A wet streak high on the driver side of a TH350, TH400 or 700R4 case, right where the shift linkage bolts on, is almost always the manual selector shaft seal. It gets blamed on the pan gasket, the pan gets resealed, and the drip comes back. This kit is the two-piece tool for that one seal: a threaded extractor that pulls the old seal straight out of its bore and a driver that seats the new one square, plus a replacement seal so the job is done in one visit.
What the selector shaft seal does in a 700R4
The manual shaft passes through the case wall and carries the outside selector lever on one end and the inside detent lever on the other. The inside lever moves the manual valve link and the parking lock actuator, so the shaft is turning every time you shift. The small lip seal in the case bore keeps fluid in around that moving shaft. On the factory overhaul procedure the seal comes out with a screwdriver only after the manual shaft, detent lever and park actuator are already removed. With the transmission still in the vehicle and the shaft in place, prying at the seal is how cases get gouged and shafts get nicked, which turns a small weep into a steady leak.
Symptoms of a leaking selector shaft seal
- Fresh fluid on the case side above the pan rail, below the selector lever
- A pan that is wet on one side only, with a dry gasket line elsewhere
- Fluid level that slowly drops between checks with no cooler line or front seal leak
- Oil flung onto the exhaust on that side, sometimes with a burnt smell after a drive
How to confirm it before you buy
Clean the case side, pan rail and cooler fittings, drive the vehicle, and look again with a light. A selector seal leak starts at the shaft and runs down; a pan gasket leak starts at the rail. Rule out the cooler lines while you are under there. Low fluid matters on a 700R4, because low line pressure is one of the main reasons 3-4 clutches burn; our 700R4 common problems guide explains what a long-running leak can lead to, and the TV cable and line pressure guide shows how pressure is set on this unit.
Fitment
The tool is listed for GM 200, 200C and 200-4R, TH350 and TH350C, TH400, the 425 and 475, and the 700R4 and 4L60. The extractor works on all of them. The 700R4 and 4L60E use a longer selector shaft, so on those the new seal is driven in with a deep socket of the right size instead of the included driver. Other 700R4 parts are in our 700R4 collection. If you are not sure which GM unit you have, the pan shape and the transmission ID tag sort it out before you start.
Install notes
- Remove the nut and outside selector lever, and clean the shaft so dirt does not get pulled into the bore.
- Tap the extractor lightly with a hammer to set it, then thread it about a turn and a half into the old seal so it has a solid bite before you pull.
- Inspect the shaft where the lip rides. A rough groove or rust will cut a new seal, so polish light marks with fine emery.
- Lube the new seal lip with ATF and drive it in square until it sits flush; on the long 700R4 and 4L60E shaft use a socket.
- Reinstall the lever and nut (23 ft-lb is the figure B&M gives for the GM selector lever nut), check that every range still clicks in, then top off with Dexron ATF to the dipstick.
The lever is off anyway, so it is a good time to look at the linkage. A worn column linkage that will not land in each detent is the time to step up to a cable floor shifter such as our hammer ratchet shifter kit. Many 700R4 owners also replace the TV detent cable during the same visit if it is frayed or sticky, since the TV cable sets line pressure on this transmission.
