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Discussion Starter #1
I just spent the last 3 1/2 hours fishing the speedo gear out from my tranny while it was still in car. A dark cloud of profanity hangs above my garage like a looming thunderstorm...I even invented a few new curse words. Just about to try and take off the tailshaft with the rest of trans still in place, when out the &%[email protected]@!!*#$!! thing popped ("Hi there!! Looking for me?!"). (Thank God...I tried briefly to knock out the reverse gearshaft pin in order to unbolt the tail, but lying underneath the car with limited clearance it was impossible to get any leverage for a good whack at the damn thing. I was sure I'd be pulling the whole #$#%*&% thing out.)

A little background: I have 2 Muncies; an M-20 in the 69 and an M-21 as a spare. The speedo is way, way off(reads too fast) with the M-20, but when I had the M-21 in the car a few years ago it was very close. So a thought comes to mind to swap the speedo gears. Unfortunately, when I pulled out the doodad that holds the gear in the M-20, the gear stayed inside, just hanging on the lip of the entrance...so ever so gently I tried to grab it with the needlenose....almost got it....closer, closer....blip, it's gone. [email protected]%$#!!!! Anyways, now that I've got it out and compared it to the the one in my M-21, I see that they are different sizes. The one in the M-20 in the car is SMALL and has 18 teeth; the gear in the M-21 is LARGE and 23 teeth.

THE question: am I correct in assuming that large and small gears are not interchangeable?? I'm afraid of sticking the large gear in and chewing up it and the trans output shaft gear the first 5 seconds I drive it. I'm guessing that the gears on the trannie output shaft come in different sizes, too. Is this correct? Is it true that these speedo gears come in only 2 sizes, or are they of different sizes, depending upon how many teeth they have??

Sorry this was such a long rant, but my adrenaline level is still quite high. The brews have not yet taken effect.

One more question: my M-20 has the speedo cable input on the driver's side, but the M-21's is on the passenger side. Why is this?? Was this something that was specific to whether you have a wide or close ratio? Or did it have to do with the car the tranny was originally installed in. I was told by the seller that the M-20 had come out of a Corvette, but my M-21 was original to the car. I never thought about this before....I'd thought they were all the same.

To anyone who goes through this in the future...it is possible to jam your finger in the hole and gently, GENTLY, slide the gear forward far enough so then you can gently, GENTLY!! loop a thin piece of wire around it to pull the end up to the hole....then you get your needlenose pliers and gently, GENTLY, DAMN IT!! pull it out. This calls for mega-patience, and failing that at least remove all children from the area. You'll still have to remove the shifter and loosen up the E-brake cable. This only holds for Muncies with the speedo cable on the driver's side...until I lose a gear inside my other one.

Ok, this one's empty, time for another, back to nursing my wounds...thanks for any help here.

[This message has been edited by Larry (edited 06-06-99).]
 

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Can't remember seeing a Chevy speedo gear on driver's side. Evidently I just haven't seen one since you have it. Anyway, the speedo gears will work over a wide range with the same output shaft gear. Seems strange but I've run several different rears in my car and used different plastic gears with no problem.
 

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Larry,
First of all, the left side speedo gear is on the early Muncies (63-65), 66-later are on the right side. The side speedo gears range from 17 to 25 teeth. There are small diameter gears with 17-22 teeth (3987917-22) which are designed to be used with a larger drive gear on the output shaft. The larger diameter gears 22-25 teeth (3860345-48) are designed to be used with a slightly smaller drive gear. I have been shifting, breaking, welding, modifying, rebuilding and swapping Muncie parts and trannys for over 30yrs. I stock, use and interchange the speedo gears, big and small, with no problems. There is only about 1/32 difference in diameter between the big and little gears. You mentioned that the 23 tooth gear gave you a closer speedo reading. The 24 tooth is yellow and the number is 3860347. Most chevy dealers stock these gears, or can order them in a very short time. They are very common because they are also used in the turbo 350.
I am aware of the problem you had, this is usually not a problem with the speedo gear on the right side since it points down.

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Discussion Starter #4
Thanks DZ...for future reference, if my M-20 is an early model, does that mean it has the smaller diameter cluster gear shaft? I thought all early units had studs on the shifter bosses to hold the arms on. Mine uses bolts. Even so, i'd guess that the tailshaft could have been swapped at one time.
 

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The answer to your question should be yes, it should have the small cluster shaft. But, because there is so much interchangeability with Muncies, it is literally possible to have a 74 main case with a 63 tail housing or vice versa (or anything in between). And on the side cover, you could have early or late covers with stud or bolt shifter shafts or one of each, they all work. You will need to check the casting number on the main case to determine if it is big or small cluster shaft. The 3831704 and 3851325 are the early main cases. The 704 is 63 ONLY, you don't want one, there are some differences. There are 3 different versions of the 325 case, but still ALL are interchangeable. And there are 4 different tail housings. The 63-64 (earlier I think I said 63-65, that's wrong) with left side speedo, 65-70 with rt side speedo, 71-74 with the big tail shaft(turbo 400 style) and last, the 63-64 LONG tail housing (about 6in longer) which requires a longer tail shaft (for full size Olds-Pont) with lt side speedo. And the there are several modifications that can be done. For example, an early, small shaft case can be bored out to accept a large cluster shaft. And, when the hole is bored out, it can be bored off center to RAISE the cluster gear closer to the main shaft gears, which increases the strength,-------------but, makes it a little noiser. Also, because aluminum can be welded fairly easy, I have had numerous repairs done to discarded cases. So, DON'T EVER THROW AWAY A MATCHING NUMBER MAIN CASE. Maybe it can be repaired. And if the cluster shaft hole in the front of the main case is so worn that the shaft is loose and it leaks oil like a seive, IT CAN BE FIXED!
Just in case you want to know any of these things for future reference.

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