The rear main seal is about 4" in diameter.  Picture.  It's maybe 3/8" thick.  The outer ring is steel.  The steel ring gets pressed into the rear housing of the engine and the rubber inner lip of the seal rides on the end of the crankshaft and keeps the oil inside the engine.  There's a spiral-wound circular spring around the lip of the seal to help keep tension against the crankshaft.

Now, driving that steel ring into the hole is tricky.  I tried to tap it in incrementally using a hammer and a block of wood, first on one side, then working around the periphery.  The problem with this is that the lip of the seal tends to roll over when the seal gets cocked.  Picture.  Every seal that I tried did this.  Finally, on the third one, in desperation I got a rubber spatula from the kitchen - the kind used to scrape cake batter from the bowl - and slipped it between the seal and the crankshaft where the seal was making normal contact, then I worked it into the problem part and managed to roll the seal lip back into proper contact.  However, this seal ended up leaking too.  I finally let the Saab dealer do it.

If I had it to do over again, I would make a tool out of a piece of plywood or something.  It would be slightly larger in diameter than the seal, and would have holes drilled for the flywheel bolts and the locating pin.  See what I'm getting at?  Put the seal in place, put the flat round disc over it, align the locating pin hole, then thread the bolts in and slowly tighten them all in an alternating pattern to draw the seal into position.

But here's a couple of very clever ideas offered by Bernie Wassertzug:

  1. Use the flywheel as the tool to pull the seal in.  He says he uses only four bolts and tightens them in a criss-cross pattern a half turn at a time.  I'd recommend using them all and tightening them in a criss-cross pattern a quarter turn at a time.  Then again, use whatever works.
  2. Put the seal in the freezer first to make it contract a bit and fit into the hole more easily.  He says you need to work fast, because the seal comes back up to ambient temperature pretty quickly, but the frozen seal does give you an edge in starting the seal into the hole.

I can see where the flywheel would be heavy and hard to handle, but this still sounds to me like a very good way to go about replacing the seal.

Should you replace the rear main seal if it isn't leaking?  I dunno ... you make the call.  If I had it t o do over again, I probably would not.  I run Royal Purple synthetic oil and I think I would trust the seal to last.

If you do decide to replace the seal, just inspect it very carefully afterward, and I do recommend trying either the flywheel method or the tool mentioned above.

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