turbo 400

[derrick79]

hey guys. i blew up my th350 in my 76 k25. dropped the pan and looked like used 10w30 and smelled terrible. i guess some clutches kicked the bucket. anyway, i found a beefed up th400 for a decent price. i know they are pretty much the same, but is there any swapping issues? or would it just bolt right up? thanks.

[TexasRed]

TH400 for a 4x4?

Different yokes on the output of the trans and then there's a strong probability you'll need to modify the length(s) of the driveshaft(s).

[heavychevy91]

TexasRed is right about driveshaft lengths also the modulator is closer to the engine so you might have to modify the vacuum line. dont know about shifter linkage. converters should be the same...

[The dream]

I'm still learning about these trucks, but i know i have a TH400 and my truck is a 1978 K20... should i be concerned?

[Donut]

@ The dream, concerned about what?

Ditto on the trans outputs, the spline count MAY be different.  (iirc my 400 had 32 splines and my 350 had 27)
Also check the adaptor for the transfer case.

In my case, the number of parts that changed made the swap not worth my time.  The old turbo 350 just got replaced with another.

[frickenbored]

th350 has a 27 spline output shaft and a th400 has a 32 spline output shaft, th350 has a square rear case bolt pattern and the th400 has a star shaped rear case bolt pattern.

So you will def need a new transfer case adapter plate and a new transfer case input gear or th400 output shaft or possibly both.  I went from a 700r4 to a th400 (pretty much the same as going from th350 to th400) so I went through all that.  One other problem is that the th400 is quite a bit longer than the th350 so you would have to move your crossmember back which means drilling new holes in your frame (get a good drill bit) and you might need a shorter rear driveshaft and longer front drive shaft depending on how much yoke play you have.  Its prob easier to just get a beefed up th350 from a good shop.

I was lucky because the 700r4 and th400 are very close to the same length so my driveshafts and crossmember didn't have to be modified.  But I did have to buy the th400 to np203 adapter plate (which took forever to find since they're pretty rare) and I changed my th400 output shaft to a 4x4 27 spline shaft so i didn't have to mess with the transfer case at all.

[76 k10]

th350 have a slip yoke

[frickenbored]

th350 have a slip yoke

On a 2wd yes, but then so does a 2wd th400.  The 4wd version just bolts on to the t-case.  I think a fixed yoke on the t-case and a drive shaft with a slip yoke is much better.

[78 Chevyrado]

The TH400 in my 2wd 78 3/4 ton had a bolt on yoke. 

If you have a 2 piece drive shaft, you should have a bolt on yoke as the slip joint is between the 2 pieces of the driveshaft.  If you have a 1 piece drive shaft, it has to use a slip yoke at the trans.