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Discussion Starter · #1 ·
I have a true 71 SS. The numbers are matching......except on the M-21 transmission. It's numbers prove it to be a 70 M-21 and it's 27 splined output shaft further confirm that. From all my research, the M-21 is correct for the car but should be a '71 model with a 32 spline output shaft. I know GM often times used parts from previous years in it's new models. Is there anyway to confirm this? I don't want to be a liar and say the car is "numbers matching" when it's not. Any help on this is greatly appreciated.
 

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Your Vehicle’s VIN number will be stamped on the main body case of the transmission. If it matches your VIN, then you have Numbers Matching (and original transmission to your Chevelle). If it doesn’t match your VIN, it is not the original transmission.
 

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Discussion Starter · #5 ·
Your Vehicle’s VIN number will be stamped on the main body case of the transmission. If it matches your VIN, then you have Numbers Matching (and original transmission to your Chevelle). If it doesn’t match, it is not the original transmission.
Thank you!! THIS is good information.
 

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Discussion Starter · #6 ·
I'm confused. You say it isn't numbers matching, but then ask if it's okay to say it's numbers matching.
READ SLOWER!!!! That is NOT WHAT I SAID................the engine, the rear end and the vin tags ALL match. The tranny does not. My question is .......Is it possible that the Baltimore plant threw a leftover '70 M-21 into the car AND is there anyway to confirm this? I also stated I DO NOT want to say it's numbers matching if it isn't.
 

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I hate to burst your bubble, but the factory would still have stamped your VIN in the transmission before installing it on the assembly line (like your engine and rear end). More than likely it was either a dealer replacement or a previous owner(s) installed it. Unless you were the original owner, I am going to bet this what happened.
 

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READ SLOWER!!!! That is NOT WHAT I SAID................the engine, the rear end and the vin tags ALL match. The tranny does not. My question is .......Is it possible that the Baltimore plant threw a leftover '70 M-21 into the car AND is there anyway to confirm this? I also stated I DO NOT want to say it's numbers matching if it isn't.
TRY AGAIN. Your second sentence IS fact. It is not numbers matching.
 

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I have a true 71 SS. The numbers are matching......except on the M-21 transmission. It's numbers prove it to be a 70 M-21 and it's 27 splined output shaft further confirm that. From all my research, the M-21 is correct for the car but should be a '71 model with a 32 spline output shaft. I know GM often times used parts from previous years in it's new models. Is there anyway to confirm this? I don't want to be a liar and say the car is "numbers matching" when it's not. Any help on this is greatly appreciated.
It does appear that your transmission is in fact not from your vehicle. GM/Chevrolet would not arbitrarily install an earlier model year transmission in any vehicle. Aside from the difference in output shaft splines and corresponding extension housing between the 1970 and 1971 M20/M21 transmissions, the input shaft also is different with 10 splines on the 1970 M20/M21 and 26 splines on the 1971 transmission. So, the driveshaft/slip yoke/U-Joints as well as the clutch disc would also have been changed. That just wouldn't have happened. Sorry to be the bearer of bad news but your original transmission is probably long gone.
 
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A lot of good information above, especially by David Bates.
Just for additional information, ALLLLLLLLLLLLLLLLLLLL 71-later cars that got a Muncie 4sp, had the 26-spline input and the larger 32-spline tail shaft. Which as David pointed out, required matching parts. When the factory made a production change to a car line, there would not have been a retro fit to "remove" earlier parts from inventory.
If it were my car---------------------------AND IT IS NOT----------------- I would round up a 71-later style Muncie and return it to correct configuration. A 71 Muncie would have a 661 main case and a 764 tail housing.
 

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Ditto for sure. While the big money is in complete numbers matching, having the same trans that the car came with off the line is almost close enough for government work.
IF, IF, IF the original Muncie is gone, and it was desirable to install and original style Muncie (661 case), FREQUENTLY some of the transmission shops (D&L, Jody, etc) will have a raw case with NO numbers stamped on it. One of these cases would not have your VIN, but at least it would not have any VIN on it. I've been doing business with Larry Fischer of D&L for about 40yrs and have bought several cases from him. Also, he has repaired a few for me and/or bored EARLY (63-65) cases to accept the larger 1in cluster gear shaft.
 
