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Piston/bore clearance


Looks for a little help guys. I had some custom forged pistons built for my Conti IO360. The piston manufacture is a well known outfit that has experience building pistons for air cooled engines (mostly VWs) but that isn’t their main gig. They tell me to set the piston/bore clearance to 0.006” which is way tighter than the 0.009-0.012” Conti specs for factory cast pistons. I expressed my concern about sticking pistons and the piston manufacture says to trust them and just run it. They actually think it is going to be a little loose. I talked to a high performance aircraft engine builder and he is concerned I’ll stick the pistons. He put custom forged pistons from a different manufacturer in a engine a decade ago. He set the clearance at 0.006” and stuck every piston. I don’t know what to think. Surely the piston manufacturer knows what they are talking about so do I go with what they say and risk squeaking a set of $1000 pistons?
I would go to continental spec sheet and see what the bore dia is and see what piston spec are and go with that tolerance.
What material are they? 2618 or 4032? 4032 run generally tighter clearances due to more silicon making heat expansion less. 2618 expand more when heated requiring more clearance. I work in a machine shop and usually follow the manufacturers clearance requirements, except when Hypereutetic call for .0015, .002 is tight as we will go. These are automotive engines mind you. Who's the manufacturer?

We set Harley pistons at manufacturers spec usually around .0025. Also depends on where you measure the piston at to come up with the final bore size. Measurements usually happen at the skirt, 1/2 inch from the bottom and 90 degrees from the pin. If your measuring anywhere else you gonna be tight!
Swamper, I don’t think going with the Conti spec because that is for their cast pistons. These are forged and have nothing in common with the factory Conti pistons.

hdsadey, These are made for 2618. Engineered and manufactured by CP Carrillo. I sent them a factory piston and had a in-depth conversation with the guy that designed them. He basically said to forget the tractor technology used when these engine were designed. He told the machinist I used how to finish the bores and he designed the piston for minimum growth. They use total seal rings and piston pins with clips. Should be a sweet setup when dialed in.

The bore is measured as you describe and all the choke has been removed from the cylinder bores.