So, working on this 270 gmc with 3.875” bore with some “pop-up” pistons and a 270H head. question is... anyone have an idea of how much to cut off the deck and head to bring the compression ratio up? Seems like gmc set the piston below deck about 0.150” and the crown is about 0.365” above that. depth of the combustion chamber is about 0.980”. Head gasket is 0.080”. Bill fisher speed manual say 0.125” off the deck (which would leave the piston about 0.025” from the top of the deck) And another 0.090” off the head to get about 12:1. All sounds good but after looking at the head thickness though the water holes, it measures out about 0.250” so in that area we’d be about 0.140” thick. Seems really thin to me... the block seems to be much thicker and I think the 0.125” should be ok. The two biggest concerns are: 1- piston hitting the combustion chamber. 2 - thin block/head material warps or deflects and we get blown head gasket and or coolant leakage. whatcha think? RichFox any input?
Your best bet is to make plaster cast of the piston top so you can get an accurate cc of the top. By putting a clear plexiglass cover makes it ez to do the head. I bought a plastic berette from Amazon to do my last one. A formula on the KB piston site really helps. One of my 270H heads had been cut .125” and the other .100” and I’ve never had problems with either. If you can, leave in the copper water directors inside the head. They aim at the exhaust valves. You must run a water line from the last 1/2” pipe plug to the thermostat housing to aid with the rear of the engine cooling. The one in front of it is for the temp sender. The 270H head has more meat on it than a 302. I personally don’t like taking any off the block especially because the front 2 head bolt threads are interrupted by holes from the thermostat housing. Anything over 10-1 gets head studs. Best head gaskets seal better than any other and if you ever go over 4.060”, which a 270 can’t, you should o-ring. My highest comp ratio is right at 13.9 to 1 for Bonneville and El Mirage. I just finished an engine for my son. Custom Ross pistons with a 61cc pop up on the piston and a stock 270H uncut with larger valves. The bore was bigger than yours and the deck clearance was .010”. It worked out to 10.2 to 1 and doesn’t not ping on premium. Photo of Ross pistons installed Good luck....Inliner #35..
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