The TH350 transmission was developed by General Motors as a replacement for the two-speed powerglide transmission. GM used the TH350 in their vehicles starting in 1968 and was found in almost all rear-wheel drive GM vehicles through 1984. If you have a GM car that falls within these model years, there is a good chance you have a TH350. With a few easy steps, you can identify whether your car has a TH350 transmission or not.
Place wheel chocks around the back tires to secure the vehicle from movement. Raise the vehicle using your jack. Place the jack stands securely under the frame rails and lower the vehicle onto the stands.
Crawl under your vehicle and locate the transmission. The TH350 is always used in rear-wheel drive vehicles, so the location is always the same, right at the back of the engine.
Count the number of bolts that hold on the transmission oil pan on. This is the pan bolted to the bottom of the transmission. If you count 13 bolts, you've narrowed your transmission model down to only two: a TH350 or TH400.
Look at the shape of the transmission oil pan. The pan on a TH 350 is square in shape, with one corner cut off, resulting in a five-sided pan.
Measure the length of the transmission from the front, using your measuring tape, where it bolts to the rear of the engine to the end of the transmission, where the tailshaft housing bolts on. Do not measure the tailshaft housing, which is the cone-shaped adaptor bolted to the rear of the transmission. This distance will be 22 ¼ to 22 ¾ inches on a TH350.
Look for the vacuum modulator. It will be on the side of the transmission and have a rubber vacuum line attached to it. If this fitting is on the right front of the transmission, you have a TH350. On the similar TH400 it is attached at the right rear of the transmission.
Look to see if there is a cable running to the side of the transmission, close to where the shifter linkage is located. Verify that this cable runs up to the engine. This is a kickdown cable, and the TH350 transmission has this feature, where the TH400 does not.
Locate the stampings on the side of the transmission. The TH350 had part codes M33, M38 & M39. The TH350C, which is a TH350 with a lock up torque converter, had part codes MV4, MX2, MX3 & MX5.
For many years, General Motors did not use the same engines in all of their model lines as they do today. For instance, a Pontiac V8 engine was a completely different design than a Chevy V8. If you're looking at a TH350 transmission for a project, make sure to get one that fits your particular engine. Tailshaft housings are different lengths, depending on the vehicle the transmission is installed in, This is why you don't include that in the length measurement when identifying a TH350.
When raising your vehicle with a jack, always check your owners manual for the proper procedure. Each vehicle is different, and failure to do so can cause vehicle damage.