.055" is nearly 1/16". The only engines I've seen with that much clearance are air-cooled radial aircraft engines, big ones! I assume you mean .0055"? Standard machine shop practice for clearancing cast pistons is a maximum of .001" per inch of bore, so a 4" bore piston could have .004" as an outside limit. It's more common to have somewhere in the neighborhood of .0025" clearance on pistons up to about 350 Chevy size, 4.00". Hypereutectic pistons can supposedly run with as little as .0005", because the have virtually no heat expansion. The forged racing pistons we run on the circle track engines have about .005" to .010", depending on the type of racing they're going to do. Anything over .005" sounds like a bunch of ball peen hammers working on the cylinder walls, and the rings don't seal well for more than 500 miles. Of course, the rings and bearings aren't intended to last over 1,000 miles on a round track race engine, and the alcohol-fueled drag engines are usually rebuilt after one pass down the strip. At the end of a roundtrack season, cylinders with zero-gap rings are barrel-shaped, with most of the wear in the middle of the stroke, due to the sloppy piston fit. Doing race-engine tricks on the street is not only nonproductive, it's horribly expensive for no good reason.
Jerry
