Stovebolt.com Forums The Shop Area The Engine Shop Got a call from the machine shop UPDATE

During a "normal" engine rebuild where the cylinders are bored over .0030 or .0060 due to normal wear and tear, how does this affect performance since the factory camshaft is usually installed.

A Chevy 350 bored .030" oversize goes to 355 cubic inches. That's 5/8 of a cubic inch per cylinder increase in volume. For all practical purposes, the increase in compression can be ignored. There needs to be a big increase in bore and/or stroke, or a dramatic reduction in combustion chamber size to make a big difference in compression pressure. That same 350 with a 76CC cylinder head has about 8.5:1 compression. A 64 CC head brings it up to around 9.5:1 or maybe 10:1. Domed pistons and a 64CC head approach 11:1.

A Chevy 230 six has flat top pistons. A 250 has a 1/4" dish in the piston top and uses the same cylinder head. That's a difference of slightly under 4 cubic inches per cylinder, from a longer stroke on the 250. The dished piston is there to keep the compression the same on both engines. Running a flat top piston for a Chevy 307 V8 in a 250 brings the compression up to about 9:1.
Jerry

Thanks.

My engine was bored .060" over, and there is .050 missing from the bottom of the head, leaving most of the quench trench by the intake valves missing. The machine shop also put in all new valve seats at an unknown level. If the new seats were ground too shallow, compression would be even higher.
I did a hot compression check on all cylinders before I pulled the head, #2 through #6 were at 240 PSI (+/- 5 PSI), and the #1 with the leaky exhaust valve was at 180 PSI.
That is about 100 PSI too high. Its a brand new gauge. I doubt that the gauge is off by 58%.

When I do some major metal butchery on that head- - - -bigger intake and exhaust valves, recessing the intake valves back to the proper depth, opening up the combustion chamber around the bigger exhaust valves to unshroud them, and a few other kinds of metal butchery- - - -I'll have one of two things- - - -a really good-performing head that can be paired up with a pretty radical cam, or a boat anchor! I won't really know until I'm about 75% of the way through the job. If I end up making the head worse than it already is- - - -no big loss, as I can use the new valves in a different head if that one can't be salvaged. I've got a few other sneaky tricks up my sleeve for the engine, other than the cylinder head mods I've mentioned.
Jerry

Yep. You are only into this for a delicious chicken sandwich, large fries, and a large Diet Pepsi.
I still say I came out ahead way ahead on that horse trade. I literally "ate your lunch."

LOL! If you've got another head handy, there's a second course waiting. There's a really good BBQ place a few miles south!
Jerry

There's a gutton born every day.

Locking this thread! Story has gone full circle.