| Sure When you git a JUN cam (for example), the head needs to be machined to accomodate the larger lobe. See pic below 
without this extra clearance the camshaft will foul on the recess. With the JWT design they actually reduce the base circle (section shaded red below) so that you don't need to machine the heads, and the cams can be changed without removing the heads. 
Pros of this approach? Easy installation. Cons? You have increased the distance between the cam and the lifter. This problem can be solved by having your machine shop reset the valve heights to compensate (depending on how thorough they are and if they know this needs to be done...) OR (as I suspect is done in most cases) the hydraulic lifter is left to take up the slack. When I put my engine together the engine builder was very much surprised to find that the JWT R4+ cam was in fact smaller from base to tip than a standard cam. After finding out why I weighed up the cost and went with JUN cams instead. Taking a little material off the heads seemed a preferable alternative to resetting valve heights or risking a noisy valvetrain due to increased clearance.
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20:20:13 10/04/12
Re: Interesting you mention lifter clearance - can you expound - 
