Question: 4-Speed Identification

[Indyhac]

I'm new to Corvettes and trying to identify what make transmission my '74 has. I viewed a post with codes for Muncie 4-speeds. Mine does not have any numbers/letters that match those. Here is what is cast into the right side of the case: 1304 065 903
W. G. Div. FC2 (the C might be a 0)

My repair manual shows 3 different 4-Speeds for 1974, Saginaw, Muncie, & Warner. I would surmise that the W. G. Div. on the case means that it was supplied by Borg-Warner. The reason I want to find out is because I want to find a competent repair shop and would like to be able to tell them what transmission I have. This is because the transmission whines or growls in 1st, 2nd, & 3rd if I don't accelerate hard. If I drive it easy, it sounds like my grandfather's old '37 Chevy or a truck transmission. I didn't notice this when the owner and I went for a test drive because he didn't drive it very slow. Should have driven it myself! A guy I ran into today told me that a Muncie Rock Crusher would tend to be noisy but didn't think that those were available for '74 Vettes. If anyone is knowledgeable on this, I'd appreciate some help. I don't know if the VIN# sheds any light on this or not. It is: 1Z37J4S430107. I haven't found anywhere yet to decode the VIN. Thanks,
Scott:confused

 

[Vettehead Mikey]

The W does indicate this being a BW transmission. No '74 Corvettes had a Saginaw unit, nor were there any M22 'rock crushers' used.

The 'J' in your VIN means that the car has a base L48 engine. The only manual transmission available with the L48 was the M20 wide ratio unit.

 

[Indyhac]

4-speed identification

Thanks for the clarification about my transmission and the engine. The seller had told me that it is a base Corvette and that was what he liked about it so that it would be somewhat easier to maintain since he was not experienced with car maintenance. When you said that the only transmission available with the L48 engine was the M20, did BW make that? I thought that those transmissions with a M designations were Muncie products. One of the guys in a car club I belong to suggested that I change the transmission oil from 90W to 140W. He has had a lot of C2 Corvettes and used to race them. What do you think about using the heavier oil as far as quieting the transmission? Scott;help

 

[Vettehead Mikey]

Thanks for the clarification about my transmission and the engine. The seller had told me that it is a base Corvette and that was what he liked about it so that it would be somewhat easier to maintain since he was not experienced with car maintenance. When you said that the only transmission available with the L48 engine was the M20, did BW make that? I thought that those transmissions with a M designations were Muncie products. One of the guys in a car club I belong to suggested that I change the transmission oil from 90W to 140W. He has had a lot of C2 Corvettes and used to race them. What do you think about using the heavier oil as far as quieting the transmission? Scott;help

The 'M' is a standardized letter used by GM to designate the regular production option (RPO) code for any and all transmission options irrespective of which supplier actually manufactured it. M does not designate Muncie, just a coincidence.

Substituting a heavier weight oil to quiet noise would be just a band-aid. Get it fixed properly now.

 

[Indyhac]

4-speed Identification

Thanks for the clarification about the "M" designation. Good advice about getting the tranny repaired. I think that's what I will do.

Scott

 

[Hib Halverson]

As "Corvette Mikey" says, with respect to powertrain RPO, "M" is the prefix for a transmission.

M20 is wide ratio and usually the "base" manual transmission
M21 is close ratio.
M22 was close ratio heavy duty.
M40 was Turbo Hydra-matic 400 3-spd automatic
MX1 was Turbo Hydra-matic 350

This numbering system is still in use today.

Corvettes used GM's in-house four-speed manual until the end of 1973 when GM stopped putting Muncies in Vetts and bought the less-expensive and not as strong Super T10 from Borg-Warner. For a period of time in 1978 and 1979, the base Vettes used a GM-manufactured transmission. Some accounts say it was a Saginaw-produced unit and others say it was made my Muncie Gear.

But regardless of which source made the trans, after 1972 they were all either M20s or M21s.