Originally Posted by dlisabeth
I am almost ready to assemble my engine but need to order a few final parts first. I am going to be running flat top speed-pro 4.030" pistons which the block has been zero-decked to. I am also using 76cc World Products S/R Torquer heads that will put my CR at approximately 9-9.1:1. I was planning on using the Fel-Pro 7733PT2 head gaskets that came in my gasket kit. They have a compressed thickness of .039" and a bore of 4.125". I have seen recommendations of using a gasket with compressed thickness of .040 or .041 with a zero decked motor. Can this small difference in thickness really make an noticeable difference? It also looks like the .040 and .041 gaskets are made of much better materials which must account for the cost difference.
I think I will order either the Fel-Pro Z1142 @ $75 or the Fel-Pro Z1003 at $35. Why the big difference in price and will either work for me?
The issue at these dimensions is less one of compression and more of potential collision between piston and head. The piston rocks around it's pin going over the top, a situation exacerbated by the pin off set which is usually .060 inch but some makers use more, others less. This rocking is from a transfer of thrust faces on the skirt; in the process of doing this, the piston tilts and its crown is high on one side of the bore. As you get under .040 inch, the risk of the piston hitting the underside of the squish/quench deck goes up, this is what people are trying to avoid with the .040 minimum clearance. .039 may be fine, but keep in mind where all this is headed. Some racers run 'em tighter but to do that they are using a zero offset pin and very tight skirt clearances. This is getting edgy but when big bucks are on the win/lose line this is the kind of stuff you try to get away with. For a street engine this just an invitation to destroy expensive engine parts.