[PeteKz]

Originally Posted by snbush67

I put in a call with William Marshall, and asked him to help me verify if my valve clearances were good. After a long explanation of what I had been doing he simply said “Go to top dead center”, I said “I’m there”, he then said “go to # 4 and measure the exhaust clearance”. I used my special Icarp magical valve lever (picture below) and noted well over .25’ of travel, I said, “over a quarter of an inch”. He said “one and done, you’re good”.

I thanked him as he had another call coming in and pondered the physics of what had just happened.

Most in this forum probably know this, but here’s my thoughts.

Check valve clearance at valve overlap for cylinder #4, with the engine at TDC for cylinder #1.

Why? Valve overlap for cylinder 4 happens when it reaches TDC during the transition from the exhaust stroke to the intake stroke. The firing order is 1-6-2-4-3-5. Cylinders 1 and 4 are on opposite sides of the engine.

When cylinder 1 is at top dead center (TDC) on the intake stroke, cylinder 4 will also be at TDC, but on the exhaust stroke. This is because the two pistons are 360° apart in the four-stroke engine cycle.

So, the piston in cylinder 4 will also be at its highest point (TDC), but completing the exhaust stroke, preparing for the intake stroke to follow. Valve overlap occurs at the end of the exhaust stroke and the beginning of the intake stroke. During this phase, both the intake and exhaust valves are slightly open to allow better cylinder scavenging and airflow into the combustion chamber.

Shane: Correct.

A couple years ago I wrote up about how to check valve clearance at overlap, because that's where the valves get closest to the pistons. You don't need the Stomski tool, although it's handy. Instead, I turn the valve adjusters down to make the valve contact the piston (gently), counting the turns. The adjuster screw pitch is 1mm, so every full turn is 1mm of valve clearance. I check both the #1 and #4 pistons at overlap because I want to be sure there aren't any differences from one side to the other, due to cam timing slipping or some defect in the cams.

In my case, using the M1 cam and the Mahle Sport 3.2 "10:1" pistons, my valve clearance at overlap was around .050". I have a tight clearance because the Mahle pistons are domed on one side without valve cutouts, and the M1 cam uses all the available lift and duration with those pistons, and I deleted the copper base gaskets to get a tighter combustion chamber and squish/quench (.030" deck clearance at the edge of the piston/cylinder/head).

You had 0.25" of valve clearance because the pistons you used have very deep cutouts. Those pistons are designed for racing engines with high lifts, much higher than what you're running.