[PetChemMan]

If you do need to bore the engine and install new pistons, buy the pistons first and let the machine shop bore and hone the block to fit the pistons. You may also need to have the piston/rod/crankshaft assembly balanced if the new pistons are of different weight than the old ones. As you can see, things get complicated fast, so go ahead and do the compression check on the existing engine to get a rough idea of its condition, since you may not need to tear into the block. Low compression in a cylinder can be due to leaking valves or to leaking compression rings, or even to a blown head gasket. However, the oil control ring can be stuck in the piston groove, and the cylinder can still have good compression, so a compression test is not definitive for all problems. An examination of the spark plugs for wet, black deposits can help tell you if the engine is burning a lot of oil. If the engine has less than about 85,000 miles on it, the block may still be in good shape and you could concentrate your time on heads, manifolds, camshaft, etc.