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Hi! I have a question involving pistons... What is the average clearance, or space, between the piston of a car that moves up and down, and the outer part that the piston moves in? The clearance obviously must be small enought to provide an air-tight seal, but I also want to know what the max. possible clearance between two objects can be to still allow an air- tight "seal"? Thanks for your help! I hope to be hearing from you soon.
Question Date: 2009-08-03
Answer 1:

The average width of the gap between the cylinder and the piston is .02 mm (20 micrometers). The reason the air doesn't escape from the piston is due to piston rings that act to seal the air from escaping. There is no maximum possible clearance between two objects that remains airtight without some sort of physical barrier.

Answer 2:

Typically piston to bore clearance is about 0.003-0.004 inches. That isn't tight enough to seal the piston to the bore though. Pistons have grooves in them and piston rings in the grooves that press outward onto the bore to better seal the piston with the bore.

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