Will I blow up my M21 4-Speed?

[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.