Repairing the internal combustion engine and condition inspection:
1) Forged pistons need 80% more bore/piston clearance than cast iron pistons. Cast iron pistons expand less.
2) Custom, extremely hard, cast iron pistons can be made by a machinist so that bore/piston clearance can be kept very low.
3) The skirt is the bottom and crown the top part of a piston
4) Clearance measurement is done at the skirt of the piston.
5) Mitutoyo bore gauge measures the inside diameter of an engine bore. It resolution is 0.0001 inch.
6) Ribbon gauges are thin long gauges that stick in between piston and bore to measure the clearance. Determine with a scale the pound/inch force needed to pull out the ribbon gauge. It must be a certain force. A 2.4D Toyota engine shall have 0.007 inch clearance between piston/bore. Use 0.002/0.004 inch ribbon gauges. If they are binded when pulled out, hone bore more. The piston is placed inside the bore without rings to measure the clearance.
7) Generally for each inch of bore diameter, the piston/bore clearance must be 0.0005 inch of bore.
8) If the piston skirt/bore clearance is to small, the piston will expand and seize the engine when hot.
9)Smear main bearings, big-ends,crankshaft with assembly grease before torquing bolts.
10)Ring gap-size is measured with a feeler gauge. It must be a certain minimum size to prevent the ends clamping together as the engine heats up during operation. The ring gap doesn't determine if the bore/piston clearance is correct. Insert an inverted piston at the top of the block to make the ring absolutely level, remove the piston and measure the gap. Rings in the first stage of failure will allow just air and/or a small amount of white gas with it. It sometimes is possible to only fit new rings and not new sleeves with first stage ring failure. The next size rings are fitted to the piston. But the ring gap must be manually resized via a file. Clamp a file onto a work bench and file of the ends of the over sized ring. Fit the ring in the piston and measure for ring gap clearance via a feeler gage. Repeat process until all the rings gap-size is within specs.
11)A tool shop bores the bore/sleeves to the exact size of the piston skirt. After this the bores are honed to the specified clearance via a honing machine. Insert the piston without rings to test for clearance as described.