This isn't really a project in metal, more of a project in rubber with a little plastic thrown in for good measure!  Also, I'll apologize up front for the lack of photos.  I was in a hurry to do this yesterday before it started to rain!

My wife and I have a 2000 Saturn SW2 wagon. This car has a rear windshield wiper, and the shaft of the wiper motor passes through a 30mm hole in the window glass, with a rubber grommet in the hole to seal around the motor's shaft.  The grommet had worn out, meaning that in the rainy Vancouver winter, our car was leaking pretty much every day!  The part is about $40 new, and you can only get them from the dealers.  Auto parts stores don't stock them.  At any rate, one fellow at the autoparts store suggested PCV grommets.  I was able to find one that was close, (the grommet fit into a 28mm hole), but the hole in the grommet was much too small, 15mm instead of the required 21mm.  Here's how I modified it for use as a wiper grommet.

First, I rough cut a piece of 1/4" PVC sheet into a disc ~60mm diameter.  I drilled a 5/16" hole in the center of the disc and mounted it on a 5/16" bolt with a nut on either side.  This was chucked into my lathe and I turned the rim down just enough to get it perfectly round.  The disc was removed from the quickie arbor and chucked into the 3-jaw with the reversed jaws.  I bored out the hole in the middle to 30mm (to match the hole in the rear window of the car), and then faced the disc to make the area around the hole as thick as the window's glass.  This finished piece was a fixture that allowed me to mount the grommet on my lathe.

Next, I used a boring bar and roughed out the grommet to about 19mm ID.  I found that, as long as I didn't take heavy cuts, the rubber cut nicely, although a slightly rough finish was left.  Of course, the whole (hole?) point of the grommet is to seal on the wiper motor shaft, so a rough bore would not do.  I chucked a piece of 100mm (4") long piece of 3/4" aluminum round bar into the lathe and turned a 25mm length of one end of the bar down to about 10mm diameter.  The bar was then flipped around and I turned the remainder of it down to 18mm.  This made an arbor that I could chuck into my tailstock that I could wrap sand paper on and use to smooth the bore of the grommet.

I rechucked the grommet in its fixture into the 3-jaw and wrapped a single layer of 220-grit wet/dry sandpaper around my tailstock arbor.  I spun the lathe up to about 500 RPM and used a small sponge brush to dab water (I placed a rag over my lathe's bed to catch any water drips) onto the sandpaper as I cranked it in and out of the grommet.  After a minute or two of this, I wrapped some more sandpaper around the arbor (I used a little hose clamp to keep it on) to thicken it, and repeated.  I did this until the ID had been opened up to about 19.5mm, after which I switched to 400 grit to bring it up to 20mm.  This resulted in a bore that was as smooth, if not smoother, than the cast outer surface of the grommet.  Definitely a good sealing surface! 

Finally, I took it out to my car, and mounted it in the windshield, using some clear silicone sealant to prevent any leaks at the grommet's OD.  I smeared some fishing reel grease in the ID, then remounted the wiper motor, wiper, etc.  It worked like a charm, and I challenge anyone to tell the difference between my replacement and the stock part.  My wallet knows the difference, though ($5 for the PCV grommet), plus I got to try my hand at a new material.  It poured all day today, btw, and there wasn't a single drop visible where the leak was before.

-Chris