Piston to Valve Clearance Calculator
Click HERE – and then download “Piston to Valve Clearance Calculator.xls”
Whenever installing a new camshaft, a set of heads, or a fresh build entirely, it is critical to check piston to valve clearance. At top dead center, at the end of the exhaust stroke and beginning of the intake stroke, the exhaust valve is closing and the intake valve is opening. Usually about 5-15 degrees before top dead center (TDC) the exhaust valve comes close to the piston, and about 5-15 degrees after TDC the intake valve comes close to the piston. Proper piston to valve clearance is required to prevent valve contact with the piston in high RPM conditions, or during an over-rev.
It can be difficult to plan for P2V clearance. Fly cutting pistons is a hassle, requires extra tooling, and adds quite a bit of labor. There is also a risk in cutting the pistons wrong and rendering them useless.
A piston‘s location up and down the bore is easily calculated if you solve it as a two bar linkage problem. There are two angles as the crank rotates, the angle that the throw of the crank creates, and the angle that the rod kicks out as the crank rotates. This is a simple trigonometry problem. When you plot piston position vs. crank angle, you will see that the piston almost dwells near the top of the bore in the 20 degrees before and after TDC.
Since I was able to ignore piston height as it relates to rod end, I was able to cancel out deck height. Simple algebra. Doing this allows the calculator to be more flexible especially in dealing with other engine types. Feel free to use this for Ford, Dodge, LT1, etc.
The valve location is a little more tricky but not impossible to figure out. Comp Cams only provides 5-6 data points that we can fit into a Nth order polynomial to interpolate and extrapolate valve position beyond and in-between the points given. Since duration is variable, we located valve events to the lobe center as apposed to start and finish of the lobe. Again, to make the calculator more flexible.
Valve drop locates the valve for the calculator. This is needed because even a valve job can change valve position as it relates to deck height. Valve drop as we define it is the distance between the tip of the valve and the deck of the head. This can be found by placing the head on a flat surface, and either measuring this clearance in the vertical plane, or measuring the drop in valve lift. Consequently, if the heads are not off the vehicle you can measure this distance by turning the engine to top dead center. With this type of measurement be sure to zero out head gasket thickness and piston out of the hole measurement. This method would also account for valve reliefs in the piston.
Enjoy! Notice how cam advance quickly eats up or gives extra P2V clearance. Notice there’s not much difference between lobe types or lift, and instead duration will make or break you!
