11 With aluminum heads, valvespring seat inserts are a must. Without them, the spring (and flat wound dampener if so equipped) can dig into the aluminum, sending metal shavings into the oil to wreak havoc in the engine. When you're checking installed height, be sure to do it with the insert installed, as this affects installed height, seat pressure, and open pressure of the valvespring. The seats on our RHS heads are 1.550-inch diameter, but can be machined to a max diameter 1.625-inch. to accommodate larger dual or triple springs, but spring pocket depth cannot be deepened.