I bought two TH400's in great condition for $100. Guy was moving and just wanted them gone. There 4x4 units. I know you can switch them to 2x2 units by changing the tail shaft housings and the output shafts. Does anyone know the length i would need on the output shafts for a 2x2 TH400 short shaft? One is a 1976 the other a 1979 if that helps. I'm gonna rebuild them anyway so i know there in perfect shape before i install them. So switching them out wont be a problem. At this point i'm on E-Bay and there are a few lengths to choose from but the people selling them have no clue which length would work. Thanks.
this won't answer your question, but maybe avoid some issues besides length. As I recall from swapping the opposite way, back around 1982...was that the TH400 was not available in 4x4 until the 1980 model year. So you may have the wrong ID on at least one of yours. My swap was easy; my friend at GM Parts showed the tailshaft was different when I wanted to swap a 79 GMC TH400 2wd to 4x4. So he ordered a shaft. I got it apart and yes the shaft was different as far as un-needed snap ring groove on the 2wd shaft, but length was the same as the stock GM 4x4 T-case from a 1980 4x4 needed. Those snap ring grooves may have been due to speedo gear? Not sure, but 4x4 shaft did not have them. my 2wd output shaft was a bolt on rear yoke for truck use. Internal threaded hole on the end of the shaft. So, there will be several different listings in the parts books, and length is not the only difference. Many TH400 had slip yoke
TH400 was always available in 4x4 behind big blocks. Some but not all have the same output shaft as a 2wheel with short tailshaft, output shaft with provisions for a bolt-on can be used with slip yoke, just did it on a trans for a '70 442, used internals from a Chevy case with MA, military, code on data plate.
So i can use the bolt on yoke as a slip yoke? I read its to short thats why the output shaft has to be replaced.
The output shaft is already the correct length for a short tailshaft 400, all you need is a stock tailhousing to bolt on and then a slip yoke to fit your application. No the bolt on yoke is not long enough to work as a slip yoke. All of the 400 outputs are 32 spline but just like a turbo 350 they made long and short tailshaft versions, they also made various lengths of slip yokes so you need to find one that best fits for your app. As long as both of your tranny's are short shaft versions its a bolt on and go swap either way, I've got 2wd tranny's that I've taken the tailhousing off of and bolted on the transfer case adapter and gone 4 wheelin. Another thing that gm made simple!
I converted a th400 from quadratrac to a dana 300 transfer case(which never came behind the th400). The cheapest longer 2wd output shaft i needed and that I could find was $75. I know it doesnt answer your question but if i were you i would look for a complete 2wd donor th400.
Thanks 33willy. I ordered two tail housings last night. I'll wait till they come in and see were to go from there. I hope your right but people have said the shaft grooves aren't deep enough u have to have the shaft machined so the slip yoke goes deep enough. I just thought replacing the shaft would be easier seeing the trans will be broken down anyway.I hear so many things at this point I'll just have to try the housing swap and go from there. Hope your right.
Hell, the Jeep Wagoneer used a Nailhead-case TH400 for 3 or 4 years in a 4x4 application from 1966-1970 or so. IIRC that one converts to 2WD with a tail housing and a yoke, it has a splined shaft that will allow for a slip yoke on it. If I remeber right the biggest problem with converting one is to change the tailshaft, you have to take every other damn thing out of the case, to the point you may as well just rebuild it while you're in there.
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