While I wait (seemingly forever) for back-ordered rocker shims to arrive from aircooled, I keep debating in my head which way to go on deck height on a 2056 rebuild I am doing:

CC volume is 59.8 (calculated by Len Hoffman)
Raby 9950 (FI) cam
JE Pistons with valve relief
Deck height w/o any barrel shims is .032.
That gives me a CR of 8.6, assuming the valve relief adds about 2 cc's to CC volume.

Adding a .010 shim puts me just shy of 8.4 CR with a deck height of .042. .042 deck height is safer. Pretty sure I wouldn't go .032 deck height with flat tops, but does the valve relief give me enough "safety?"

This is my first Type 4 rebuild, so maybe I should go conservative? Am I crazy considering going less than .035? I've done the "clay test" with deck height at .042, and there's no issue there.

What would you do?

Go with the shim

valve relief cuts no mystery...find a local shop..you can split the cr 70/30

So I should probably add: I did go to my "local" machinist (about an hour a way) and had him cut .035 off the jugs a few weeks back. I should have had him take a bit more, but I didn't think it all the way through -- that's my fault. Having him do additional work is a couple of 2 hour trips, $100 in services, and, likely, 2 to 3 weeks waiting. None of that is a deal breaker, but just trying to determine my options.

So I guess the straightforward question is: what is safe/acceptable deck height with valve relief? Still .040? .035?

Thanks for the input, fellas.

ya shims

Ok, you and Rich are going to keep me on the straight and narrow.

Shims it is.

The old rule of thumb was that .040 was the smallest safe deck height. Tighter then that you needed to measure and be SURE.

Some guys would measure by putting a little lump pf clay/play-doh on the piston and rotating the engine by hand. Then pulling the head off and looking at the clay to see how much room they had after it was squished by the valve.

Are you running a head gasket?

Zach

Thanks, Zach. No head gasket. I did the play-doh with the barrel ship installed and deck height of 042. No issues. Just wondering if .032 was too close even with valve relief. I should have planned it a little better ahead of time with the machinist, but it should still be all good.

Add the shim.....

I doubt you need valve reliefs with the FI camshaft either way.

Yeah, live and learn (which is a large part of what this hobby is for me). If the worst outcome is I left roughly .1 CR on the table then I will still be pretty happy with my first Type 4 build

Piston rocks at TDC, .040" is my minimum.
That cam I don't know the specs, but you said FI, so you don't need pockets.

When I set up the valve train with that cam (9590) the max lift I got was .426

Your results may vary.

Off the top or the bottom? If the top, make sure the heads don't hit the top cylinder fin.

Off of the bottom...

....and, I went out and purchased a set of Keith Black flat top pistons. I'm hoping it doesn't radically change existing deck height, but we'll see. I guess I'm willing to start from scratch to get closer to 8.6 CR after all.