The lower end of the reamer was made to fit nicely in the guide for the valve seat. One of the plugs in the engine directly over the valve was bored out to fit the upper part of the reamer.

As an example, if the valve seat on a small-block Chevy iron head ... That may sound good, but the tradeoff is that the mid-lift flow numbers at 0.350-, 0.400-, and 0.450-inch suffer horribly.