Greg Bull

Anybody know the easiest way to tell the difference between an M20 and M21 (close ratio) 4 speed trans? Don't know which is in my '67.

Thanks in advance,

Greg

knight37128

http://www.5speeds.com/muncie2.htm

JohnZ

If it's in the car, you can't tell without pulling the side cover and counting teeth; the Muncie "P-code" didn't have a ratio designator until October, 1968. However, you CAN tell while driving - if the rpm drop from 3-4 is about the same as it is from 1-2 and 2-3, it's an M21 (close-ratio); if the 3-4 rpm drop is significantly more than it is from 1-2 and 2-3, it's an M20 (wide ratio).

richbopp

Is there any difference in the gear shift throw length? I thought M21 had a short throw and M20 was longer.

knight37128

No.

The only difference is internal.

Frankenvette 63

Your the man John!

Don SSDD

I saw a calculator somewhere on the internet that would calculate MPH, RPM, tranny ratio, rear end ratio, and tire size. It would solve for what was missing and gave you a choice of the 2 Muncies. So if you know your tire size, rear end ratio, speed at a given RPM, it would tell you what your tranny ratio was in each gear.

Might have been NastyZ28 or somewhere like that- maybe it was a drag site? Anyone seen that too?

Don

Allcoupedup

If the transmission is out of the car, count the teeth on each ring of the cluster gear.
Brian

Allcoupedup

Long/short throw is determined by the holes on the shift levers. Using the outer holes results in a long throw. The holes closer to the shift shafts give you a short throw. The long throw is REALLY long in my opinion.

The same thing can be done with the clutch.

Brian

richbopp

Here's one of the calculators.

http://www.angelfire.com/fl/procrastination/rear.html

There's another one called "RPM Calculator Ver.1.5" from GearZone.net that you can download.

good luck

richbopp

Are you saying I can adjust the throw by changing the hole positions used on the linkage?

IGO200

If the trans is out of the car you can tell by the I.D. grooves on the input shaft (circumferential around the splines). 2 grooves= M20, 1 groove =M-21 (I think)

splidecision63

What about 3 grooves?

sting ry

Yes and no!! The 64 arms do not have the extra hole but you can buy 65-67 arms and put on your transmission and then change to the inner holes for a MUCH shorter throw. I did this to my 64 and WHAT A DIFFERENCE!!! It is really a pleasure to shift now, short and crisp. One of the best things I've done and cheap also.

Here they are from Corvette America http://www.corvetteamerica.com/cf/di...ord%3A%2035189

Nos 35189 and 35190

Cheers
Steve

buns

I know I will catch some flak for this, but, what the hey, I'm only quoting from a book.

According to Noland Adams, starting in 1966, an I.D. tag was attached to one of the side cover mounting bolts, and contained the part number of the transmission.

#3870354 RPO M20, wide ratio
#3880853 RPO M21, close ratio with L79
#3880855 RPO M21, close ratio with L36 and L72
#3879987 RPO M22, heavy duty gears

But if Bubba worked on your trans, it is gone for sure.

Allcoupedup

I didn't know that '64 didn't have the extra holes - very interesting.

One more thing - you will need to realign the neutral gates on your shifter which can be a real PITA. This will need to occur because the geometry of each gear linkage will have changed.

brian

sting ry

Actually I didn't have to readjust the rods, thought I would but didn't.

Cheers
Steve

66since71

I have found the adjustment is really simple (30 minute job, including jacking up the car) if you use an alignment tool to block all of the shifter plates in the nuetral position. It is just a small rectangular plate. I made mine form a piece of 1/8" aluminum. Seems like the shop manual has details? (loosen the rods, block the plates, tighten the rods...)

SMR 67

I'd have to check one of my Corvette books at home that I got this information from, but I'm almost positive that if you have a '67 327/300, then the only manual transmission option was an M20. This is how I determined what my original tranny was since my '67 was a 327/300 before the engine swap. The RPM drop from third to fourth confirmed this. If it's another engine/hp combo, I'd follow the recommendations in this thread.



Edit - I checked my reference book (Fact Book of the 1963-1967 Stingray by M.F. Dobbins). Chart on page 235: with a 327/300 you could get a three or four speed manual transmission or the Powerglide. The only 4-speed offered with this engine had the 2.52:1 low gear which identifies it as the M20. Not sure if this is helpful to you since your profile doesn't include your engine type.

Finn Jensen

If memory serves me right, there were no "three groove" Muncies. Zero grooves indicate M22 ("Rockcrusher").