#1  
Old 01-14-2022, 02:42 PM
bhill86 bhill86 is online now
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Default Driveshaft differences 77 fbody MT vs AT

Can anyone confirm whether the length of a driveshaft is different between manual and auto transmission cars? Iíve seen people measure theirs at 48Ē and 48.5Ē but with not concrete reference as to which is which or if there really is a difference.

I also looked in my parts manual and see they show the same part number for the driveshaft for auto or manual cars (77 fbody) but a different part number for manual cars with the close ratio. I found one suggestion online that thought that may be due to different yolks being required for different MT output shafts but I donít know if thatís the case.

Iím leaning toward the length being the same either way but I wanted to see what you thought. I also searched here without finding a definitive answer.

Thanks in advance.

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Old 01-14-2022, 03:10 PM
rustedgoat rustedgoat is offline
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I believe for a m21 and th350 I used the same driveshaft on my gto. What transmissions m21, t-10, th350, th400?

M21 and th350 use the same yoke, M22 and th400 use a larger yoke.

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Last edited by rustedgoat; 01-14-2022 at 03:16 PM.
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Old 01-14-2022, 04:14 PM
bhill86 bhill86 is online now
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What I showed in my manual for 77 f series is M21 getting one part number and M20 and TH350 getting another. I didn’t see a reference to M22 but that may still point toward the difference in part numbers being only the yolk differences and nothing to do with driveshafts.

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Old 01-14-2022, 04:26 PM
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For what it’s worth too, I double checked my t10 (although it came out of a 76) and it’s a close ratio T-10 (M21?) and it required the same yolk as a TH400. When I swapped that in to replace my TH350 the auto yolk didn’t fit.

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Old 01-14-2022, 09:08 PM
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I swapped a ST10 in my Camaro that had been a TH350 car and used the same driveshaft. Just swapped a TH400 yoke on it. I think 48.5 is factory length.

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Old 01-14-2022, 09:27 PM
bhill86 bhill86 is online now
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Thanks for the reply. I keep reading 48” and 48.5”. I didn’t have the time to measure mine today. I will though

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Old 01-14-2022, 10:25 PM
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Quote:
Originally Posted by bhill86 View Post
What I showed in my manual for 77 f series is M21 getting one part number and M20 and TH350 getting another. I didnít see a reference to M22 but that may still point toward the difference in part numbers being only the yolk differences and nothing to do with driveshafts.

In 77...M21 was an ST-10.....The Muncie 4 speeds went away in mid 74....and option code for all ST-10s remained as M21..till 79 anyways

M20...was likely a Saginaw in those years....which used the smaller TH350 yoke...

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Old 01-14-2022, 11:01 PM
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Quote:
Originally Posted by Green Hornet View Post
In 77...M21 was an ST-10.....The Muncie 4 speeds went away in mid 74....and option code for all ST-10s remained as M21..till 79 anyways

M20...was likely a Saginaw in those years....which used the smaller TH350 yoke...
Ok thanks. Iím leaning toward that being the only difference in the driveshafts at this point.

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Old 01-15-2022, 12:44 AM
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You can also tell by buildsheet call out for which tailshaft and the colour coding.

For example 75-79 models, M15, M38, M20 and M33 used the same shafts with 2x Brown stripes (#7819344). The BW M21 was by itself with 2x Yellow stripes (#7819345).

I think the Chev-engines may have been odd ones out? Someone may confirm.


Last edited by Aus78Formula; 01-15-2022 at 12:59 AM.
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Old 01-15-2022, 10:45 AM
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Old 01-15-2022, 10:53 AM
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Went and did some measuring. 76 th350-48.5", 78 st10-48", 70 th400+12 bolt rear-48".

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Old 01-15-2022, 02:53 PM
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Weird. Maybe there is a half inch difference between them.

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Old 01-16-2022, 10:32 PM
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It seems there probably is a difference. I climbed under my car this evening and measured my driveshaft which had been paired with a TH350. 48.5”. Seems maybe auto cars did have a particular length and manual had another. Now the question is whether or not that half inch makes that big of a difference. My yolk is about a half inch stuck out from the edge of the tail shaft. I set up some Jack stands and my phones video camera and let the axle hang and lifted the car by the rear axle. I only saw minimal travel on the yolk. Even if I were to hit a dip in the road that caused the suspension to fully compress I believe that would pull the axle away from the transmission by design of the leaf springs so I’m thinking I’m ok?

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Old 01-17-2022, 08:47 AM
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Is the tube diameter the same for all shafts?

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Old 01-17-2022, 10:39 AM
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FWIW I took the 81s factory shaft 48.5 to a driveshaft shop that also makes them to replace the U joints and give it a really good balance. Replaces the front yoke and mentions the wear pattern n the old yoke actually looked a hair short. Factory trans, driveshaft and rear yoke.

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Old 01-17-2022, 12:41 PM
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Quote:
Originally Posted by Skip Fix View Post
FWIW I took the 81s factory shaft 48.5 to a driveshaft shop that also makes them to replace the U joints and give it a really good balance. Replaces the front yoke and mentions the wear pattern n the old yoke actually looked a hair short. Factory trans, driveshaft and rear yoke.
Is that 48.5 center to center of U joints? My 81 came with 3 drive shafts and I have no idea which is correct.

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Old 01-17-2022, 06:05 PM
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Yes center to center on mine.

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1981 Black SE Trans Am stockish 6X 400ci, turbo 301 on a stand
1965 GTO 4 barrel 3 speed project
2004 GTO Pulse Red stock motor computer tune 13.43@103.4
1964 Impala SS 409/470ci 600 HP stroker project
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Old 01-17-2022, 08:44 PM
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Quote:
Originally Posted by Skip Fix View Post
Yes center to center on mine.
Thanks

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