350/TH350 swap to a 455/TH400 - Tailshaft Length?

I'm thinking to do a swap here within the next few weeks for my 72 skylark, and was wondering if I should pick up a TH400 with either the 9" or 4" tailshaft.

I used the search function, but couldn't find the answer. So, apologies if this has been answered here before. I imagine that this swap must be fairly common.

(I would also be open to suggestions for other possible tranny swaps. Improving gas mileage would certainly be nice.)

Kind thanks for the help.

 

Short shaft 400 would be for a GS.

Long shaft 400 would be a big car.


A 4L60E / 4 Speed w/ Overdrive would help MPG. Depends a lot on rear end ratio.

 

Thanks for the great input. I sincerely appreciate it Doo Woop.


Edit: It looks like the 4L60E transmission is electronically controlled by an electronic GM Controller... and that carbureted engines would also require a Throttle Postion Sensor... (from my brief stent of research)

I wonder if anyone has done this swap locally on the board... seems like it has the potential to be a headache. I'm not turned off to the idea though. An overdrive gear would certainly be worth an extra bit of work. Off to search...

 

A short tail TH400 used with the shortest yoke, ought to fit
your drive shaft. Needs an electric kickdown switch, no carb
linkage. The mount is back about 6 3/8". A switch pitch will
go so extra go, even with the most economical axle ratio.
The same shifter linkage will likely be fine.

Any 4 speed with lockup has potential to save gas. But they
need big buck upgrades to survive big block torque. Some
versions only have Chev patterns.
The extra gear means revisiting the optimum axle ratio.
good luck,
Bruce Roe

 

The "shortshaft" 4" tailshaft was used in some big cars, and all the little cars.

The "longshaft" 9" was used only in the big cars. People say it creates a bad geometry situation in A bodies.

There was also an 11" shaft that came in some Caddys.

I have had the best luck finding BOP shortshafts under performance-oriented Pontiac products from the mid seventies.

 

I'm planning to swap a 455 and TH400 from a '71 Riviera into my '71 GS 350. I know the driveshaft will have to be shortened, but I'd like to know what is meant by creating a "bad geometry situation in A bodies."

 

The geometry is the pinion angle.....the longer the tailshaft the worse the pinion angle....causes vibration problems and pre-mature u-joint wear.....use a 4" for the Skylark.

Yergi,
Come to our local Buick club meeting the first Monday of the month, we meet at Cracker Barrel on Ulmerton Rd/SR688 and 40th St at 7pm....we can answer all your questions

 

Do you consider this more of a problem for cars that are raced, or street cars as well? I'm no expert, but just by eyballing the line of the driveshaft and pinion on my Skylark it seems the angle change from a longer tailshaft would be pretty slight. Also, I saw an earlier thread where a couple of people said they had been running longshaft 400's in their A bodies without any problems. I'm not saying this to dispute anyone's viewpoint, but my longshaft 400 is all rebuilt and ready to go into my GS, so the idea of tearing it apart again to replace the tailshaft puts me in a very grumpy mood.

 

Yes, you can use the longshaft....but if you have resonance/vibration and u-joint wear/failure...you now know the reason.

When these cars were built....the mechanical engineers stated shortshaft for a reason......I never understood why people try to re-engineer something that has worked for 40+ years without problems.

 

I appreciate the information and advice, and I obviously should have posted the question here, because when I asked about swapping the 350 for a 455 and TH400 the only issues mentioned were crossmember location and driveshaft length. As far as trying to re-engineer something, I couldn't do that if I wanted to. I flunked out of engineering school and ended up in the construction field, where you just give something a whack with a hammer if it doesn't fit quite right. :Brow:

 

Larry, are you running a Gear Vendors overdrive unit ? If so did it changed the length of your tail shaft longer changing the drive line angle? I bought my car 25 years ago with a long shaft 400 already in it. I know its not original, but I never had vibration problems or lost a u joint. The drive shaft is not the same as the originals in my parts cars. So the previous owner had it made I guess. I can see where it was balanced as the weights are not the same as the factory originals on most GM,s. Ill keep running it untill I do the 2004R conversion. And I have ran it hard. But It would be best to go with the correct Transmission, Especially if you dont have you modify the drive shaft. But mine will be changed sometime next year.

 

Not really any point in a 4L60E swap, it would be really expensive.... If you want the best overdrive trans then go with a 4L80E, more work but at least you have a good trans.

If you want a simple overdrive swap then use a 2004R trans.

 

:gp:
Larry, Would it be wise to adjust pinyon angle just running the 400 with the 9" tail shaft then? I have adjustable upper rear controll arms also. But their set in the stock length. Yergi could use his long shaft 400 and buy adjustable upper controll arms if needed, correct?
Also what type pinion-angle gauge did you use, Or did you use somthing else? I have an old guage somewhere.
And last, Is -2 the recomended angle on a stock or modified set up, or is just with in an exceptable range? Im trying to learn somthing here.:Smarty: Thanks...