I heard Kevin mention his article on cork bonding a sleeve. I see it's locked so I can't reply on that thread but I would like to pass on some of my experience with this technique. I had talked to Jim Wilson about this process so I've been working with his suggestions.

I have been polishing my sleeves with a "cork" (yes, some people do actually use cork) using some Delrin rod. I machine it for a tight fit in the sleeve and split the end with two crosses using a hacksaw. I found I needed the give at the end doing the "cork" as the Delrin expanded and locked up in the sleeve. I use 3000 grit diamond powder that I get from Ken's Gems in NE Calgary. I wet it out with WD 40 and put a dab on the cylinder. I lock the Delrin rod into my lathe chuck and at slow speed, I work the sleeve in and out over the rod and let the diamond dust polish the sleeve. Checking the sleeve after a minute of work you will see the sleeve start to loose the honing marks from the factory. The diamond dust impregnates the Delrin so you don't need to reapply very often.

To get the sleeve to fit, I put a bit of the diamond/WD40 mix on the piston and work it into the sleeve with a twisting motion back and forth. Rotate the sleeve and piston to make sure both are fitted so you don't feel any tight spots as you rotate which would indicate the sleeve is out of round. The cork bonding will take out some out of roundness but not all. Watch how close the piston gets to the top of it's stroke as you work it. Stop just 1/16 short of where TDC would occur. Pull the piston out and clean thoroughly with brake clean and try a dry fit. The dry fit will go in a bit further than with the diamond dust so don't work in the piston all the way to TDC. The piston will now have a very short distance to seal at the top and will break in naturally after that.

I have found small engines do keep their fit but at the same time, you don't want it to 'squeak' going over TDC. That puts a big load on the rod and bearings. I've found larger engines like 67 and up actually have very loos fits when the run best. I had a K engine that I could push the piston through the sleeve yet it ran great. The thick Castor seems to provide a good seal which is a good reason to run 20% levels.

With this method, you will greatly shorten the break in period and save your engine.