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First off, pistons are not round when cold - they go circular when warm. (if you look underneath the crown you'll see steel expansion strips cast into the aluminium or there'll be expansion control slots in the skirt, usually at the bottom of the oil control ring groove) They are generally "barrelled" also, (i.e. wider toward the middle, the top and bottom being narrower) so it is important where clearance in the bore is measured on the piston. The only really accurate way is to measure the bore precisely using an internal gauge then transfer that measurement to an accurate micrometer. Then measure the piston about two thirds down the skirt at right angles to the gudgeon pin. Take the first measurement from the second and divide by 2 - that's the clearance. Absolute accuracy is imperative and difficult to achieve without practise.
The actual clearance varies according to engine type but should be printed either on the boxes the piston kits came in or on the information sheet with them (if they came with them). Typically, its either 0.0002" or 0.0004", the greater clearance for injected engines which generally run leaner and have higher compression ratios than carb engines, consequently run hotter.
When you fit them, use a proper ring expander to fit the rings to the pistons, oil the piston and bore well, use a proper ring compresser, make sure the ring gaps are staggered (including the rails of the oil ring set), that the top ring gap faces away from the exhaust valve, no gap is on the thrust side, and the ends of the oil ring set expander aren't overlapped. Put some electrical tape over the big end bolts also to avoid the possibility of marking the crank with them when pushing the piston and rod into the bore. When fitting the piston, it takes a bit of a push but don't force it - if it doesn't want to go pull it out and fe-fit the ring compresser. You'll hear the rings enter the bore with a light "click". Tap the pistons in with a wooden handle once the the rings are in the bores. Phew! hope that helps. Cheers.
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