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Discussion Starter · #13 ·
It does appear that your transmission is in fact not from your vehicle. GM/Chevrolet would not arbitrarily install an earlier model year transmission in any vehicle. Aside from the difference in output shaft splines and corresponding extension housing between the 1970 and 1971 M20/M21 transmissions, the input shaft also is different with 10 splines on the 1970 M20/M21 and 26 splines on the 1971 transmission. So, the driveshaft/slip yoke/U-Joints as well as the clutch disc would also have been changed. That just wouldn't have happened. Sorry to be the bearer of bad news but your original transmission is probably long gone.
It's not bad news sir. Just cleans things up for me. Like I said, I have no desire to claim my car is something it isn't.
 

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I know GM often times used parts from previous years in it's new models.
GM did not use parts from previous years in its new models. As a matter of fact after the last car for a model year was built they got rid of all that years components. The interior materials/parts were burnt and metal parts (all parts) were scraped.
If your car has a numbers engine then your car is numbers matching.
 

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GM did not use parts from previous years in its new models. As a matter of fact after the last car for a model year was built they got rid of all that years components. The interior materials/parts were burnt and metal parts (all parts) were scraped.
If your car has a numbers engine then your car is numbers matching.
I find that very hard to believe. First off, their shouldn't be that many USABLE extra parts at the assembly plants once all the scheduled production has been completed, and anything further back in the supply chain could just go to parts distribution. As for NEVER using previous years parts in the next year's stuff, well, I have an all original 1973 454 from a C20 pickup with a steel crank. Every source I have ever found says that ALL 1973 454's got cast cranks. Where did that steel crank come from? Please don't read any of this to mean that I think the OP's car has the as built transmission in it.
 

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DZAuto made a great point about the OP's transmission. The splines on the output shaft changed year over year 70-71, so substituting an older transmission on the assembly line would have required a driveshaft with a different yoke. EXTREMELY unlikely.
 

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DZAuto made a great point about the OP's transmission. The splines on the output shaft changed year over year 70-71, so substituting an older transmission on the assembly line would have required a driveshaft with a different yoke. EXTREMELY unlikely.
I agree in that case, with all the related parts that would need changed, it would be highly doubtful. I wonder if all the donor parts came from a wrecked car or something, and why that was cheaper/easier than getting the correct tranny? It didn't really matter with the cars WE were working on, but in 1985, I worked in a transmission shop for about six months. Unless it was an oddball or only needed minor repairs, almost no car that had the tranny pulled (automatics) got their original tranny back. By then you didn't really see muscle cars come into a shop like where I worked, (franchisee of a national chain) but I wonder what was going on say in 1975? Did a LOT of otherwise numbers matching cars get done in by tranny shops that wanted to get them back out the door the same day?
 
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Just one simple example. When the change was made in 1971 to a Muncie with the 26 spline input/larger 32 spline output----------------------REMEMBER, the clutch also changed to a 26 spline. Thus, when it was time to replace the clutch and you ordered a clutch for a 71 and it came with a 10spline disc, it would not work.Thus, replacement parts had to be compatible for the car they were ordered for.
SOOOOOOOOOOOOOO. I will not accept that a 71 Chevelle was built at a factory assembly plant with a 10 spline input/27 spline tail shaft. Didn't happen.

Now, with all of the above comments considered, here is a piece of false information that has floated around for years.
MANY NON-knowledgeable "armchair experts" have professed that if a Muncie had a fine spline (26 splines) input and the big (32 spline) tail shaft, then it was an M22. WRONG AS HECK!!!!!!!!!!!!!!!! Beginning with the 71 model cars, ALLLLLLLLLLLLLLL, repeat, ALLLLLLLLLLLLLLLL Muncies, REGARDLESS of M20, M21, M22, had the 26/32 spline shafts.

SO, with that tidbit of information, as always, there is an exception. For the 70 model Chevelles, IF, IF, IF it was built new with a 454 and M22 (NOT M20 or M21), then the M22 did get the 26/32 spline shafts. This is NOT hearsay or speculation, it is documented information.
 
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