[Grady Clay]

PBH,

The “dam seal” doesn’t function as a seal. It is simply a dam that prevents oil from draining out the guide tube and into the clutch. It is important that the lip of the seam NOT contact the input shaft as there shouldn’t be any lubrication there.

The very important other half of this is to drill a drain hole in the guide tube.

Any oil that gets past the input shaft seal would drain into the unsealed cavity between the input shaft seal and the dam seal. The drain hole in the guide tube allows the oil to drain out and dribble down the surface of the bell housing and make a spot on the floor.

It is important the dam seal be located slightly toward the clutch from the edge of the ground surface of the input shaft for the input shaft seal. The reason for this is to allow any oil that would travel along the ground surface to drip into the cavity. On the other hand, the dam cannot be so close to the clutch to have the hub of the disc ever contact it.

The dam could just as well be a machined ring of aluminum or steel. A press-fit commercial seal is an inexpensive solution. I’m not comfortable with simply gluing in some RTV that doesn’t have a real interference fit. The consequences of the RTV going into the clutch aren’t good.

I developed this in ’72 with the poor design of the early 915 where the input shaft seal installs from the INSIDE of the transmission and the gearbox is assembled around the seal. The whole point of the dam seal is to prevent oil contamination of the clutch IF the input shaft seal leaks.

I think later seals are better and more long-lived material. The biggest feature is you can easily (without disassembling the transmission) replace the seal whenever the engine is out. That said, I look at the dam seal as simply an inexpensive added level of protection. One of the insidious aspects of getting oil on a clutch disc is it starts to fail to release completely and damages the syncros.


I’ll get back to work on a through post.

Best,
Grady