C3 Drive shaft

[John Hames]

I bought a 76 L-48 automatic and the drive shaft was removed. The owner gave me two drive shafts, because he was unsure of which one was correct. The sizes of the two shafts are 2 1/2 wide 29 1/2 center to center, and 2 1/4 wide and 29 3/4 center to center. This car is a automatic with a 350 turbo. Any suggestion on which is correct?

Thanks

John

[David D.]

Re: C3 Drive shaft

John, The shorter one is for an automatic trans. Dave

[Joe L.]

Re: C3 Drive shaft

John and Dave-----

We have conflicting information here. I show that the driveshaft applicable to all 1971-79 Corvettes, 4 speed, THM-400, and THM-350, as being 29-3/4" center-to-center.

[Chuck S.]

Re: C3 Drive shaft

Joe, do you have info on the driveshaft tube diameter?

With only 1/4" difference in length, the diameter may be the deciding factor. In 68-72, the tube diameter is 2.25" for the THM400, but I have no info after 72. The only 2.5" diameter Corvette driveshafts prior to that go all the way back to the C1s.

[David D.]

Re: C3 Drive shaft

Joe, I believe they are the same length from 68-79 with an automatic trans, not just 1971-1979. I have a drivehaft off of a 1974 454 Coupe with TH400, it is 29.5 inches, center to center of the u-joints. It has a 2.25 tube diameter, tube length is 26 1/16 inches. It uses a larger U-joint than the 2 inch tube diameter driveshafts that I have. One 2 inch driveshaft is from a 1968 and a one from a 1966 Corvette, both with a Muncie 4 speeds, which I believe were the same shaft, from 1963 to 1970. They are both 29.75 inches, center to center of the U-joints, tube length is approx 26 5/8 inches, as close as I can tell, + or - 1/16 inch. That is what I based my response on. FWIW. Dave

[Joe L.]

Re: C3 Drive shaft

Dave----

Yes, 1968-70 with THM-400 driveshafts are the same as 71-79 with auto trans. However, the reason that I used the 71-79 year model range is because the original question involved a 76 AND the fact that for 71-79 ALL of the driveshafts were the same, 4 speed, THM-350, or THM-400 OR SO I INCORRECTLY THOUGHT.

In any event, I did a little more research on this one and the situation is more complicated than I thought and I learned a few new things. Previously, I had thought that there was just one driveshaft used for all 68-70 with THM and ALL 71-79. That's not how it is, though. As it turns out, there were THREE different driveshafts used during the 71-79 period. The first was GM #3924148 (driveshaft alone) or its PRODUCTION assembly, GM #3921675 (with u-joints and transmission yoke, but NEVER available in SERVICE regardless of what some GM parts references say). This driveshaft was used for 68-70 with THM and ALL 71-75 Corvette applications. It was also used for 1976-77 with M-40 or 4 speed and 78-79 with M-21 (BW-T-10). This shaft has a center-to-center length of ?????, a tube length of 26-1/16" and a tube OD or 2-1/4".

The second driveshaft used was GM #367903 (driveshaft alone). This driveshaft was used for 1976-77 with THM-350. I don't have CONFIRMED specs on this driveshaft.

The third driveshaft was GM #378231 (driveshaft alone). This driveshaft was used for 1977 with 4 speed or THM-350, 1978-79 with THM-350 and 1978-79 with M-20 4 speed ("Muncie"; Saginaw design). I don't have CONFIRMED specs on this driveshaft, either

In November, 1982 the GM #3924148 driveshaft was discontinued from SERVICE and replaced by the GM #378231. In January, 1983, the GM #367903 was discontinued from SERVICE and was also replaced by the GM #378231.

I feel pretty confident that the 2 driveshafts which were the original subject of this question represent TWO of the above-referenced driveshafts. I just don't know which two. And, I don't know how the THIRD driveshaft differs from the other two. I KNOW that the GM #378231 has REARWARD interchangeability with the first 2 (because GM says they do). However, I don't know whether the other 2 have FORWARD interchangeability with the 378231 or whether they are interchangeable with each other. My expectation is that the SHORTER driveshaft (whichever part number it is) is fully interchangeable and will work for ALL 71-79 applications (and 68-70 with THM) since there should be enough range with the transmission yoke/transmission output shaft. This would lead me to believe that the 378231 is the shorter driveshaft since we know that it's compatible with ALL 71-79 applications. I'm not sure if the longer driveshaft (whichever one(s) it is) will work for applications which originally used the shorter driveshaft, though. I expect it will, but I'm not certain.

The other part of this whole thing that I don't undertsand is why there were originally used different length driveshafts for the THM-350 and M-20 applications. The THM-350 and M-20 (78-79) are exactly the same length as the THM-400 and M-21 (77-79 BW). It may have something to do with the different yoke used for the THM-350 and M-20 or it may have something to do with the output shaft length for the THM-350 and M-20.

I'm going to have to do further resarch sometime to clear this all up.

[Joe L.]

Re: C3 Drive shaft

Chuck-----

I agree; the original 68-70 THM 400 driveshafts and ALL 1971-75 driveshafts used 2-1/4" OD tube. Between 1976 and 1979, I don't know what they were and I don't know what they were for 80-82, either. See my post below for more info on what I do know (and, what I don't know).