TheSamba.com :: View topic

Wed Oct 22, 2008 10:45 am

Why do high performance builders flycut heads to raise compression, then add shims or headgaskets to increase deck height? Doesn't flycutting bring the valves closer to the piston in the same way as reducing deck height?

Jeff

Wed Oct 22, 2008 11:53 am

When you're building a big stroker motor with longer than stock rods, without cylinder shims the piston will actually stick out past the top of the cylinder so the cylinder shims will compensate for that. I used to be a VW engine builder and what I liked to do when I was building a big motor was to shim the cylinders until I had 0 deck height and use .060" copper head gaskets because you need at least .050" of deck and then flycut the heads to whatever head cc I needed for the desired compression ratio. As far as bringing the valves closer to the piston you're right but the only time it's ever really a concern is when you've got a motor with a huge amount of compression and then you've got to cut valve pockets into the pistons. I hope that was some help.

Wed Oct 22, 2008 3:53 pm

Isn't the rule for needing "at least .050 for deck height" to account for potential valve interference with the piston at TDC?

My question is, how does fly cutting the heads differ from simply using less shim or head gasket thickness to reduce deck height?

Wed Oct 22, 2008 4:20 pm

The .050" deck height is to allow for any heat expansion in the valve train and the piston. And yes, some times you can get the desired compression ratio by using cylinder shims but most cases when building a high compression engine you have to flycut the heads. There's simply a lot more volume to remove in the combustion chamber than in the cylinder deck. For instance, I built a 2276 for a buddy a few years ago with 5.6" rods and 11:1 compression and I had to take quite a bit out of the heads to get that. Without cutting the heads it would have been a little over 8:1 with .060" head gaskets alone.

Wed Oct 22, 2008 4:25 pm

The pourpose of .050" min. deck height is so the top of the piston will not smack the head.

Wed Oct 22, 2008 5:40 pm

For close valve to piston Clearence you have to notch the pistons like this
This is our Piston Vise @ ACE in UTAH

Thu Oct 23, 2008 7:53 pm

Is it O.K. to use head gaskets to compensate for deck height?

Jeff

Thu Oct 23, 2008 8:12 pm

Jeff Pyzyk wrote: Is it O.K. to use head gaskets to compensate for deck height?

Jeff

Minimum deck is actually .040" and yes you can use copper head gasket to compensate for deck.