Once again this post is meant to help those who wish to rebuild their own transmission, however it is not meant to replace a quality rebuild manual. I recommend using an ATSG rebuild manual, HP Books "How to Rebuild and Modify the TH400", and this post for those new to transmissions. I will be covering some modifications that are not usually required but good to do. After you have disassembled your trans and cleaned the mostly bare case, I recommend you change the shift shaft seal, kickdown connector o-ring, and rear case bushing. Pictured here is a tool that allows removing the shift shaft seal without removal of the parking linkage. It is relatively inexpensive and can be used without pan removal (in the car). Be sure to lube the seal before re-installing, and take care not to cut it on the edges of the shifter shaft. If you do not have the tool or wish to buy it, now is DEFINITELY the time to change the seal, it requires removal of the linkage. You want to do this before you have the trans internals assembled. If you drop the "nail" in the case, or anything else, it is very frustrating later on to disassemble. I use a universal cam bearing tool to replace many bushings in the various automatics. It will not do very small bushings. Pictured here driving the bushing out towards the front. A TH400 and 4L80-E used in heavy towing will sometimes cause the rear bushing to "walk" to the rear, ruining the governor gear. There is a "fix" bushign for this that has a thrust surface on the front that prevents the bushing from walking. I use a stock bushing, red loc-tited in place, and staked in place. If you plan on using a rear torrington type bearing you want to place the bushing ever so slightly protruding to the front, so that it will locate the bearing. I use a TH350 pump -direct drum bearing from a later style TH350. These can be ordered new (preferred). This torrington bearing replaces the stock thrust washer and rear selective tang washer in the TH400. Pictured here, bearing is on top: You set rear endplay using TH350 pump shims as pictured on the left here: The total thickness of the bearing as compared to the thrust and selective is very close, it is usually about .150". It is best to adjust rear endplay on the tight side. On a Th400 (unless it has straight cut planetary gears) the planets will try to thrust load against the case bearing and the center support.