Very high lift cams can cause the retainer to hit the top of the guide. Mopar recommends at least .050-inch clearance, with the seal included. The clearance can easily be checked by rigging a valve with a checking spring and reading with a dial indicator to see how much travel there is until the retainer hits. Be sure to make an allowance for the valve seal, and determine the amount that needs to be removed to provide the recommended clearance. The machining-down is usually done when the tops of the guides are cut for the positive valve seals.