Fly-cut pistons or shim cyl base?

[tpknet]

I posted previously about my piston to valve clearance problems. The shop that did the cam timing for me confirmed with a guage what I had found with the screw adjuster method that they had screwed up the timing the first time. They took the motor and "fixed" the timing as best they could. The best they could get things are as follows:

Set at 1.7mm ( the spec with the cam said 1.8mm to 2.2mm)
Intake valve clearance 1.4mm
Exhaust clearance 1.25mm

There has apparently been too much taken off the heads to get things right. He has agreed to take the motor and either shim the cylinder base with thicker gaskets or fly-cut the pistons.

Am I going to be able to get the clearance I need by fly-cutting the pistons? Am I going to have too much compression if I don't shim the cylinders? How much should I fly cut the pistons?

If I go with the shims, how much shim will I need?

Thanks,
TPKNET

[350HP930]

Shim it. The last thing you need is to do is hack up a perfectly fine set of pistons when a set of gaskets will do the trick.

[Steve@Rennsport]

Good question, and the answer is somewhat complex.

You are juggling several parameters here; piston-to-valve clearances, compression ratio, and deck height.

You need to know exactly what your CR is and where your deck height would be with some thicker base gaskets so you do not exceed .040 deck height.

Knowing these variables will tell you how best to resolve the valve clearance issue.

[Henry Schmidt]

Shim the base. If the problem truly is that the heads were fly cut too far, the engine is out of dimension. What I mean by this is that the dimension between the crank center and cam center is fouled up. Probably too short. Some people shorten these engines in a crude attempt to raise the compression ratio. This adds problems that can be compensated for but why bother. What you want is to get the proper dimension so the cam fits squarely in the center of the cam chain housing. There are ways to compensate for a short engine but getting it back to stock is what I believe is the best remedy for your problem. When the engine is dimensioned correctly, retest your valve to piston clearance. If at that point you don't have the clearance you want and the compression you seek is close just pocket your pistons. Before you pocket the piston check the piston dome thickness to make certain that after the material is removed you have at least .250" na and .280" turbo. Valve pockets only effect the compression ratio slightly.
It is my opinion that you should not use more than 2 shims. If you need thick shims they are available in .25 mm, .50 mm, 1.0mm. If you need more than 2.0mm of shims it's time to reconsider the parts you are using.
Good luck.
PS: Valve timing is a relatively simple process. If your mechanic screwed it up, perhaps a different source for expertise is in order.