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Author Topic: Muncie question M20 and M21  (Read 1365 times)
69Z28
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« on: February 27, 2013, 12:45:38 PM »

Hey guys. Set me straight on this will ya. I have a Muncie, P9E28B, 660 case, that has 2 rings on the input shaft. I don't know the history of it. I thought the M21 had 1 ring. Am I missing something? or is it possible the trans has been gone over? Appearance wise it looks like all Muncies I have come across that are dirty and crusty with road grime. Any thoughts on this? I was told it was an M21. Is there another way to tell without opening it up? Can the input shaft be spun and the output be counted for so many turns like you can do with figuring what gears are in a rear?
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GaryC

'UNRESTORED' 1969 Cortez Silver X33D80 Z28
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« Reply #1 on: February 27, 2013, 12:53:04 PM »

Gary,
The 'B' on the end of P9E28B' says it was an M21 when manufactured (that part of the trans), but after 44 yrs, most of the transmissions have had something done to them!  Smiley   break off ears, change the front housing, etc.    Is the VIN on the trans consistent with your car VIN?     Yes, you should be able to put the trans in first gear, and turn the input shaft until you get 1 complete turn on the output shaft.   At that point, have you rotated the input shaft about '2.5' turns (2.56 first gear meaning M20), or about 2-1/4 turns (meaning 2.20 first gear or M21 trans).
PS.  I've heard so many different things about the 'number of rings on the input shaft' which differed, and with so many possible changes in the trans over the years, I don't pay much attention to that particular feature.
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Gary W.  /  69Z28-RS, 72 B 720 cowl console rosewood all tint
69 Corvette convertible, silver/black 350 hp,
60 Corvette white/red, 72 Corvette coupe (2), 
90 ZR1 red/red #246, 90 ZR1 white/gray #2466
72 El Camino, '55 Nomad, '57 Nomad, '57 B/A Sedan
69Z28
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« Reply #2 on: February 27, 2013, 01:09:30 PM »

Hi Gary. There is no VIN on this trans. It was with my Z when I bought it though. I'll do what you said and see what I com up with.
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GaryC

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« Reply #3 on: February 27, 2013, 02:06:05 PM »

Input shaft grooves and transmission codes are discussed here...

http://www.camaros.org/trans.shtml#4speed

Paul
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69Z28
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« Reply #4 on: February 27, 2013, 03:07:50 PM »

Yeah. I read all that stuff. Been looking on line as well. This is what I found, the case is 3925660, the tailcone is 3857584 and the front baring retainer is 3915020. The side cover is 3831707, which is 63-65 with studs and which it does have. The input shaft has 2 ring grooves but the case is stamped P9E28 "B" for M21. However, doing what Gary said to do to find out what first gear is in the trans and it comes out as 2 1/2 rotations on the input and 1 full rotation of the output. So clearly this tran has had something done to it in the past. Not what I was expecting.
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GaryC

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« Reply #5 on: February 27, 2013, 04:05:37 PM »

Gary,

As discussed earlier, there are so many possibilities of combinations of Muncie parts (once they have been broken, repaired, combined, etc), it's difficult to deterine from external id cues.    Your trans seems to be a 'wide ratio' unit now based on your testing.   It also seems to be a 'combination' of parts, rather than a complete trans as manufactured by GM-Muncie.   Since it isn't your original trans (NO VIN), it doesn't much matter;  clean it.. check it. and use it as it is if it's in good condition.   (Wide ratios seem to work better with the Z28's anyway - if the 302 is present - as it gives you a bit more gear when pulling off - which the 302 needs unless it's revved above 3-4K..  *G*   OR unless you have very big gears (4.11 or greater ratio).
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Gary W.  /  69Z28-RS, 72 B 720 cowl console rosewood all tint
69 Corvette convertible, silver/black 350 hp,
60 Corvette white/red, 72 Corvette coupe (2), 
90 ZR1 red/red #246, 90 ZR1 white/gray #2466
72 El Camino, '55 Nomad, '57 Nomad, '57 B/A Sedan
69Z28
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« Reply #6 on: February 27, 2013, 04:50:01 PM »

Well I was hoping it was an M21. My car does have an M20 in now. I don't really need 2 of them. I had 4.10's in my rear with the M20 and loved it but I was getting tired of not being able to cruise at higher speeds so I went back to the 3.73's. Much better in my opinion. Now I need to sell this other one.


Gary,

As discussed earlier, there are so many possibilities of combinations of Muncie parts (once they have been broken, repaired, combined, etc), it's difficult to deterine from external id cues.    Your trans seems to be a 'wide ratio' unit now based on your testing.   It also seems to be a 'combination' of parts, rather than a complete trans as manufactured by GM-Muncie.   Since it isn't your original trans (NO VIN), it doesn't much matter;  clean it.. check it. and use it as it is if it's in good condition.   (Wide ratios seem to work better with the Z28's anyway - if the 302 is present - as it gives you a bit more gear when pulling off - which the 302 needs unless it's revved above 3-4K..  *G*   OR unless you have very big gears (4.11 or greater ratio).
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GaryC

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« Reply #7 on: March 01, 2013, 10:09:10 AM »

Gary,

Why not replace the input and cluster gear ? That's the only difference in the M20/21 - a lot cheaper than trying to track down another M21 (and one without a VIN like your case is -). Does it work (date-wise) for your car ?

Regards,
Steve
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1968 Z28 BRG/W
1969 Z28 X77 LeMans/W
1969 X66 L78 Cortez/BVT
1969 Z11 L48
69Z28
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« Reply #8 on: March 01, 2013, 12:09:34 PM »

Wondering what that would cost to do. I think I'll stick with the M20, though. I like how first gear is in the M20. It seems easier on the clutch.


Gary,

Why not replace the input and cluster gear ? That's the only difference in the M20/21 - a lot cheaper than trying to track down another M21 (and one without a VIN like your case is -). Does it work (date-wise) for your car ?

Regards,
Steve
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GaryC

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« Reply #9 on: March 01, 2013, 05:45:46 PM »

Best reason to use the close ratio is to keep the rpm's up in the power band in between shifts, especially using a 4:10 and up rear, but they work well with the 3.73's ( that's what my '68 has as a combination, my '69 is a 4.10, l78 is a 4.10). To answer your other question, you can sneak up on an input and a cluster gear sometimes fairly priced, or you can buy a new set (Italian built, I believe) for a nice higher price. Add the cost of a rebuild kit ($150.00 or so), and you have a good freshened M21. 
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1968 Z28 BRG/W
1969 Z28 X77 LeMans/W
1969 X66 L78 Cortez/BVT
1969 Z11 L48
69Z28
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69 X33D80 Z28


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« Reply #10 on: March 02, 2013, 01:46:11 PM »

Yeah man. I like the 4.10's as well with the M20, but I wanted to get back to original and went back to 3.73's Really not a whole lot of difference but I can feel it it in the seat of my pants when I let out the clutch. I have the trans for sale in a few places. It appears the market is saturated with Muncies and the prices are all over the place. If it doesn't sell maybe I'll keep it and change the cluster to 2.20.
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GaryC

'UNRESTORED' 1969 Cortez Silver X33D80 Z28
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