ? Did you leave the blue gear in place or go with the red 21-tooth to get a slightly better speedo reading ? Which color 22-tooth driven gear is in the trans now ?

Sounds like a small dia. shaft gear combined with a small dia. plastic driven gear that is not engaging enough and eventually wears under cable load, then slips.

Hi ,

I think after you look at the photos, this problem I am having may be a problem with many others . I did not change the drive gear on transmission when rear was changed. It chewed the green gear and gray gear, do I need a 23 tooth? But look at photos

If you look at the photos of the driven gear photos you will see that the plastic gear moves in and out of the fitting toward end spot in transmission. The fitting is set in housing by and lock key the puts the fitting in the same spot (depth in housing) when inserted in housing each time.

The photo shows the ware spot on my housing. I am sure the engineers at GM machined the end spot that the gear rests on, to put the plastic gear in line with the drive gear on shaft. If the plastic gear can move further in because of ware on end shelf, it will no longer be in line with drive gear. I think,

Could this cause the plastic gear to be chewed even if gear (drive and driven) are matched.

What is the allowable amount of end play of driven plastic gear before it gets chewed up by drive gear?


What ever it is , I have more I think

Jack

I have to submit photos on new thread.

[Wayne M.]

Re: Muncie M-20 Speedo question

Here's another shot of a 24-tooth yellow gear (for 4.11 rear in '65-6, maybe others). I rotated pic so it is in actual position relative to the road. The bullet is fairly tight and will not wiggle. I'll check the surface the plastic gear turns against and report.

[Jack P.]

Re: Muncie M-20 Speedo question

Here's another shot of a 24-tooth yellow gear (for 4.11 rear in '65-6, maybe others). I rotated pic so it is in actual position relative to the road. The bullet is fairly tight and will not wiggle. I'll check the surface the plastic gear turns against and report.

See photos on new thread "Speedo phots muncie ""

Mine is about more that an 1/8 away

Jack

[Joe L.]

Re: Muncie M-20 Speedo question

Hi ,

I think after you look at the photos, this problem I am having may be a problem with many others . I did not change the drive gear on transmission when rear was changed. It chewed the green gear and gray gear, do I need a 23 tooth? But look at photos

If you look at the photos of the driven gear photos you will see that the plastic gear moves in and out of the fitting toward end spot in transmission. The fitting is set in housing by and lock key the puts the fitting in the same spot (depth in housing) when inserted in housing each time.

The photo shows the ware spot on my housing. I am sure the engineers at GM machined the end spot that the gear rests on, to put the plastic gear in line with the drive gear on shaft. If the plastic gear can move further in because of ware on end shelf, it will no longer be in line with drive gear. I think,

Could this cause the plastic gear to be chewed even if gear (drive and driven) are matched.

What is the allowable amount of end play of driven plastic gear before it gets chewed up by drive gear?


What ever it is , I have more I think

Jack

I have to submit photos on new thread.

Jack------


A 23 tooth gear (or any other tooth count gear) will not solve the problem you have. What's important with respect to your problem is the SERIES of the DRIVEN gears. Inasmuch as you've, apparently, tried both series 22 tooth gears (the ONLY tooth count gear that's available in both series) and that has not solved the problem, I'd say the problem must lie elsewhere.

The only thing that I can imagine that would affect the positioning of the DRIVEN gear with respect to the DRIVE gear would be wear on the cast-in thrust block in the extension housing. However, I've never heard of this being a problem, and I don't see how a nylon gear could wear this surface. Some "oddball" steel gears once used for "end-of-spectrum" rear gear ratios might wear the surface, but your application should never have used those.

Your fitting looks to be correct. However, if it were me, I'd try replacing it anyway. It's GM #345215 and it only GM lists for about 20 bucks.

[Jack P.]

Re: Muncie M-20 Speedo question

Jack --- you could be in luck. Currently, 3.70 is the only ratio that has 22-tooth plastic driven gears available for both the large diameter mainshaft steel drive gear (factory for the small plastic gear family [3.08, 3.36, 3.55]) and the small diameter steel gear (factory for the large plastic gear family [3.70 and 4.11]). First pic shows the two gears, both 22-teeth. The gray gear, Gr 4.337 # 3987922, was not available until the '70s (Joe L. could verify). Your car would have originally had the large(r) dia. steel gear for the blue, small dia., 20-tooth plastic gear.

But need more info -- did you change the steel gear when you went to 3.70's a long time ago ? Did you leave the blue gear in place or go with the red 21-tooth to get a slightly better speedo reading ? Which color 22-tooth driven gear is in the trans now ?

Sounds like a small dia. shaft gear combined with a small dia. plastic driven gear that is not engaging enough and eventually wears under cable load, then slips.

I want to agree with you. I believe the drive gear to be original as you stated. If I went with a blue gear 20 gear would the tendency to strip uder cable load be less? I don't care about the speedometer accuracy at this point, just the ability for the speedometer to work for more that 90 miles.

Jack

[Joe L.]

Re: Muncie M-20 Speedo question

I want to agree with you. I believe the drive gear to be original as you stated. If I went with a blue gear 20 gear would the tendency to strip uder cable load be less? I don't care about the speedometer accuracy at this point, just the ability for the speedometer to work for more that 90 miles.

Jack

Jack------


I don't think that using a 20 tooth gear will make one iota of difference with respect to the problem you're having. Maybe Wayne will have a different opinion, but I doubt it.