Rebuild Your GM Transmission

We ordered the full rebuild kit from B&M, which included this extremely thorough instruction book. Other resources are available-rebuild manuals from HPBooks may be available at your local library or bookstore. Also, professional rebuild manuals can be purchased online from Automatic Transmission Service Group (ATSG). We're not going to do a full step-by-step build in this article. It's been done before, and the rebuild manuals will do a better job than we can here. Instead, we'll show some of the problems we encountered and how to fix them.

Transmission work requires some specialized tools. Be sure to read the rebuild manual first to see what's you'll need. In addition to the various sockets and wrenches, here's what we bought for our TH350: a set of picks, snap-ring pliers, a clutch spring compressor, a slide hammer, various flat-blade screwdrivers, some C-clamps, and a container of Trans Jel assembly lube.

This transmission had been sitting in the corner of our shop for about two years. It worked when it came out of the car. It was probably not long for this world though-note the metal shavings on the filter screen. When you see this, be sure to look for wear on all the bearing surfaces in the trans.

We're warning you now. After removing the valvebody, the gaskets between the valvebody, separator plate, and transmission case always rip and stick to the fluid passage walls, no matter how carefully you try to peel them off. The valvebody and trans case are aluminum, so be extra careful when scraping the residue off. Use a sharp razor blade held at a shallow angle to avoid gouging the passages.

The valvebody will contain several check balls in the fluid passages. Our TH350 had four. They are meant to impede or restrict flow through the fluid passages. The instruction manual will tell you where they are. Some rebuild kits will come with new ones, and if you're installing a shift kit, you may not replace all of them.

The next problem we encountered was when trying to remove the shifter selector shaft. It wouldn't pull out of the case all the way, and we needed to deburr the end with a file.

After performing all the work we could do with the trans on an engine stand, we had to come up with a method of holding it upright with the tailshaft off the ground. This seemed like a good idea at the time-we drilled some holes in a length of angle iron and bolted it to the trans pan holes. It was just wide enough to fit in one of the trash cans in our shop. The oil pump fits tightly in the case, so you can't pull it out by hand. Two of the bolt holes in the pump cover are threaded to facilitate removal of the pump with a slide hammer.

Here is the first bearing surface we encountered. It's a bronze washer that separates the input shaft assembly from the input ring gear. Be sure to inspect all bearings for wear and make sure to orient them correctly when reassembling the transmission. Fortunately, most of the bearings will only fit one way. As you can see, this washer has tabs that fit into the ring gear.

Lots of parts inside a transmission are held in place with snap rings that may be hard to reach. The best solution is to have a variety of snap-ring pliers with different-shaped tips. That can be expensive, though. Pliers with removable tips will do in a pinch, but they are flimsy and the tips have a tendency to loosen just as you've almost gotten the snap ring out.

Other snap rings are bigger and can be removed with flat-blade screwdrivers. The last clutch assembly in our case, the low/reverse clutch, required a special spring compressor, Hayden Transtool No. T-0151. Ours didn't arrive in time, but the guys at Trans1 Transmission in Gardena, California, helped us out. The tool engages the lugs and pushes the piston return spring enough to access the snap ring. The whole process took less than a minute.

Once the pump is removed, you have access to all the pieces that make the transmission work. In the TH350, the first component is the intermediate clutch assembly. Behind this is the intermediate overrun brake band that encircles the forward clutch. Once the band is removed, pull the entire input shaft out, taking the direct and forward clutch assemblies with it.

The low/reverse clutch piston is also a little tricky to remove, as it fits very tightly in the case. The instructions recommend blowing compressed air through a port in the valvebody. This worked great; just be ready to catch a flying piston and dodge a stream of transmission fluid.