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700-r4 tranny swap

Discussion in 'Chevy Trucks' started by Matt M., Feb 12, 2001.

  1. Matt M.

    Matt M. Junior Member
    Messages: 2

    My buddy owns a 1985 GMC Sierra. 2wd 350 c.i.d. 1/2 ton.
    His truck has a th-350 tranny that has gone south(many miles no maintaince)Front pump is shot. The engine and tranny are not original to the truck. Emission is not a concern. He wants to swap in a 700-r4 overdrive tranny. Expense and reliability is the main concern. He wants overdrive because he logs a lot of highway miles. The truck is not used for towing and has a stock engine so the 700-r4 should be reliable enough.

    This is what we have found
    The best v8 700-r4's were made between 1985-1990 or 1987-1990 depending on who you talk to.

    We are looking at getting a junkyard tranny. What all do we need?
    Will the old driveshaft work?
    Fly wheel/flex plate?
    Shift linkage?
    Kick down linkage?
    What else do we need to know to acomplish this swap quickly easily and most important cheaply?

    Thanks for your time.

    Matt Merry
     
  2. MTCK

    MTCK Senior Member
    Messages: 346

    OK, here's my 2 cents worth. I believe that the TH350's had a couple different length tail shafts on them, so the only way to really know is to measure as far as the drive shaft goes. You will need to hook up a TV cable for the 700r4, which is like a kickdown, but controls a lot more stuff. I don't know about the linkage, but an easy fix for about $150 bucks would be a B&M Light truck Megashifter. It's cable operated, and I have one of these in my 4x4. I like it a lot, it's pretty fun. The only other thing is the lockup control for the torque converter in overdrive, which is electric. You can purchase from summit racing a vacuum sensor that controls this and wires right up made by painless wiring specifically for that transmission. The flywheel and bell housing should bolt up. I'm not sure about the torque converter, but one of these should come in the tranny you find. Good luck and keep us posted on any hang ups, or stuff I might be wrong about.
     
  3. John DiMartino

    John DiMartino PlowSite.com Veteran
    Messages: 2,154

    Ok,the 700 should bolt to your flywheel,if not buy one for a 700R4,it bolts to the engine,you will need to modify the cross member's location,the holes are probably already in the frame,you need to slide it rearward about 3".The driveshaft needs to be shortened,unless you have the long stock TH350,you need to measure to be sure.If you have a TH350 with lockup torque convertor,the wiring should plug in from th350,and work fine.If it doesnt you have to splice the wires to the 700R4 plug,but its all there as long as it has lockup now.You'll need a kickdown bracket from a truck with the 700r4,your might already have it.Make sure your tranny is out of a truck,or have A shop install a truck 700R4 tailshaft,the truck tail has 4" of splines,the car unit has 3",the vette has a little over 2" since it has IRS.If you put a car tranny in a truck,you will be deshafted when the driveshaft falls out on the road,trucks have more suspension travel than cars and need the extra inch yoke.The best years for a 700r4 are 1988 and newer,they have the 10 vane pump and aux valve body.Good luck,its not s hard as it sounds,but its worth it when you get 20-30% better highway mileage
     
  4. Matt M.

    Matt M. Junior Member
    Messages: 2

    Thanks for the info guys. So far we have located an '86 2wd truck to get the drive shaft brakets and tv cable and other related linkage from. We also found a rebuild tranny from a '90. The tranny has 30,000 miles on it before the truck was totaled. even has a shift kit. All for $350. This weekend 0224