Will I blow up my M21 4-Speed?

[Dub J]

Im worried that the M21 Muncie 4-speed that I bought isnt gonna last behind my small block 350. I was reading some horror stories on the board about guys that tear their M21 4-speeds all to hell when they power shift or dump the clutch (which I plan to do). I really like the idea of a straight shift in my truck, and dont really want to spend $2000 on a TKO500. Whats the next best alternative? Will I definitely blow the muncie up? I'm building a 350 .60 over with patriot 64cc aluminum heads, Comp 286 cam, Edelbrock Air gap intake, shorty headers and 750 double pumper. Im thinkin this motor will put out atleast 350-400 HP right? Is that too much for this lil tranny?

Has anyone put a T56 in from a Late model camaro? I'd love to do that, and saw somewhere that it could be done but couldnt find anything on the board where anybody has done it.

[Chevymann1968]

This is some info i would love to find out about as well!!!! I think that it would last!!! It should unless you start to run drag radials or slicks then i think that you will have a problem!!!

[bhoeger70]

It is really hard to break a m21 behind a small block. I had a m20 behind a big block in a chevelle and the only way I hurt it was doing stupid stuff (shifting without using the clutch for one).

[Dub J]

Really cause I was reading threads about guys tearing the teeth off the gears every so many thousand miles. It kinda got me worried. Do any of you guys know what 4th gear will cruise at on the highway say if your running 3:73 gears and 20 inch tires? It shouldnt be 3K at 75 or 80 should it? Im trying to get an idea of what I should do here, either stick to my Muncie or try to go 5 or mabye even 6 speed if I can get my hands on one for cheap enough.

[truckpoorpackrat]

If you are planning on dumping the clutch / speed shifting you will probably tear it up. Why don't you put a lenco or other race trans in it, then you will be here asking about the driveshaft or axles. Sorry, but if you abuse it something has to break.

[697498chevy]

Had a factory 4 speed in my '67 Chevelle many years ago . Did a bit of power shifting , even blew up the motor once . Never had any problem with the trans . My 396 had headers , Eledbrock , Holley , etc .

[BurnoutNova]

yes it will blow... send it to me before you ruin it!

I think it will be fine... some Muncies have had way more ower and abuse put through them than you will with your 350, and they lasted.

[truckpoorpackrat]

Guess I should ask what you have done to the motor (cam, nitrous, blower)? nowdays some smallblocks put out more power than bigblocks.

[Dub J]

350 .60 over with patriot 64cc aluminum heads, Comp 286 cam, Edelbrock Air gap intake, shorty headers and Holley 750 double pumper.

[special-K]

These Muncies were made for this.They were muscle car transmissions,tough as nails.That M21 will hold up better than any car 5spd.

[thirdstreettito]

My dad had a 68 Camaro with a 425hp 350 and a stock M-21. He never blew it up. He put 10's of thousands of miles on it and had no problems. I guess it all depends on who is driving and how they drive/shift.

[Dub J]

Well I dont plan on intentionally pushing the limits of it, but If I were to be involved in a lil street scurmish, I wanna know that I can bang on it (within reason) if I need to and not disentegrate the guts inside.

[Dub J]

How much stronger is the M-22 Rockcrusher, than the 21? I know their supposed to have straight splines which are supposed to be better I hear.

[truckpoorpackrat]

You should be OK & parts for it should be reasonable .

[SactoJim]

Beat the hell out of one in my 68 Camaro for years. Managed to break the input shaft off one night when a couple friends were in the back seat and I said, "watch this"! Extra weight made it hook, and...... SNAP!

Big advantage in a truck is lack of rearend weight. If you load that thing up with a bed full of dirt and then doing 4,000 rpm clutch dumps, you will probably find any weak points in the tranny.

[ChevLoRay]

My memory says the thing most people complained about with any Muncie transmission was the stock Muncie shifter. Hurst made a lot of $$ selling shifters to replace the stock ones. Seems the shifters got the blame for missing shifts and 86-ing the gearbox as a result.

[Dub J]

What RPM do you think I'll be at in 4th gear driving around 70-80 mph on the highway with 3:73 gears on 20" tires? Anybody know?

[special-K]

Nah,you see muscle cars,hot rods,and old stuff in the right lane on interstates all the time.3,000 rpm is cruisin' by old standards.You`ll roll along at 65-70 ok.

[weircc]

well for starters i would say it depends on what kind of shape its in. is this an original? has is ever been rebuilt? im asking cause if its worn out because over the last thirty years someone has wore and tore that thing up there's a good chance it wont hold up.but if it has been professionally rebuilt you should be good to go i have a m-21 behind a 468BBC pushing around 500-550 hp in my camaro and believe me it gets ran through the ringer every time it sees the road.And yes 3500rpm is probably about right at 70mph.And just to make another note yes the m-22 is a lot stronger and will make a howling noise don't ask why they just do for some reason.

[thirdstreettito]

Just talked to my dad, he thinks you should get an M-22 "Rock Crusher" simply because of the fact you wont be babying it.

[68C15]

the M21 and M22 are identical except for the 22 having straight cut gears. the M21 has the typical helical cut gears.

I see no reason why a M21 wouldn't hold up. unless it is either been rode hard and put up wet or you have a 500 caddy with 14 PSI of boost. (that would be ultra cool)

as for the RPM at speed http://www.csgnetwork.com/multirpmcalc.html

[Gray Ghost]

No reason why the M21 won't hold up. They were made to be shifted hard and cruise long miles at 70. Just be somewhat sane about clutch operation and you will be fine.

[chevy29267truck]

also make sure the grear oil is full too

[spinning wheels]

An M21 will hold up, providing it's in good shape and you don't grind the gears.
I have one in my '70 Z28, running factory LT-1 engine that was rated at 360 Hp (gross). The car also has the factory 12 bolt with 4:10 gears and it screams down the highway, with 26" tall tires. It came stock with a Hurst competition shifter.
I also run a Valley gold series 10 inch clutch, that were intended for Bonneville runs.

The M22 would be a better trans for what you want to do, but you will be fine with the M21. You may want to invest in a scatter shield, just incase your clutch flies apart. I sold my 2 piece Lakewood scatter shield, cause it hung lower than my headers, and the headers were already scrapping.
So unless you have your truck lowered into the weeds, a single or 2 piece scatter shield will work great in your truck.

Good luck

[BurnoutNova]

Originally Posted by spinning wheels

The M22 would be a better trans for what you want to do, but you will be fine with the M21. You may want to invest in a scatter shield, just incase your clutch flies apart. I sold my 2 piece Lakewood scatter shield, cause it hung lower than my headers, and the headers were already scrapping.
So unless you have your truck lowered into the weeds, a single or 2 piece scatter shield will work great in your truck.

Good luck

To give my input on this... The M22 would not be a good investment at all, They are so hard to find and valuable that it isnt worth beating one up. They aren't much stronger, the angle on the gears is diffrent making them have less thrust loading on the main shaft, so it is a bit stronger... but not enough to spend the money on a rare trans. They can be worth $1500 to $2000 in come cases!

They do make aftermarket cases, that are much stronger, and as stated above a scatter shield for safety reasons, but a M21 in good shape will hold up fine.