I’ve done several spread sheets on Wiseco pistons and found that the same part number can vary in size up to .0007 in diameter. I’ll bore a cylinder without a piston in hand but only to find where the cylinder will clean up. For example, if I have a stock cylinder with deep gouges in it, I’ll set it up in the boring bar and bore to the next available size - .003” and see if it cleaned up. If it needs more material removed to clean up, I’ll keep repeating the process until the bore is clean. Once I receive the piston, I’ll measure it and add the required clearance to it. Wiseco recommends .002” for a Blaster. Once I I know what my final measure needs to be, I’ll remove the last amount of material with the cylinder hone. This usually ends up being .0025” - .004”. If your machinist bored the cylinder 68mm and you order a 68mm piston after the fact, the piston skirt to cylinder wall clearance needs to be measured and can be no less than .002”. Knowing that pistons can vary in size by .0007”, you could end up with a tight bore if the cylinder was finalized without the piston in hand.
