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 71 Lemans 400/th400 driveshaft length?
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Numbah_wahn
Tribal Scout

USA
2 Posts

Posted - 17 Nov 2017 :  3:49:20 PM  Show Profile  Reply with Quote
Do any of you know how long the driveshaft for the th400 needs to be? I know since its longer than the th350, ID need to shorten it a bit, but I can't seem to find that information anywhere

Bill Boyle
Horse Feathers (Charter Member)

USA
4834 Posts

Posted - 18 Nov 2017 :  07:57:57 AM  Show Profile  Visit Bill Boyle's Homepage  Reply with Quote
Number 1--not sure if you'll ever find reliable information on the correct length of a 1971 Lemans driveshaft equipped with a THM-400 transmission. Advice is typically to measure and provide that crucial information to a driveshaft builder. For instance--

Consider www.dennysdriveshaft.com

The site provides information on how to measure everything needed to build a correctly sized driveshaft.

This is not the answer you want, however, it's the only answer, which is a suggestion, on what to do.

Watch this informative video: https://www.youtube.com/watch?v=-VAdXKCrIvw

"Dedicated to keeping the classic Pontiac engine alive."

----
400 bored +.030, forged TRW pistons, ported 62 heads, Hedman headers, 2.5 SS dual exhaust X Pypes, Comp 276AH10 cam, Scorpion 1.65 RR, 850 Q-jet, stock intake & tuned HEI; original owner.

Edited by - Bill Boyle on 18 Nov 2017 08:02:02 AM
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69lemans
Two Feathers

USA
27 Posts

Posted - 19 Nov 2017 :  08:57:14 AM  Show Profile  Reply with Quote
I just had my driveshaft cut down last year when I went to the TH400 from the TH350. Unfortunately I don't remember the length but I do know I measured it because I did not trust all the info online and I am glad I didn't. I can measure mine if you would like it is a 69 lemans so it should be the same.
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Phil
The Great White Buffalo

USA
7228 Posts

Posted - 20 Nov 2017 :  12:10:15 AM  Show Profile  Reply with Quote
Many moons ago when I still had 3 speed autos I swapped the TH-350 in my Formula for a 400 and seem to remember shortening the shaft 15/16" in addition to swapping to the larger 400 yoke.

Bowties are for Pee-wee Herman. "Chevy": even the name sounds cheap, but not as cheap as your Pontiac will be with an LS transplant.

Edited by - Phil on 20 Nov 2017 12:11:33 AM
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Numbah_wahn
Tribal Scout

USA
2 Posts

Posted - 21 Nov 2017 :  11:24:10 AM  Show Profile  Reply with Quote
I kinda eyeballed the tail shafts from both my th350 and th400, and it seems that the th400 had a shorter tail shaft than the th350. With that logic, should I be able to use the factory driveshaft since the th400 is longer, but the tail shaft is shorter?

P.S I will try to take detailed measurements of both.
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69lemans
Two Feathers

USA
27 Posts

Posted - 22 Nov 2017 :  8:32:29 PM  Show Profile  Reply with Quote
I will measure mine this weekend and post it. It is definitely not the same as the stock one.
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