True Connections

This month we continue with the second of our three part series on transmission swaps in Chevelles and El Caminos. Many people prefer to use the TH400 transmission in place of the powerglide or TH350, due to its superior strength. To remove your existing transmission simply follow the removal instructions from last months issue. To install the TH400 it is basically the same process as installing the TH350, except there are a few key items you need to be aware of. One is the transmission mount, itself. The rubber mount on a TH400 is different from all other mounts, so that is something that would need to be purchased. However the crossmember, where the transmission mounts, is the same as on a powerglide or TH350 so you can reuse your original piece. The only problem is that the tailshaft on a TH400 is a few inches longer. On an open channel frame, this problem is easily solved by installing your TH400, mounting the crossmember to the transmission, and drilling new holes in the frame once you make sure everything is lined up. If your lucky, the holes might have been already drilled from the factory. If you have a boxed frame, such as on an El Camino or Convertible, there will be steel plates welded onto each side of the frame where the crossmember mounts. Some frames will have enough material to just drill new holes, others will need to have more steel welded on to support the crossmember.

The second problem is the drive shaft. Since your new TH400 is longer and the output shaft is larger, your original drive shaft will need to be shortened and you will need a new yoke. For this task it is best to call a dependable driveline shop. They will tell you how they’d like it measured up, just make sure they balance it for you. The driveline shop should also be able to come up with a new yoke for you since they are readily available.

The last problem with the TH400 is that the kickdown is now handled electronically, where the powerglide uses rods and levers and a TH350 uses a cable. In 1967 and 1968 Chevrolet used a plunger type kickdown switch mounted to the carburetor, and although the brackets are available reproduction, the switch itself is very hard to come by. In 1969-1972 Chevrolet changed to a throttle position type switch mounted to the gas pedal which can still be accomplished by modifying the 1970-1972 cowl induction throttle switch. Your only other options would be to use a manual valve body or contact an aftermarket performance transmission company such as B&M or TCI. They sell aftermarket kickdown switch kits that can be adapted to any setup.

Everything else on a TH400 trans swap is easily accomplished. The only other parts you should keep in mind is the dipstick tube, and mating the torque converter to the flexplate. Either of these parts can be found new at an aftermarket performance parts supplier. Shift linkage from a TH350 will work just fine on a TH400, so if there are any questions there, just review last months article. Next month we’ll continue on to install a 4 speed Muncie.