im installing E7 heads on my 68 302 block that im rebuilding. shoul i just install flat top pistons to raise the compression or have the E7 heads shaved?
my stock heads had 58cc chambers but they are shot..the E7 heads are 63cc and rebuilt.
Pistons are the better option, but shaving is much cheaper (much depends on the quality of the pistons, cheaper pistons and rings might not runa whole ton more than the machinework involved with shaving). Beyond a point though shaving won't help. For most heads, to drop the combustion chamber by 1 cc you have to shave approx. .005-.006. I don't think you'd want to shave much more than .030 (though Hearne did his .060).
5-6 CCs is worth about a half a point in compression max, and get you roughly where you were with those 58 cc heads. If you have good stock pistons (68's should be pretty good), that might be fine and I would think get you in at least the mid-9s and potentially into the 10s.
I'd say if the things already apart, go ahead and get some inexpensive performance speedpro pistons and then you can choose whatever compression ratio you want. It won't cost you much more or take much more time.
If you're like me and don't want to dissassemble the rotating assembly, shave it .030 (i did just that a few months ago). You'll save a little money and a lot of effort.
If you choose to buy pistons (you'll need to anyway if it needs an overbore) Pay close attention to the compression or pin height.. Not all flat tops will give you a zero deck. The pin heights of 289/302 flat tops varies from 1.585 to 1.619. The taller, the better. The 1.585 usually end up .020-.030 down the hole at TDC, that's an additional 9-10 ccs volume added to the combustion chamber. And a full point compression loss. Do your home work before buying pistons. Do not rely on the piston mfg's compression ratio chart for that piston. If the pistons you choose don't get you a zero deck, you can also mill the heads and block decks some to get it there. That probably needs to be done anyway to get the decks square and the heads flat. But only remove what's necesssary in doing it.
definitely buy new pistons. buy some flat tops that come up level with the deck block and have 2 valve reliefs. IMO, don't start shaving heads and risk causing problems. Besides, if you get pistons, you're putting money towards an upgrade instead of blowing it away on a set of stock heads. In fact, if you're really looking for power and can afford it, buy some aftermarket heads with 58cc chambers.
+1 for pistons. Don't buy pistons before the engine is taken apart; if you have worn cylinder walls it is not wise to install there new pistons without reboring the block. measure the distance between piston at TDC and top of the block and based on that find the piston with the best deck height. Also test valve to piston clearance (with a plastic mold or so) if you have an aftermarket cam