Tips for a Transmission Swap 350 to 700R4by Richard Rowe
The TH350 (Turbo-Hydramatic 350) and TH700R4 (Turbo-Hydramatic 700-R4) can be thought of as relatives of a sort: uncle and nephew, if not father and son. The venerable TH350 saw duty in some of GM's most prestigious hot rods and is strong enough for most non big-block applications. However, its 700R4 replacement had one thing that its precursor didn't: overdrive. With today's emphasis on fuel economy and a comfortable ride, it should come as little surprise that so many are turning to this fairly simple and inexpensive swap.
In stock form, the 700R4 is capable of handling around 300 foot-pounds of torque. The 1987 and later 700R4s are stronger than older units, and 4L60Es from 1992 and later (which are internally almost identical to the 700R4) are a good option if you don't mind electronic controls. B&M and TCI both sell 700R4 upgrade kits capable of handling over 450 foot-pounds of torque.
If your original TH350 came with a 6-inch tail shaft, you'll have to cut the drive shaft by 3 inches. If your old transmission had a 9-inch tail shaft, then the drive shaft should fit. The 700R4's drive shaft yoke is identical to the TH350's, so your old yoke should work nicely with the new transmission.
The 700R4 was produced after all of GM's engines went to the "corporate" Chevy pattern, which means that the transmission has a standard small-block Chevy bolt pattern. If you're adapting the transmission to an older Buick/Olds/Pontiac or Cadillac engine, then you'll need to use an adapter plate. You'll also have to move the cross-member by 2 to 3 inches, depending on stock location.
You can reuse the rear passing gear (throttle kickdown) cable bracket on the engine and carburetor if you have the original carb. If you have a Holley or Edelbrock fuel mixer, you'll need a throttle kickdown adapter bracket fitted to the transmission. This bracket helps to correct for the aftermarket carb's longer throttle levers, preventing a hard, light 1-2 kickdown. You'll need to plug off the TH350's vacuum control line (the 700R4 doesn't need it), and you'll also need a new 700R4 throttle valve cable and dipstick.
You can reuse your factory three-speed shifter, but you'll lose control over first gear. The original "Drive" will become Overdrive, second becomes third, and "Low" becomes second.
If you want the torque converter to lock up for better fuel mileage, you'll have to install a lock-up control kit in the transmission pan. The lock-up controller is required equipment for 1982 through 1984 700R4s with a 27-spline input shaft; going without will result in a freshly-fried transmission.
- shifter image by Brett Bouwer from Fotolia.com