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Discussion Starter #1
I have a th400 out of a truck I think. Being a convertible will a stock sedan driveshaft fit my car (i'm guessing not)? I can have a custom shaft made for about $350 but have access to plenty of sedan shafts close by. I could even possibly get one for $50 and have it modified and balanced for about $100.

any other ideas? Thanks! Asking a lot of questions today!
 

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The truck that the TH400 came out of could answer your question. Long wheel base trucks (long bed) had an extra three inches added to the end of the TH400 tranny. Short wheel base trucks (short bed) didn't and can be used in a car. You can use the longer tranny but you have to shorten the drive shaft. Heavier duty trucks (one ton and up had a bolt on yoke instead of a slip yoke, but it can be modifided with a little work to accept a slip yoke by disassembly of the tail shaft and grinding down the O-ring collar).

There are internal differences as well reflecting the GVW of the vehicle. A tranny rebuilder can combine the best of the 1965-'69 with the 1975 and up TH400 internal parts (sprags and pistons, etc.) to make a bullet proof monster that will work in a car.

No difference in drive shaft between convertible and other two door cars. Drive shafts interchange. BUT, the end of the drive shaft is different between a TH350-PowerGlide car and one with a TH400. The TH400 was a bigger (physically) and stronger (made of cast iron instead of aluminum) transmission; as such it had a bigger stronger rear yoke to fit the bigger stronger out put shaft. You have to match the yoke to the output shaft size (spline count).

Big Dave
 

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Discussion Starter #3
Great info! I actually do have the bolt on yoke in the th400 so it must be from a heavy duty truck. When I took my bolt on Yoke to the driveshaft shop he said he could put a slip yoke on it and do something to the yoke to make it not leak. I dont remember what he said. But he didn't mention anything about modifying the tail shaft. The tranny with yoke were installed in the car when I got it, minus the drive shaft.

I actually went at lunch and bought a very similar length (64") shaft with what they said is a 350 slip yoke, and it has the correct yoke for the rear end I believe. It might be the right drive shaft for the car the salvage guy suspected. It was just in a pile of driveshafts.

So I now have a salvage shaft about the right length and I have the bolt on style yoke that is loose. What do I need to do next? Just buy a slip yoke for the th400 (and have it installed onto the shaft)? why can't the bolt on yoke be modified (plug the bolt hole) so it won't leak and will slide as needed?
 

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Bolt in yoke was used in trucks because the slip is in the middle of the two or three piece drive shaft where the carrier bearing is located.

There is no room behind the bolt on yoke. It has a chunk of aluminum in the tail shaft housing to hold an O-ring to seal the tranny. You have to remove the tail shaft and machine (hog out) the O-ring seal portion and install a standard slip yoke seal to the rear of the casting (more machining). Then there is both enough internal room for a slip yoke and a means of sealing leaks.

Far easier to find a passenger car TH400 tail shaft and verify the output shaft is the correct length for the car (might need to replace it with a car output shaft).

I would rebuild any transmission I bought (minimum od boiling the case new steels and clutches along with new seals), and verify it has the parts I want inside (34 element Sparg out of a '91 4L80, 10 clutch piston, new steels and TransGo red clutches to begin with). Much cheaper and less aggravation than rebuilding it later when whatever you had goes bad (it was sitting in a junk yard for a reason when you found it: would you throw away a perfectly good tranny or a broken one?).

I learned to rebuild automatics after i gave up on four speeds for racing. If you do not have the few required specialized tools as well as a large parts cleaning tank or the knowledge on how to rebuild an automatic (it is a lot easier to rebuild than a Muncie) a fully rebuilt tranny might be the better choice.

https://www.jegs.com/i/J+W+Performa...Yo2yXVFHuAvBhkKPxqoaAqX4EALw_wcB&gclsrc=aw.ds

You can use yours as a core umless you can sell it for more than the core charge.

Big Dave
 

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Discussion Starter #5
I suspected this one had been rebuilt. Since it was installed already and attached to a freshly rebuilt 454 bbc that was done very well. But there is no way to know for sure. I'm good at pulling them out if needed to rebuild later, but have never rebuilt one myself.

So I understand your thinking perfectly and agree. But hopefully this one works. I know it pressures up and turns and shifts, just haven't been able to drive it yet due to driveshaft issue.

So my confusion now is that this tranny already had a standard rear slip seal in the tail shaft housing, with the bolt on yoke attached to the shaft. I wonder if someone modified it already like you said. I don't know enough to tell the difference but I will try to post a picture of it.

I also found a special slip yoke that says it is designed to convert the bolt on style yoke to slip on car style. Would this help my situation?

https://www.dennysdriveshaft.com/p6..._slip_yoke_1310_series_32_splines_fits_t.html

Thanks for sharing your wealth of information!
 

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Please note the forth line of the ad that states:

>>>> Please consult a transmission rebuilder to see what is required to convert it to a normal light truck tailshaft and tailshaft housing.
Transmission shops are a dime a dozen (AMMCO for example). I can not offer advise over the internet. You need a transmission tech standing next to you looking at what you have.

Big Dave
 
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