My ECZ-G valve seats/bosses need to be cut for beehive valve springs. The spring OD is 1.25/ID is 0.845. The viton seals are 11/32 x 0.500 with an OD of 0.546. 

What size valve seat cutter should be used? 

I think I'm making this more complicated than it has to be for my target level of performance for a street engine. 

Spring seats or spring cups? Each can potentially require a different cutter.

If spring seats, then the ID of the cutter is important as the spring seat needs to fit around the guide in a ‘just fits’ manner.  The O.D. on that cutter just needs to be large enough that the spring seat fits in the head.  If the valve springs are slightly larger in diameter than the spring seat, then the O.D. of the cutter preferably needs to match the O.D. of the valve springs rather than that of the spring seat.  Also keep in mind that the particular spring seat being used needs to be specific to the valve spring being used as there will be a step on the inner portion of the seat to allow the spring I.D.  to fit without moving around.

If spring cups, then the inside lip at the outer edge of the cup needs to fit the outer diameter of the spring.  The spring needs to fit within the cup without an excess of free movement but just loose enough that spring rotation within the cup is not inhibited.  There are two different ways to approach which cutter you use.

For approach #1, the inner I.D. cut at the guide needs to match the hole in the cup.  The outer diameter cut needs to be at least as large as the outer diameter of the cut but being slightly larger will not be a problem.  Just like the spring seat, it’s important that the cup fit snuggly around the guide boss so it does not move around.

For approach #2, the outer O.D. cup needs to match the O.D. of the cup so that the cup is solidly located in the head so it cannot move around.  In that instance, the center hole in the cup can be larger than the guide that it has to fit over.  If the inner I.D. and the outer O.D. of the cutter matches both dimensions of the spring cup being used, then all the better.

Be sure to cut the valve guide tops accordingly for whatever seals you are using.  My preference is to use the 0.530" or 0.562" seals over the 0.500" seals when possible, as using the 0.500" seals does make the top of the guide marginally weaker.  Also make sure that there is adequate retainer to seal clearance to ensure to contact issues there when the valve is at full lift.  Cutting guides for seals takes a different cutter than those used for the valve spring seats and/or cups.

Lorena, Texas (South of Waco)