4-speed terminology & "fit"

[Mark H.]

Rebuilding transmissions are new to me; before I decide whether to dive into this project (my '64 has a '68 4-speed) I'd appreciate it if someone can tell me if I have the terminology straight. The input shaft (aka main gear?) is described in terms of splines and teeth. I believe the spline count describes the portion of the shaft that sticks out the front, and must be matched to a clutch with the same count. The tooth count refers to the gear on the end of the input shaft inside the main case and is relevant to calculating the gear ratios. The output shaft also is described in terms of the number of splines, and this needs to match the driveshaft slip yoke that slides over it.

So, if one were to swap out an M-20 from 1964, which has a 10 spline input shaft and a 27 spline output shaft, in favor of a 1968 M-21, which also has a 10 spline input shaft and a 27-spline output shaft, there shouldn't be any need to change any other parts, and it should fit with no problems. On the other hand, swapping the same M-20 from 1964 for an M-20 from 1970 (which has a 26 spline input shaft and a 32 spline output shaft) would require a different clutch and a different driveshaft slip yoke. (the forgoing ignores any shifter issues and the side of the transmission the speedo cable attaches to).

Is that right? or have I missed something?

[Joe L.]

Re: 4-speed terminology & "fit"

Mark------

You're pretty close. You can interchange any 64-70 Muncie transmission AS A UNIT into any 64-70 Corvette and the unit will fit and function properly without modification of other parts. For Corvettes, the last use of the 10 spline input/27 spline output Muncie was 1970. For 1971, the input changed to 26 splines and the output changed to 32 splines. So, a different clutch friction disc and a different transmission output yoke is necessary. In addition, the 1971-74 Muncie is about 3/4" longer than the 64-70. So a commensurately shorter driveshaft is also necessary.

There is a great deal of interchangability with respect to INTERNAL Muncie parts, too. 1964-65, though, have many unique internal parts which are not interchangeable with later Muncies and vice-versa. Also, many internal parts have to be mated with specific other internal parts. For example, a 66-74 input shaft/gear is specific to M-20, M-21, and M-22. These MUST be used with a specific countergear. However, a 1966-70 10 spline input shaft for a specific variant can be interchanged with a 26 spline input shaft of the same specific variant. In that manner, a 10 spline gear box can be converted to a 26 spline and vice-versa.

By the way, some 1970 Chevelles with 454 used the 26 spline input/32 spline output Muncie. For Corvettes, though, the change occurred for 1971.

[Duke W.]

Re: 4-speed terminology & "fit"

The part you refer to is called the "clutch gear". It and the countershaft (referred to as the "layshaft" by some English speaking peoples) are what differentiate a wide ratio and close ratio transmission. Everything else is essentially the same, so it is relatively easy to change one from another by swapping in the correct clutch gear/countershaft combination.

The best way to handle the spline count issue is to just install a proper clutch disk and transmission yoke to match the spline count on the clutchgear and mainshafts.

Duke

[Wayne M.]

Are your trail housings both "584's" ?

... which have the speedo sender output on the passenger side ? Your '64 trans would have originally had a "429" tail housing (output on driver side). As Joe L. says, over the years these trannys could have been subject to much internal (and external -- housings, cases, sidecovers) modifications.

[Steve D.]

Re: 4-speed terminology & "fit"

Mark

You might want to check out 5speeds.com. They have a video (over 1 hour)that shows step by step disassembly and re-assembly of a Muncie 4 speed. And if you call with questions, the guy in the video answers the phone. I found it extremely helpful in my rebuild.

Steve

[Joe C.]

Re: 4-speed terminology & "fit"

Mark:

The '68 unit is much stronger than the '64 unit. The cluster gear shaft is 1" as opposed to 7/8", the synchro hubs wave wider shoulders, and the maincase has added stiffener webs.
As you probably know, the transmission is not scrutinized during Flight judging. The only reason, really, fot swapping out your '68 unit is "if it makes you feel better".

Joe

[Mark H.]

Re: 4-speed terminology & "fit"

Thanks to all. The issue is primarily driven by wanting to change from a close ratio to a wide ratio. I know Duke likes his this way, but I guess I'm a lousey driver and I'm tired of burning the clutch or squeeling tires or stalling when I start up from a light (3.08 rear). Having made the decision to switch, the question becomes whether I change some internals on what I have to create a wide ratio, or buy a replacement.

[Joe L.]

Re: 4-speed terminology & "fit"

Mark-----

I consider the wide ratio 4 speed to be the best for street operation. With 3.08:1 rear gears, it's practically essential unless you have a high torque big block. Personally, I wouldn't even consider running a small block with 3.08 gears and a close ratio 4 speed. GM didn't think it was a good idea, either; in PRODUCTION, they NEVER produced such a combination.

As described previously, all you need to do is change the input gear and the cluster gear to change ratios. These gears are available for all 66+ Muncies (and, perhaps, earlier models, too) as manufactured in Italy by Massiero S.A. These are VERY high quality gears and they are reasonably priced, too. Contact Larry Fischer at D&L Transmissions or Ed Hartnett for them.

[Wayne M.]

Factory 3.08's w/small blocks

Joe -- according to NCRS C2 Power Train tables, from '62 thru '65, the solid lifter 327's could be ordered with this rear end ratio, and I believe Duke's 63 is one of these. My first car was a '65 L79 with 3.08's (P-O-P proof), although this combo was not "officailly" available.

[Joe L.]

Re: Factory 3.08's w/small blocks

Wayne-----

Yes, I meant to post a correction earlier. 1962-65 Corvettes with small block and close ratio 4 speed were, apparently, available with 3.08:1. Perhaps it took GM that long to figure out that this wasn't a very "streetable" combination. After 1965, though, no 3.08:1 ratio was available with a close ratio 4 speed and small block engines.

[Wayne M.]

It certainly was a challenge

Summer of '68, Montreal Quebec; learned how to drive 4-speed on this car; inoperative parking brake; at stoplights on hills, would reach down and depress brake pedal with right hand while trying to smoothly launch.

[Loren L.]

The 1963 convertible Z0-6 that recently surfaced

has 3.08 + close ratio.

[Mike Cobine]

Re: The 1963 convertible Z0-6 that recently surfac

A good combo for Daytona. You could sacrifice a bit in the infield to have the much higher speeds on the oval. And I believe the infield was longer back in the early '60s such that the straight from Turns 1/2 (Pit out) to the first hairpin was much longer and not the short chute it is now.

There was probably also an SCCA or FIA requirement that the gear combo be available for them to run it. Back in the early '60s, Production was essentially what Showroom Stock was in the '70s and '80s.

That is why they could drive them to the track, race on the weekend, and then drive to work all week. The presence of race use only cars that were trailered to the track meant everyone had to eventually.