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.
