Hey i was wondering if anyone knew if i could remove my 700-R4 Transmission and put a TH-400? Would i have to modify anything and how easy is it?

Thanks Matt

 

yes you'd have to make a modification or two. i'd rate it as fairly easy

 

There's a swap kit designed for relocating the tranny mount and torque arm. The TH400 uses an electric detent so a switch needs to be installed on the throttle linkage somewhere. I'm pretty sure it's part of the kit.

Hardest part is the driveshaft. The TH400 is a long tranny plus it uses a large diameter tailshaft. A TH400 slip yoke is required for the tranny. No other GM tranny uses this large diameter so your stock slip yoke won't fit. Just about every truck TH400 has a bolt on yoke so that won't work.

 

I have a th-400 in my 88. I bought the $125 hurst swap kit. I wish I had known about spohn at the time. His is higher, but looks alot better.

This next info is only usefull if you have the short shaft th-400 (which most are)

The trans face to mount lenght of a th-400 is 26-3/4"
700R4 is 22-3/8"

That shows you the crossmember has to be moved back. In older cars you could just re drill holes in the frame. Cant do that with our cars, so you need an off set crossmember (swap kit)

Also th-400's dont have bolt holes on the tail shaft to hook the torque arm too, so the swap kits gives you a place to hook it. The hurst kit I bought has a thick metal bracket, that goes on the trans tail shaft, you take 2 of the bolts out of the tail shaft, put this mount in place, and put the 2 bolts back

The trans lenghts are diff too.
Th-400 over all is 28-1/4"
700R4 is 30-3/4"

You will need a driveshaft thats 43-7/8" from center of u-joint to center of u-joint

There's 3 ways of getting that.
1) have your driveshaft cut down
2) buy a custom driveshaft
3) get a driveshaft from an 82 or 83 camaro/firebird with the 3 speed auto trans

I went the #3 route. The shaft aint quite the dead right size, but the th-400 yoke makes up for it, and all turns out good.

You'll also need a front bastard u-joint (conversion U joint) It has small ends on one side, and bigger on the other. Depending on year of th-400 yoke you may need inside or outside clip U joints.

My th-400 came from a 1974 corvette and it takes outside clips, my driveshaft had to have inside clips.

30 min. worth of digging through auto zone's u joints with the help of a parts guy that was a real car guy and we came up with the right joint.

For mine is was brute force part # 2-1569BF for $14.95
(I strongly recommend at least a brute force joint up front. They are forged solid body u joints that are much much stronger than any zerk fitting greaseable joint. Also has the bigger bearings in them)

The rear U joint will be stock part number for 82-92 camaro or firebird (I also went with the brute force in the rear)

The kickdown on a th-400 is elec. (tab on driver's side of trans, right above pan rail) you can put a stock kickdown switch on the throttle pedel from any th-400 car, or use a mirco switch on the carbs throttle linkage, or just put a toggel switch inside the car for the kick down. Thats what I done.

You'll need to run a vac. line from full manifold vac down to the trans gov. 700r4 dont have this. The hurst kit came with some vac line and fittings. I didnt feel safe with all rubber, so I bend me up a pc. of steel hard line, and used rubber on each end.

You'll also need a th-350/400 torque converter. Your 700R4 converter will not fit a th-400

You'll need to bend the cooler lines alittle by hand to get them at the right angle for the th-400

Thats about all I can think of.

It's real an easy swap, and well worth it if you got some power, or planing on a build up, and alot of hard use