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M21 Close ratio transmission, etc.

KOPBET

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M21 Close ratio 4-speed transmission.

1973 - 3,704 made that year and a little more than 3/4 of the BBs had them.

Good thing or bad thing? Is there anything special about this transmission?


And the CV5 3.36B CARRIER?
 

bossvette

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I think there were three Muncies two close ratios and one wide. Nothing wrong with a muncie they were the trans to have back then. Close ratio means the gear spacing is closer together (2:20-1 first) then the wide ratio (2:54-1 first) among the gears.
 

Mac

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From what I've read, the close ratio M21 worked well with the torque-monster big blocks engines like yours whereas the M20 worked well with the high-revving small blocks like mine. I don't know that there was any other advantages. The M22 (aka the rock crusher) had straight-cut gears which were supposedly bulletproof.


-Mac
 

Vettehead Mikey

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3,704 made that year and a little more than 3/4 of the BBs had them.


Where did you get this info?

A close ration gearbox has a much higher first gear, making the 'drop' in RPM smaller between the 1-2,2-3,3-4 gear shifts. Great for road racing, but a bit of a pain and no real benefit in street driving.
 

Crimson Thunder

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The Corvette Black Book was my source.My Uncle(sort of)had a 73 L48 coupe that he thought was special because it was an M22 car.I tried to find out how rare that was but couldn't find any information on any M22 73's.He sold the car 2 years ago.I never got to see the car in person.
 

KOPBET

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Where did you get this info?

The Black Book shows the LS4 .... 4,412 and the M21 .... 3,704. So if ALL the M21s went in behind the BBs (which I figure they would), that would be around 83% maximum.
 

KOPBET

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The Corvette Black Book was my source.My Uncle(sort of)had a 73 L48 coupe that he thought was special because it was an M22 car.I tried to find out how rare that was but couldn't find any information on any M22 73's.He sold the car 2 years ago.I never got to see the car in person.

As sharkcar71 said earlier, M22s were not offered in '73.

:w
 

Vettehead Mikey

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The Black Book shows the LS4 .... 4,412 and the M21 .... 3,704. So if ALL the M21s went in behind the BBs (which I figure they would), that would be around 83% maximum.

There's no reason to think that the only people that ordered M21s are the BB guys. The M21 was also popular with the L82 crowd.:beer
 

Hib Halverson

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M21 Close ratio 4-speed transmission.
1973 - 3,704 made that year and a little more than 3/4 of the BBs had them.
Good thing or bad thing? Is there anything special about this transmission?
And the CV5 3.36B CARRIER?

Nothing special about either. In fact, I'll bet the a lot of the BB's sold with M21 had that 3.36 axle.

It was said earlier in this thread that M22s, the heavy-duty four-speed some call a "rock crusher" has straight-cut or "spur" gears. That is not correct. M22s have helical gears but the helix angle is less than that of the standard-duty transmissions the M20 and the M21. Less helix angle makes for stronger gears but increased noise, hence the name "rock crusher"
 

Crimson Thunder

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There are numbers on the side of the case near the vin stamping that will tell you which one you have.I let a friend of mine borrow my book that has all the codes but,if they are the same as 69-70 the A=M20 B=M21 C=M22.Some late 74's had a Borg- Warner Super T-10.
 

Mac

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It was said earlier in this thread that M22s, the heavy-duty four-speed some call a "rock crusher" has straight-cut or "spur" gears. That is not correct. M22s have helical gears but the helix angle is less than that of the standard-duty transmissions the M20 and the M21. Less helix angle makes for stronger gears but increased noise, hence the name "rock crusher"
Thanks for the correction! Just out of curiosity, were the M22's gear ratios the same (or similar) to the M20 or the M21?

I read somewhere recently that the designation(s) M20, M21 & M22 were used on other GM product lines for transmissions which were nothing like the Muncies in the Corvette. In other words, the designation was simply a corporate label, not a product description so getting an M21 from a Pontiac was not a guarantee that it was anything close what was installed in the Corvette. M simply meant 'manual' and the number designation represented which option was chosen.

Perhaps JohnZ or someone can confirm this?

-Mac
 

Hib Halverson

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Thanks for the correction! Just out of curiosity, were the M22's gear ratios the same (or similar) to the M20 or the M21?

I read somewhere recently that the designation(s) M20, M21 & M22 were used on other GM product lines for transmissions which were nothing like the Muncies in the Corvette. In other words, the designation was simply a corporate label, not a product description so getting an M21 from a Pontiac was not a guarantee that it was anything close what was installed in the Corvette. M simply meant 'manual' and the number designation represented which option was chosen.

Perhaps JohnZ or someone can confirm this?

-Mac

The M22 had the same, close-ratio gears as the M21. The difference was the 18 deg helix angle rather than a 22.

During the period of time Muncie or four-speeds were produced, the M20,21 and 22 RPOs were used in many other GM vehicles but they always designated the same type of trans, ie: M20, std-duty/wide-ratio; M21, std-duty/close-ratio or M22 HD/close-ratio. I should add that prior to MY66 the "catch-all" RPO for a four-speed was M20 and the differentiation between a close or wide-ratio must have been handled some other way. Also, after about MY74 the transmissions were either Warner "Super T10s" or Saginaw four-speeds and not Muncies.

The "M" in those transmissions RPO code does mean "manual", however, it is true that during that period most transmission RPOs had the "M" prefix, such as the M40 (Turbo Hydra-matic 400).
 

Mac

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Thanks, Hib!

I went searching for the article but no luck. Too many magazines, too little time. Going from memory, the guy was talking about GM's various productions, not only in North America but globally. The example he used was a Holden pickup which had a transmission designated as an M22 but which was a three-speed built for an inline four or something like that.

-Mac
 
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Here's an M20 gearset, about a 42* helix angle on the teeth:



Here's an M22 gearset, about a 20* helix angle on the teeth:



Although the ratios were the same as the M21, the M22 gears were made from a premium alloy, had a different heat-treat, and the lower helix angle was used to reduce the fore-aft thrust loads on the case.

From '62 to '65, the engine option determined whether you got a wide-ratio (M20) or close-ratio (M21) gearset - it wasn't a customer choice, and the M21 option code which provided that choice didn't appear until 1966. All 4-speeds prior to '62 had close-ratio gearsets.

All Corvette 4-speeds from 1975-1981 were Warner Super T-10's.

:beer
 

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