Th700r4 swap to Th400

Kray

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so I have a 1982 k5 and it has rear wheel drive. Right now I am swapping my tranny from 700r4 to the 3 speed 400 and need advice on the actual swap itself. I'm gonna need a new driveshaft and how should I go about choosing the correct one please help thank you
 

obijuank5

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Pretty easy overall. The th400 is vacuum controlled so you'll need to hook that up.

Your shifter mechanism shouldn't change.

As for the driveshaft, the slip yoke is different so that will need to be bought with a new driveshaft. There might be a local shop who will make the shaft you need. I'm fortunate enough to have two in town.

The crossmember might just move to a different set of bolt holes.


Don't pinch any ignition wires installing the new trans.
 

andyblack

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I did this exact swap in 2012, except it was a 4wd truck. I used a 4wd TH400 from an '85 CUCV with the same NP208 transfer case and the slip yoke was exactly the same. Not sure if the 2wd TH400 ever came with a slip yoke in any year. The 400/208 was an inch longer but that didn't affect the slip yoke or the slip shaft on either shaft. The linkage was the same for the shifter. I got the downshift switch and bracket off the donor blazer and ran a wire to my fuse block under the dash. I had to put some rubber hose on it to make it engage when I wanted it to. For the vacuum modulator I bent some 3/16" brake line and ran it up to a vacuum tee that was on the intake manifold right by the distributor. I'm real pleased with the swap. Way better than the 700. I think I did a thread on it with some info you might use. PM me with questions.
 

andyblack

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Oh yeah I did have to drill new holes for the crossmember since the trans moved back an inch.
 

andyblack

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As far as a new driveshaft, could you get one off a pickup in the junkyard maybe and have it shortened?
 
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