patmills

I want to trade a 1967 M-21 for a M-20. Are they interchangeable? My M-21 is in my '66 442 and I want to replace it with a M-20.
Thanks

2blu442

Hi Pat
The differences between the M20, 21 & 22 are internal. The outsides are the same. I believe some of the 1960's full sized cars may have had longer tail shafts so make sure that's the same. As I think about it there are two different tail shafts, one that matches the TH350 and the other is larger like the TH400. You might have to change the driveline yoke if the tailshafts are different sizes, but that's all.

John

patmills

Thanks John. My car is a standard transmission car stock so from your information, it looks like the transmission swap will be pretty straightforward.

Pat

stan 65 cutlass

hows it goin up there pat still runnin the the tri carb?

My442

Muncie differences:

Input shaft can be corse or fine.

Output shaft can be big or small (some M22s are big)

The shift levers cab be stud mount or bolt on.

The cases are the same size.

patmills

OK Stan. Still running tri carb but need to get rid of my high first gear and reverse gear. I hate riding the clutch to get a vehicle going.
How's your car and how are you doing?

Pat

patmills

Thanks MY442 for the info. It sounds like it might be a better idea to just switch first and reverse gears to lower ratio.

stan 65 cutlass

Im fine pat, and my car is still
In pieces, but ive been gathering lots of good parts, 425 all new ready to drop in, hurst comp plus shifter, need to finish welding and get some paint on it, i have an m20 in my car and have heard from a guy with a 65 gto and an 63 impalla both manual shifts, the same thing about the gearing . My car launches hard even with 3.23 gearz

patmills

Your car sounds great Stan. I've got to give you a call and drop in and see you.

Pat

Chesrown 67 OAI

They are a direct swap.
M20 has 2.20:1 1st gear
M21 has 2.54 (or maybe 2.56)1st gear.

Thats the only difference -- depending on year you may have a difference in sidecover shifter arms in which case all you need to do is swap the side covers "in toto". In 1970 they went to the fine spline input shaft so if yours are pre-70 they are a direct easy swap. Trust me, unless you are running a really high rear gear ( like 2.56 or 2.73) you will barely notice the difference when driving. (MOO)!!

RandyS

I think the gear ratio's on the M20 & M21 are just the opposite of what you posted. The M20 has the 2.52 or 2.56 1st gear. 2nd, 3rd and reverse gears are lower than the M21 also.

Chesrown 67 OAI

My bad - ( I know better) Thanks --- Heres a link tha should answer all of your Muncie questions:


http://www.chevellestuff.com/qd/muncie.htm

Another article also says you should have 3.73 or lower to warrant the use of the M21 --- Ironically the M22 only comes in the same gear ratio as M21.

Octania

Agreed on the ratio error

M22 & M21 are "close ratio" - the lowest [first] is CLOSER to top gear at 1:1...

The M20 has more reduction in first, it's a wider spread of ratios.

Because the final drive has more multiplication built-in at 3.73 and numerically higher, you don't need as much multiplication in the trans's 1st gear, thus the numerically closer (to 1:1) first gear ratio found in the M21/22 typically mated to the performance rear ends in factory combinations.

allyolds68

Just out of curiosity, what gear rear end are you running?