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700 r4 swap

JohnGMC

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warren New Member

warren

Posted
Posted

What modifications to the driveshaft and cross member do i have to make to swap a 700 r4 in place of a th400?

The truck is a 78 stepside...

Thanks...

warren

Guest Friz

Guest Friz

Posted
Posted

I'm not sure about the mounting mods, but have you figured out the lock-up converter? It needs to have an electrical input to lock at the correct  load, rpm, and gear combo.

Why are you changing? The TH-400 is a really solid tranny, but I've never met anyone with a 700 that didn't have trouble. If it is strictly the OD that you're looking for, you can buy an aftermarket OD to put on your rig.

76BBSub Enthusiast

76BBSub

Posted
Posted

Warren,

Make sure you get one of the newer ('87+) units, or if you have an older one make sure it has the heavier duty parts installed.  In fact, whichever model you choose I would recommend having some aftermarket heavy duty stuff installed, these trannys are known for failing after less than 100K miles when stock.  I would try to get one out of a newer Suburban or pickup and have a professional rebuild with hi po parts.  I have been considering this same swap for my 454 Suburban.  There are a couple of manufacturers out there making electronic lockup kits for the 700R4.  By the time you have bought all the stuff you may be near the price of a Gear Vendors Overdrive.  Bolts onto the back of the TH400, driveshaft mod, and you're ready to go.  Stout as ####....

Something to think about.

Jeff

www.jeffsclassics.com

JohnGMC Enthusiast

JohnGMC

Posted
  • Author
Posted

Warren,my 89 700  r4 is stock nd original with 145 K on it,and i plow and tow.I even gross out at 9000+ all winter with my V-box spreader in the back and Fisher plow.I dont think you'll have troublewith one if its set up properly and you install a large auxilary cooler.You need to get one out of a truck to work best,and dont worry about the lockup-its not a huge deal to hook up,it is essential though,you can buy a lockup kit from B&M if you dont know how to make it work.You can probably slide the stock crossmember back about 3" on the frame and drill new holes.The driveshaft,try to get the one with the 700-and you need a truck output shaft and yoke-it has 4" of travel-the car ones only have 3".You will be deshafted (dropped driveshaft )if you use the car shaft and yoke,so dont make this mistake on a truck.The single most important thing on this swap is correct TV cable adjustment-if the kickdown cable is out of adjustment-the tranny will burn up-in as soon as 50 miles if its really out.The tv cable(kickdown) is the only wa the tranny knows how far into the throttle you are-it has no vacuum modulator like the th400/350.so if its out of adjustment and your into the throttle the line pressure will be low,and it will slip the clutches in the tranny-a few times and they are toast,so get the right bracket for your carb,and get that puppy adjusted right.If it feels like its short shifting its probably out of adjustment.Good luck ,

  • 2 weeks later...
summitXMK Enthusiast

summitXMK

Posted
Posted

Check out the new Jet 700R4. They make one without the electronic lockup and its supposed to be bullet proof.  The web site is www.jetchip.com All of the 4wheel drive magazines I have gotten have raved about it.  It is designed specifically for what you want to do. :cool:

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