General Faults: Head Fractures

Effect: A piece broken away from the valve head roughly in the shape of the chord of a circle.

Cause: Very high cylinder pressure and valve temperatures; incorrect valve material; etc. This problem is nearly always confined to exhaust valves.

Effect: Concavity or tuliping of the back of the valve head (piston side).

Cause: Very high cylinder pressures and valve temperatures incorrect valve material.

Effect: A piece broken away from the valve head roughly segmental in shape, or a radial rim crack propagating inwards.

Cause: Excessive valve temperature together with unequal cooling or cyclic engine loading

Example: Exhaust gas leaks past the valve seat, causing it to expand into an oval shape, this creates Hoop Stresses in the valve head. After a few cycles of uneven expansion and contraction stress cracks form at the outer edge of the seat, these cracks progress inwards towards the center of the valve until they are approx. 1/4" inch long then turn 90 degrees and grow around the valve head, when two cracks meet a piece of the seat breaks off.