Author Topic: M20 or M21  (Read 5292 times)

firstgenaddict

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M20 or M21
« on: August 13, 2008, 04:40:44 PM »
How was it determined whether a car got a M20 or a M21?
I am restoring a numbers matching 69 and the car has a 4.10 rear and numbers matching trans...
The stamp on the trans is correct with no A, B, C suffix... (Trans assembled October 19th 1968).
Who or what options determined the trans selection?

James
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KurtS

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Re: M20 or M21
« Reply #1 on: August 13, 2008, 06:48:51 PM »
Depends on the car. An L48 or L35 could only come with a M20.

For a Z like yours, it was up to who ordered it.
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firstgenaddict

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Re: M20 or M21
« Reply #2 on: August 13, 2008, 07:13:41 PM »
As in the customer or the dealer?
Why would this be left up to the dealer?
I thought it would have been rear end ratio regulated... as a M21 with 3.55's would be hell to drive...
James
Collectin' Camaro's since "Only Rednecks drove them"
Current caretaker of 1971 LT1's - 11130 and 21783 Check out the Black 69 RS/Z28 45k mile Survivor and the Lemans Blue 69 Z 10D frame off...
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JohnZ

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Re: M20 or M21
« Reply #3 on: August 14, 2008, 04:45:04 PM »
As in the customer or the dealer?

It was up to the customer - he could order M20, M21 or M22. Although the 3.73 was the "standard" Z/28 axle if no other ratio was ordered, Chevrolet's AMA submission for the Z/28 option shows availability of the 3.55 as an "economy" option, the 4.10 as a "Performance" option, and the 3.07 as a "Special Order" option.

The 3.55, 3.73, and 4.10 were available with either M20, M21, or M22, but the 3.07 "Special Order" axle was only available with M20 (for obvious reasons). :)
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firstgenaddict

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Re: M20 or M21
« Reply #4 on: August 14, 2008, 05:45:25 PM »
So if a guy ordered a M20 with 4.10's... he knew what he was doing... right?
James
Collectin' Camaro's since "Only Rednecks drove them"
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JohnZ

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Re: M20 or M21
« Reply #5 on: August 15, 2008, 03:10:12 PM »
So if a guy ordered a M20 with 4.10's... he knew what he was doing... right?

That combo would definitely provide the best launch.  :)
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ericisback

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Re: M20 or M21
« Reply #6 on: May 02, 2020, 10:41:29 PM »
As in the customer or the dealer?

It was up to the customer - he could order M20, M21 or M22. Although the 3.73 was the "standard" Z/28 axle if no other ratio was ordered, Chevrolet's AMA submission for the Z/28 option shows availability of the 3.55 as an "economy" option, the 4.10 as a "Performance" option, and the 3.07 as a "Special Order" option.

The 3.55, 3.73, and 4.10 were available with either M20, M21, or M22, but the 3.07 "Special Order" axle was only available with M20 (for obvious reasons). :)

Forgive me for following up on such on old post, but I thought it easier to show the thread history, than restate it all. So here is my question:

If I have an original 1968 “MW” coded L35 with an original transmission, was it possible to order this car with an M21 from the factory?

Also, is there any way to determine an M20 or M21 by just looking?!? Or perhaps a stamp somewhere?  Thanks?
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bertfam

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Re: M20 or M21
« Reply #7 on: May 02, 2020, 11:44:11 PM »
No, the 1968 L35 was only available with the M20. As for determining what you have, without removing it from the car and counting the rings (or better yet, the teeth since the transmission could have been rebuilt at some point), there's no way to know for sure.

If the transmission is out of the car, then count the rings on the input shaft, or like I said above, the teeth.

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ericisback

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Re: M20 or M21
« Reply #8 on: May 03, 2020, 01:25:02 AM »
No, the 1968 L35 was only available with the M20. As for determining what you have, without removing it from the car and counting the rings (or better yet, the teeth since the transmission could have been rebuilt at some point), there's no way to know for sure.

If the transmission is out of the car, then count the rings on the input shaft, or like I said above, the teeth.

Ed


Thanks so much!
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67 GTO 400 conv. - 68 Camaro 396 conv.
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bcmiller

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Re: M20 or M21
« Reply #9 on: May 03, 2020, 02:34:45 AM »
As in the customer or the dealer?
Why would this be left up to the dealer?
I thought it would have been rear end ratio regulated... as a M21 with 3.55's would be hell to drive...

Hell to drive? Not really, but depends on the combo.

The combination of 550 hp big block, an M21 and 3.31 gears works just fine with a 28 inch tire. No it won’t have a rocket launch but with a good clutch the combination works and will light the tires up through 2nd gear.
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