The measurement is the measurement
Nothing calculates piston to valve clearance from first principles in a way you would bet an engine on. It comes from clay on the piston crown, two turns of the engine, and a razor through the impression — or from a dial indicator on the retainer with a light checking spring, pushing each valve down until it touches at a series of crank angles. What this page does is take that measurement and carry it forward when something in the stack changes, so you do not have to strip and re-clay for every gasket you consider.
It carries a number. It does not produce one, and if the cam timing or the rocker ratio or the valve size changes, the whole thing has to be measured again.
Four movements, three of them projected
Everything that changes the gap does it by moving the head relative to the piston along the bore, or by moving the valve relative to the head along its own axis. A thicker gasket lifts the head away and opens the gap. Metal off the block deck raises the piston and closes it. Metal off the head face brings the valve down and closes it. A valve sunk deeper into the head after a seat cut opens it.
The first three happen along the bore axis. The clearance is measured along the valve axis. On a canted-valve head those are not the same direction, and only the cosine of the valve angle turns into clearance — at 23 degrees, cos is 0.9205, so a 0.010 in thicker gasket buys 0.0092 in of clearance and a 0.010 in skim off the deck costs the same. An 8 percent difference is easy to lose in a sum and it always runs against you when you are close.
The default case, worked
Take 0.110 in intake and 0.135 in exhaust measured, a 23 degree valve angle, and a 0.010 in allowance for rod stretch and thermal growth. With no change to the stack at all, the results are 0.100 and 0.125 — the allowance is the only thing acting. Now go a size thinner on the gasket, 0.010 in less, and the projected loss is 0.0092: intake lands at 0.091, exhaust at 0.116. Against an 0.080 in intake minimum that leaves 0.011 in in hand, and it is the intake side that ran out first, which is normal.
The sideways number is a warning, not an answer
Moving a head down the bore does not just bring a canted valve closer to the piston, it slides it across the piston. At 23 degrees, sin is 0.3907, so a 0.010 in movement drags the valve nearly four thousandths sideways in its relief. On a piston with generous pockets that changes nothing. On one where the valve is already close to the edge of its relief, the closest point on the crown after the change is not the point you measured, and the axial arithmetic reads optimistically. The page prints the figure so you know when to stop calculating and go back to the clay.
The check happens either side of top dead centre
Both valves are furthest from the piston at top dead centre, which is why checking there tells you nothing. The intake is tightest some way after TDC as the piston comes down and the valve is still opening; the exhaust is tightest before TDC as the piston comes up and the valve has not finished closing. The two worst angles are different and the two clearances are usually different by a good margin. Any measurement fed into this page needs to have come from the right angle for that valve.
Questions people ask
Can this calculate piston to valve clearance from scratch?
No, and nothing sensible can. It needs a measurement — clay on the crown sliced with a razor, or a dial indicator on the retainer with a light checking spring at a series of crank angles. What the page does is carry that measurement through a change to the stack: a different gasket, a skim off the deck or the head, a valve sunk deeper after a seat cut. If the cam timing or the valvetrain geometry changes, the measurement has to be taken again.
Why does the valve angle matter?
Because a gasket change or a deck skim moves the head along the bore axis, while the clearance is measured along the valve axis. On a canted-valve head those are different directions and only the cosine of the angle turns into clearance. At 23 degrees that is 0.9205, so a ten thousandth gasket change is worth about nine thousandths of clearance. Ignoring it overstates what a gasket change buys by roughly eight percent.
Where is piston to valve clearance tightest?
Not at top dead centre. The intake is closest to the piston some way after TDC, when the piston is coming down and the valve is still opening; the exhaust is closest before TDC, with the piston rising and the valve not yet shut. The two angles are different and so are the two clearances. Any figure entered here has to have come from the correct angle for that valve.
What minimum clearance should I use?
Whatever your engine builder or your cam supplier specifies for the combination, which is why both minimum fields are inputs rather than constants. It depends on the rod material and length, the engine speed, the valve size and how much the valvetrain floats, and this page has no basis for a number and offers none. It subtracts what you enter and prints the difference.
Why does the page subtract an allowance?
Because a clay check is done cold, by hand, on an engine that is not making power. At speed the rods stretch, everything grows with heat, and the valvetrain does not follow the cam exactly. Engine builders take a margin off the static figure to cover that, and the field is there to subtract whatever margin yours works to. The value in the field is a placeholder, not a recommendation.