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Angled gears to not automatically indicate an M22. The angle is less in an M22 than M20/21 - more straight cut than angled. The lack of angle is what caused all the "rock crusher" noise.
https://www.hemmings.com/stories/art...ie-four-speeds
...an extension housing casting number of 3857584 indicates the tail shaft housing is from a 1965-'70 passenger car with a 27-spline output shaft. The side cover castings will tell you the type of shift shafts that were used internally.
The main case oil pans on the M20 and M21 did not have a fluid drain plug before 1970, whereas the M22 came with the drain plug in all years. Because aftermarket input shafts do not have the "grooves" designations, the only real way to know if you have the M22 ratio is to open the unit and check the cluster gear. If the helix angle is shallow (around 20 degrees), chances are you have the M22, or someone has upgraded to the M22 gears. M22 gears are also made of a higher-grade nickel alloy. The M20 and M21 cluster gears are cut at a 45-degree angle. The reduced gear angle of the M22 created a problem with noise (hence the name "rock-crusher"). Performance seekers decided to live with the noise when the benefits of increased durability from the gear-tooth angle outweighed the gravelly clatter. If you are not able to locate one of these units, and you really have your heart set on one, there are parts available from several Muncie parts specialists to upgrade your M20 or M21 into an M22.
Last edited by barkingrats; Nov 24, 2020 at 10:16 PM.
Yes Joe, can you share a few pics of the trans please?
Greg
So here are the famous pictures. But first, I'd like to thank all of you guys for trying so hard to help me figure out all this old school stuff I forgot years ago. One update is the factory forged steel crank has already been cut to about the max allowable. So "Boat Anchor".
Hi Joe,
Welcome to C3.
Yes, good info here from most of the guys.
Here's a quick break down:
3872702 head, oval port, 1966 used from 396 325 hp up to 427 390 hp. Valves 2.06/1.72, 98 cc combustion chamber.
3917215 head, oval port, late 1967 through 1968 used from 396 325 hp up to 427 390 hp. Valves 2.06/1.72, 100.9 cc combustion chamber.
intake manifold, early 1968 single 4bbl (Rochester) used on 427 390 hp.
Does the distributor still have the aluminum band wrapped around it's neck? If yes, then post that number here please and I can give you more exact info on that part as well.
If you can post a pic of the transmission input shaft I can tell you the probability of it being an M20 (wide ratio) or an M21 (close ratio).
All in all these are good parts. Not "rare break the bank" kind of parts by any means, but decent stuff that is certainly saleable to some of your Corvette brothers out there.
Just curious, what are your plans for the engine? Are you building some type of street rod?
Cheers, Greg
Greg, I cleaned up the distributor and yes it has that band you asked about. The number on it is 1111296, It's a tach drive with cable gear. I'm not going to use it on this motor. Any idea what it is worth? Maybe it'll help pay for a new crankshaft. I also saw a guy on eBay selling the intake I have 3919849 for like $1600.00, Is that even possible? And thanks for all the info on the heads too. Think I'll lose those and get a set of aluminum ones so I don't have to lift these again, way too heavy.
These are all good parts Joe. I'm sure you can recoup some build cost with them. When I have extras like this, I always put them on eBay with a $5 opening bid. I get the widest possible audience and the highest realistic market value. Corvette parts are always in demand so there's always multiple bidders. I'd think the distributor should bring a couple hundred dollars pretty easily.
The distributor is a good one. Originally from a 1968 427 435 hp car.
At $1,600 I think the guy is fishing. $500 is probably a more realistic selling number for this intake..
I agree with you on the heads. They will sell easily too.
I'll dig out my trans book and post more re: your 4 speed.
Cheers, Greg
Joe, I believe your transmission is an is an M21.
I base that partly on the angle of your input shaft gears in this pic: From the pic this looks more like the 39 degree gear angle of the M21 rather than the straighter 21 degree angle of the M22. Also the main case has no forward drain plug. All M22's had the forward drain plug drilled and tapped. Plug is magnetic with the letter P stamped into it.
Hard to tell them apart externally as the casting numbers are the same for the M21 and M22 as follows:
Main Case 3925660
Extension Housing 3857584
Side Cover 3884685
No grooves on input shaft and 26 teeth through me at first so I can only assume it has been replaced at some point. The M21 and the M22 share the same gear ratios so tooth count won't differentiate between them.
Date code is May 13, 1968 built at the Saginaw, MI. plant.
All in all, a good transmission.
Greg
I also saw a guy on eBay selling the intake I have 3919849 for like $1600.00, Is that even possible? And thanks for all the info on the heads too. Think I'll lose those and get a set of aluminum ones so I don't have to lift these again, way too heavy.
