Ask the Editors: Are Pistons Truly Round?

By Jason Giacchino - November 15, 2015

Dear ATVC: Sort of a strange question- When rebuilding the top end on my Can-Am DS, I noticed that the piston doesn’t appear round so much as bowed out for lack of a better term. If the cylinder bore is perfectly round, how can this possibly work? Or did I just get a poorly cast piston? RK1977

What you’ve discovered is something called piston taper; which means that common to popular misconception pistons are not perfectly round but rather manufactured with a slight taper.

Were you to measure the piston, you’d discover that the diameter at the lower part of the skirt is actually larger than the diameter of the piston head. Or another way of saying it, the piston bows out as you travel down its length.

So why the bow? The answer has all to do with heat and its affect on metal. Aluminum expands as engine temperatures rise. The piston head, exposed directly to the heat of combustion, will always expand more than the cooler skirt below. To compensate for this expansion up top, the piston is constructed with a taper. Once up to operating temp, that same bowed piston expands to become round in the cylinder then returns to the taper once things cool down.