The Stovebolt Site Forums The Shop Area Engine Shop measuring compression distance

I imagine this should be basic knowlege, but in the past I've used stock compression ratio pistons and never bothered to measure the compression distance. I would guess it's the measurement from the top of the wrist pin bore to the top of the piston, but i'm not certain. I have a 230 I plan to rebuild and I think I may have some options using pistons availible for 283 engines to get a small bump in compression. Mostly I want to be sure that when I measure my old pistons I use the correct method so I figure my clearances and compression ratio correctly. Thanks in advance for any help.

If you measure the old pistons from the pin to the top, and if the new pistons are higher from the pin to the top the piston will move higher into the head firing chamber. more compression more clearance issues with valves and stuff.

I believe the published compression height in most of the piston catalogs is measured from the wrist pin center to the piston top. The only time compression height becomes an issue is if you're changing stroke, and using the same rod length. For instance, both 302 and 327 cubic inch Chevy V-8's use a 4 inch bore, but the 327 has 1/4" more stroke. They both use rods 5.7" long. A 302 piston in a 327 would project 1/8" out of the top of the block unless the compression height of the piston was shortened by 125 thousandths. The two pistons look identical, until they're both slid onto the same wrist pin. Then the difference is obvious.

It's highly unlikely pistons for the same displacement engine will have different compression heights, unless the rod length is also changed. Pistons for a 350 with 6-inch rods will have a noticeably different (shorter) compression height than a piston designed to work with a 5.7" rod.
Jerry

Edit: According to some quick thumbnail calculations, the 283 pistons would stick out the top of a 230 block about .015" or so. That amound could be milled or shaved off without affecting the strength of the piston, I believe. The pistons probably wouldn't hit the cylinder head, even if they weren't shaved off, because of the thickness of the head gasket, but I'd be more comfortable having a little bit of deck clearance, at least getting the piston flush with the top of the block!
Jerry

Thanks for the replies. That's what I'm after, Jerry. Originally, I was thinking it would measure from the center of the pin but wasn't sure I could get an accurate measurement. Shouldn't be hard to measure from the top of the pin bore to top of the pistons with calipers then add 0.4635" (half the pin dia.). Right now I'm exploring ways I might be able to get from 8.5 to a 9.5 compression ratio to optimize the cam I would like to use. May not be much to gain with different pistons where the standard is a flat top. Some of the pistons I've seen listed claimed to fit 153, 230 and 283 cid. engines, but didn't list a compression distance. Others listed for a 283 show compression distance from 1.780" to 1.800". I really need to find a book or other source with more detailed specs to make this a little easier to figure out. Maybe the way to go would be to look at the stroke end of the equation. I noticed the pre-68 250 has the same journal sizes as the 230 but with 0.280" longer stroke. If a 250 crank could be made to work, I could get there using flat tops.

I believe the 230 and 250 cranks will interchange. The blocks are basically the same. You'll just be making yourself a 250 engine, which will probably get you more power gain than hotrodding the 230. BTW, 307 V-8 pistons fit the 250, and convert the 250 dish-top pistons to flattops, with a good compression boost. The last one I built that way also got a .050" head milling job, and it still runs OK on regular gas. Find yourself a good 250 crank, and grab a set of 307 pistons.
Jerry

Now you've got the wheels in my head going. I even have a 250, but I'm pretty sure it's thrown a rod and broke a cylinder wall.(had antifreeze in the pan)May not be able to save the crank but might be other parts I can use. I at least plan to get the HEI distributor off of it.

Thanks for the info.