The GM TH-400 transmission is considered the heavy-duty version of the Turbo Hydra-Matic 350, and the two transmissions are similar in many characteristics. They were used in many of the same vehicles; however, you would normally find the TH-400 in larger engine applications because they are able to function more reliably at higher RPM. They also use slightly more engine horsepower; a TH-350 typically uses about 36hp as opposed to the TH-400 using about 44hp. Used extensively by all GM divisions in the 1960s and '70s, the TH-400 was also commonly used by Jeep. GM continued to use this unit in Chevrolet and GMC trucks into the 1990s after renaming it the 3L80 (three speeds rated to 8,000 RPM). One of the most appealing aspects of the TH-400 is availability. It is estimated that millions are still on the road or easily found in your local salvage yard.
TH-400s were made in a couple of different versions: The first generation units were released in the 1964 model year and only used in Buick and Cadillac. Oldsmobile and Chevrolet began using the TH-400 in 1965, and a variable-pitch torque converter was added. These units are not the best candidates for exchange. This variable-pitch version of the TH-400 was only used from 1965 to mid-1967. Later units used a fixed-pitch converter. Another quick way of identifying a newer TH-400 is to look for a VIN on the side of the transmission. For a car newer than 1968, the vehicle's original VIN will be stamped on the casing. In addition, on the early units, the transmission filter was metal with an intake pipe attached to it. This filter could be washed and re-used. The 1968 and up filter was flat metal and paper with no intake pipe and was not reusable. All TH-400s, regardless of year, used a large black can-type vacuum modulator located on the passenger side of the transmission by the dipstick tube.
TH-400 transmissions are three-speed automatics, with a 2.48:1 first gear. Reverse gear is 2.08:1 and third gear is an even 1 to 1. Turbo 400s use three tail shaft lengths; the most preferred units for a GM swap would have the short 4-inch tail shaft.
You can find a donor TH-400 in one of these vehicles:
The TH-400 was strong enough to be used on 4WD vehicles as well, and was original equipment on 1973-'79 Jeep CJ series. These were mated to a Borg Warner Quadra-Trac or Dana 20 transfer case. The TH-400 was also used behind the Buick 350 in Jeep J-series pickups and SJ-series Cherokees and Wagoneers. Chevrolet and GMC used the TH-400 in 4WD truck applications with New Process transfer cases well until 1991.
Once a donor vehicle has been located, it would be a good idea to grab all necessary installation parts that may be required. You will also need the driveshaft, flexplate, crossmember and neutral switch.
For installation into GM products, the transmission bolt pattern makes a difference. The Chevy pattern is different from everything else. This is true, not only with the TH-400, but with all the other GM transmissions. The Chevrolet bolt pattern is compatible with all Chevrolet V-8, V-6 and inline six-cylinder engines. The "B-O-P" bolt pattern on TH-400 transmissions is compatible with any Buick, Olds, Cadillac or Pontiac V-8 engines. You will occasionally stumble across a bellhousing that has both bolt patterns and these can be used in any GM engine. Early AMC/Jeep engines used the Chevrolet pattern with an engine adapter while later Jeep applications had their own AMC bellhousings.
Converting a Powerglide or TH-350 trans-mission to the TH-400 in GM cars, the crossmember needs to be moved back in order for the TH-400 mounting pad to align properly. Most GM mid-size and full-size cars already have the frame drilled for this, because the cars could have come with either transmission, depending on engine size. The rubber mounts are different, so this would have to be changed as well (GM number 22188496 is the newest TH-400 mount). The output shaft spline on the TH-400 is 32-spline and finer than the shaft in the Powerglide, TH-200-4R and the TH-350 (27-spline), so a new 32-spline yoke would be needed for the driveshaft. GM still sells the correct yoke, part number 14075214. Depending on the application to which you are adding the TH-400, you will need to have the driveshaft shortened, as only the TH-200-4R driveshaft is similar to the TH-400 in length. A chart of some common transmission dimensions is posted on our www.hemmings.com/parts-locator Web site for subscribers to access on the "parts locator" page.
Driveshafts using a smaller U-joint than that for the replacement yoke can be modified with the purchase of an adapter U-joint to solve this. Spicer (5-3022X) makes a U-joint that has the correct yoke dimensions and cap sizes. Your original floor shifter could still be used, but we recommend the purchase of a changeover kit that would contain new shifter cable brackets and a new indicator lens. A TH-400 neutral switch should be used as well. Most GM and aftermarket flex plates are now drilled for both the TH-400 and the TH-350 bolt patterns, but make sure your donor flexplate is or a new plate may be necessary.
Converting to the stronger TH-400 transmission will allow you to make many engine performance modifications without the fear of over-abusing your drivetrain components. Because of the availability of donor cars, the cost of converting to a TH-400 should be relatively inexpensive and could be completed over the course of one weekend.
