driveshaft length?
 

We are all guilty, and I am no exception...

What would I be guilty of(?);
I occasionally forget something.

Maybe I have forgotten this, and I'd appreciate anyone who knows this to speak up.

I had been given the understanding that second gen Firebird driveshafts regaurdless of drivetrain would interchange.

Disclaimer(!!) This should hold true as long as the driveshaft in question is set up for the correct size U-joints for the differential and input shaft.

Also, I had understood that 1970 model year cars had an unusually long input shaft.

I saw THIS "for sale" thread, and today something clicked;

If this driveshaft is too short for a manual transmission, then is my previous understanding incorrect?

I have four driveshafts;
I haven't run anything other than the 1972 M22 that my car came with (circa 1995), and since taking it off the road I have collected parts to run a different transmission - namely one with a 27 spline output.
i.e.: swapping to a TH350 input shaft

I had assumed up to recently that the 1970 Trans Am which my car has parts from, donated it's driveshaft to run the 1972 (Firebird coded) M22 that I previously ran - because of the oversized (i.e.: longer) input shaft.

I just measured the four driveshafts that I have (my measurements might be imprecise):
my 1970 driveshaft: 50-3/4" out to out of U-joint holders
two 2nd gen TH350 driveshafts: 50" out to out of U-joint holders
one unknown TH400 driveshaft: 49-1/2" out to out of U-joint holders

Question:
What should a s/t driveshaft for a 1970 Firebird measure?
vs
What should an a/t driveshaft for a 1970 Firebird measure?

Attached is a picture of the four driveshafts that I just measured - with my "original" (as acquired) driveshaft on the left.

Here is the AMA spec for 1970...

Why are there two different lengths for auto trans?

The TH400 is longer than the TH350.

ok - thank you VERY much HFR!!

Is it correct to assume that the 49.2" vs 48" is a center to center of u-joint measurement?

Yes.


:)

Correct. Here is the 1970 Firebird Service Manual and 1971 and 72 AMA charts too.

And one has to have the correct T-400 yoke otherwise will be too short w the 48" driveshaft.

Notice the 1.2" difference in lengths between T-350 & T-400 for '70 models?

The '70 F-body T-400 driveshaft takes the long Spicer 1310 T-400 yoke.
The machined surface, aka, the shaft, portion of the yoke is near 6 1/2" long. That's the longest version of over a dozen different styles of GM usage T-400 yokes.

For '71 model Birds, Pontiac went to a longer driveshaft for T-400 & M20/M22 usage. That little over 48 1/2" long driveshaft utilizes 3R u-joint, as does its mating ("med length") T-400 style yoke.

'72 models Birds factory equipped w T-400's & Muncie 4speeds reverted back to the small Spicer 1310 u joint & that style was used through the '74 models.

Have pulled several clean original F-body T-400 driveshafts over the years, still have at least one '70 version. Back in the early '80's, had a '75 up V8 T-350 Bird driveshaft cut down 5/8" for my '80 when I briefly swapped out its Turbo 301/T-350 for a '70 YZ 400 & PQ T-400.

Hey OPH, I sent you a PM last week.

Incase you missed it, I shared an image of my "original" for my car as acquired circa 1995 driveshaft:

https://forums.maxperformanceinc.com...4&d=1780526604

Jon, not a vapid reader on this site (it's gone to H#ll).

Only contribute, if posts lack clarity.

Try to keep posts brief.