choosing correct piston size

[demonpixel]

I know this is going to sound stupid to a lot of you, but I've always wondered how people choose the correct size pistons for their cylinder bore diameter.



I mean if you have a 4.185" diameter bore, do you get pistons that have a 4.185" diameter? or do you get some that are ever so slightly smaller so that rings have some clearance? how much clearance should there be between the slug and the cylinder wall?

I understand you have to match connecting rod length, stroke of crankshaft, and cam size, but I've just always wondered how engine shops always say "you need these size pistons for your newly honed/bored block."

Thanks for any help/advice!

[Paul_J]

If you do it the way we did it, we started with the piston first and then bored and honed the block to match. The piston bore is larger than the piston and is determined per application. A cast piston expands less than a forged piston, race engines vs street engines are set up differently, etc..... If you were to purchase a new block ready to assemble and bought pistons you would need to determine what the correct bore size needs to be for the specific piston. If it needs to be increased then a local shop can hone the block to the correct size. This is done with a special hone and not the cheap one they sell at the auto parts store. What type of machine and the use of torque plates is a whole issue in it self.

[demonpixel]

Gotcha. So I guess it's best to start by finding out the maximum bore your block can handle without significant weakening, and then find pistons that will work, whether cast or forged, and get the cylinders properly honed/bored. Does that sound about right?

[jerome]

going to maximum bore will preclude any rebuilds

For example, stock SBC is 4.000
1st rebuild is usually 4.030
2nd is usually 4.060

after that, you may not be able to bore it out more. This doesn't cover all situations, there are sizes other than 4.03 and 4.06, but in general unless this is an aftermarket block designed for big bore, and it comes undersize, I would not go much bigger than stock.

[demonpixel]

Great info guys, thanks!