The last time I rebuilt a supercharger (before this most recent rebuild) about 6 months ago, I used a high performance VITON seal. It leaked for quite awhile, at a very slow non-scary rate before it 'settled in' and sealed properly.

The LAST rebuild (last weekend), I rebuilt another SC with the same number of miles and used the less-expensive NITRILE seal. No leaks whatsoever.

My theory is this:

On higher-mileage nosedrives, the OEM seal will wear a small groove in the input shaft (call it POLISH instead of WEAR if you like). The high-performance VITON seals seem to be too wear resistant, and don't let the seal 'settle' into the groove as well as the Nitrile.

Something to think about when ordering a new seal or nosedrive rebuild kit.

Wilwren...You are correct in your assesment regarding the behavior of the seal material. That is the reason why I used the double lip version of the Viton seal and did not expereince any leaks.

Sounds like the consensus would be to stick to the Nitrile, or the DOUBLE lipped Viton. Not the single-lipped Viton.

I'm happy with the Nitrile this time around. We'll see how it goes long-term, but I didn't have the INITIAL leaks this time. That'* a good start.

Where did you source the double-lipped? A local bearing distributor?

At work, i stock all kinds of seals
Viton, Nirtle, all that good stuff..Vitons we use mainly for Pinion seals in some cases a speedi sleeve is used as well to bring the shaft size back up to spec...
Im kinda supised that Viton leaked before hand and then stopped..hmm

I've had different experiences replacing the nose seals.

I've replaced 2 seals with the double-lipped Viton versions, and neither leaked. I doubt that the 2nd lip affects whether or not it leaks, because it'* just a dust shield.

I've replaced 1 seal (on a 3rd unit) with the single-lipped Nitrile version, and it didn't leak initially but began leaking after few hundred miles, and still leaks a small amount every few hundred miles, so I keep an eye on it regularly.

I wonder if the leaking might also have to do with the oil level, and the internal pressure at certain temps and RPMs. I suspect this because after I rebuilt the one with the Nitrile seal (but before I installed it) I added another 1/4 bottle even though the oil level was at "the bottom of the threads" before and after adding the extra 1/4 bottle. I haven't added any more oil to this unit since it began leaking a couple of months ago, and the level is still at "the bottom of the threads".

Tossing this in to go with the extra 1/4 bottle theory.

When I have read up on the M90...the little cap at the end is called a breather by all documentation. I didn't look or pay enough attention to understand if it was allowing anything to breath....

Anyone?

The seal I got was from a bearing/seal supplier that carries MIL-Spec and aerospace seals. Yes, there are double lip seals that one of the lips act like a dust shield. However, this particular Viton seal is a real double lip seal used for shafts that carry high pressure hydraulic fluids and which by design, needs to have a very low leakage rate. Next time I go to the shop, I will retrieve the part number. and post it. I think I paid around $12.00 for the seal.