1967 M21 close ratio transmission questions

[Jeff B.]

Re: 1967 M21 close ratio transmission questions

Roy:
I have always thought the M21 transmission was available ONLY with rear end ratios of 3.55 and lower. Those with 3.36 and higher would get the M20. My car (437-390 4 spd, #8902) has the original AZ (3.55) rear end, and transmission thought to be an M21, based on the trans tag (3880853). It still has the original driven gear (green - 21 tooth). I've owned this car since 1978, and have talked to all previous owners. Am certain the drivetrain is original and untouched. Maybe this info can be of help.

[Joe L.]

Re: 1967 M21 close ratio transmission questions

Roy:
I have always thought the M21 transmission was available ONLY with rear end ratios of 3.55 and lower. Those with 3.36 and higher would get the M20. My car (437-390 4 spd, #8902) has the original AZ (3.55) rear end, and transmission thought to be an M21, based on the trans tag (3880853). It still has the original driven gear (green - 21 tooth). I've owned this car since 1978, and have talked to all previous owners. Am certain the drivetrain is original and untouched. Maybe this info can be of help.

Jeff-------

The 1967 L-36 and L-68 could have been equipped with an M-21 with 3.08: and 3.36:1. In fact, the 3.36:1 was the standard ratio supplied if no other was ordered

[Roy S.]

Re: 1967 M21 close ratio transmission questions

Roy:
I have always thought the M21 transmission was available ONLY with rear end ratios of 3.55 and lower. Those with 3.36 and higher would get the M20. My car (437-390 4 spd, #8902) has the original AZ (3.55) rear end, and transmission thought to be an M21, based on the trans tag (3880853). It still has the original driven gear (green - 21 tooth). I've owned this car since 1978, and have talked to all previous owners. Am certain the drivetrain is original and untouched. Maybe this info can be of help.

Jeff, Thank You for the confirmation. I'm sure you are not the only individual with a 3880853 close ratio transmission big block Corvette if they retain their original ID tag and are M21 with 3.55, 3.70 or 4.11 they most likely have that tag.

[Patrick B.]

[Mark L.]

Re: 1967 M21 close ratio transmission questions

Roy,
I agree. My 67 L71 had a transmission tag 3880853 and I'm pretty sure it was original to the car.

[Joe L.]

Re: 1967 M21 close ratio transmission questions

Joe—-I have a 67 L-71 that came with a 3:70, but I changed the ring and pinion to a 3:36. Did GM or any one else ever make a plastic driven gear for this application?

Patrick------

If you're talking about a DRIVEN gear using the existing speedometer DRIVE gear from a 3.70:1 application for an application of 3.36:1 the answer is not that I'm aware of. For sure, GM never offered such a driven gear and I don't think anyone else ever did, either. That's because I don't think it's possible to make such a gear. If it could have been done, GM would have done it.

The 3.36:1 application requires a 20 tooth DRIVEN gear. The lowest tooth count DRIVEN gear compatible with the DRIVE gear installed in your transmission is a 22 tooth green gear.

[Gene M.]

Re: 1967 M21 close ratio transmission questions

Was there a time when you could/could not order a 3.70/4.11 gear with a wide ratio?

Was there a time when you could/could not order a 3.08/3.36 gear with a close ratio?

I was under the impression a 3.55 gear got a wide ratio tranny.