Most of the super-expensive pistons are race equipment. For instance, the rotating assembly for a Chevy 350 race engine I priced out recently went something like this: Crankshaft: custom-machined billet, 38-Lb. weight, $2,800.00. (A stock crank weighs 55 Lbs.) Connecting rods: Tool steel H-beam rods with Honda-size rod journals, $1,100.00. Pistons: Forged, lightweight, short-skirt, weight-matched to crankshaft counterweights, $1,000.00. Total-Seal piston rings, $240.00. Once this assembly is balanced and installed in a specially-clearanced block, all we need to buy is a set of race-prepared cylinder heads, $2,500.00 (minimum), and a cam, lifters, and pushrods, etc., $1,500.00 or so, plus a few other high-dollar bits & pieces. All this for an engine that will need rebuilding about mid-season, or 1,000 miles, whichever comes first.
Street-type forged pistons will usually run about 50% or more cost than similar cast pistons, and won't require special thickness rings, etc. One telltale property of a race piston will be a 1/16" thick, file-fitted compression ring. The rings will come with slightly more length than necessary, so the ends of the ring will butt together when the engine builder tries to install it in the cylinder. The ends of every ring will have to be filed to a specified clearance, depending on the application, closer for drag-race engines that don't get up to much running temperature, and larger clearances for roundtrack engines that come up to running temp and stay there for a while. Then, there are the "zero-gap" rings, that have a second piece in the top, and sometimes the second ring, that overlaps the conventional end gap. These must be file-fitted, also.
None of the above is desireable on a street engine. It's just not necessary, and does nothing to help reliability, or long life. Speed-Pro makes both race and street pistons, but it's pretty easy to spot the difference. If the pistons cost less than $100.00 each, they're probably for street use.
Jerry
