What is the outside diameter of OEM valve guides? I see 0.500 more than anything else, but mine measure closer to 0.575. I bought 11/32 x 0.500 viton valve seals, but I think 0.530 would require less cutting of the guides in order to fit. Sometimes less is more. 

I get 0.585” at the upper part of the guides on a pair of heads sitting in front of me.  I typically get away with using 0.562” seals but there are some heads that have just enough of an offset at the hole that necessitates using a 0.530” seal.  I do not use 0.500” seals unless there’s a spring fitment issue when trying to use the larger diameter seals.

Lorena, Texas (South of Waco)

Thank you, Ted.