I need some help understanding this. Both statements imply that decking the head, decking the block, or head gasket thickness reduction will have different impact on the combustion chamber. Taking .010" off the head surface, the cylinder block, or using a .010" thinner head gasket all have the effect of bringing any reference point in the combustion chamber .010" closer to the crank centerline (and, therefore, the piston crown). The piston would come .010" closer to the squish area and .010" closer to any open valve (assuming no changes to valve timing). I would expect any method used to make the .010" reduction would have the same impact. I suspect that decking the head does have a secondary impact on combustion chamber shape, since you would presumably be slightly reducing the percentage of surface area projected below the concave portion of the chamber vs. the projection below the machined flat area (over the piston). This would seem to be a small difference, though. I really don't see how decking the block or reducing the head gasket thickness would give different results. (For this discussion, I'm assuming all components were straight to start with and we are only looking at the impact on the combustion chamber, not on component alignment, surface flatness, etc.) Obviously, I'm not an engine builder (at least for 4-strokes). But I am interested and would appreciate any further explanation.
OK. I'm an idiot... Obviously, the clearance to the machined surface above the piston remains the same after milling head. Thanks. :up:
Exactly right, you can remove 2 inches from the head and the piston to head clearance would not change. Piston to head relates to the flat portion of the head only, ie the squish pads. Think about it.
I understand. The only engines I've seriously modified are 2-strokes, and that was a long time ago. Their squish bands were in the concave portion of the head, so milling the head did bring the piston closer to the recessed combustion area in the head. Sorry, I should think longer before posting! Thanks again.
Just remember even though it doesn't change squish/head clearance, it does change piston to valve clearance.