When you swap a bigger cam, mill a head, or lose timing on an interference engine, the first thing that grenades is piston-to-valve (P/V) clearance. This is the minimum distance between the valve head and the piston crown during the overlap event near TDC — when the intake is opening and the exhaust is closing, and both are hovering above a piston that's racing toward them.

The critical moments aren't at TDC itself. They're roughly 10° BTDC for the exhaust valve (still closing as the piston approaches) and 10° ATDC for the intake valve (already opening as the piston descends). Bigger cams with more duration and higher lift push the valve deeper into the chamber at these exact crank angles, eating clearance fast.

The industry rule of thumb:

• Intake side minimum: 0.100" (steel rods) or 0.120" (aluminum rods, which stretch more)
• Exhaust side minimum: 0.120" — exhaust valves grow more with heat and the exhaust event has less margin for a timing slip
• Add another 0.030–0.050" if you're spinning past 7000 RPM (valve float risk)

How to actually measure it: Use modeling clay or a solder wire technique. Roll out clay strips onto the piston crown at the valve pocket locations. Assemble the head with a used head gasket, light valve springs (checker springs — around 40 lb seat pressure), and rotate the engine through two full revolutions by hand. Pull the head, slice the clay flats with a razor, and measure thickness with a caliper.

Real-world example: A common LS build swaps a stock 196°/201° duration cam for a 231°/247° "truck cam" with 0.617" lift. Stock LS pistons have shallow valve reliefs sized for the OEM cam. Result: exhaust P/V drops from ~0.180" to ~0.080" — below the safe minimum. The fix is either flycutting the pistons deeper (loses compression), advancing the cam a couple degrees (moves the valve event away from TDC), or buying pistons with deeper reliefs.

The quick math for whether you're in trouble: valve lift at overlap TDC is roughly (max lift) × 0.15 for a moderate street cam. A 0.550" lift cam puts each valve about 0.083" into the chamber at TDC. Subtract that from your deck clearance plus gasket thickness plus valve pocket depth to see if you have real numbers or fantasy numbers.

Skip this check on an interference engine and one bent valve is the best outcome. The worst is a valve head snapping off, hammering the piston crown flat, and destroying the block through the cylinder wall.