Correct MM for Cam rollers thingie bopper

[flmont]

I cant think of the proper term,..its the round metal part that keeps the valve tappit in place.The roller that takes a 6 mm nut on one end and a 4mm nut on the other end. Damn I hate to forrget things like that ,...But I cant find my re-build book to look up the correct Hex head size to loosen it up...Mine had slipped alittle and I need to re-set it Thanks Frank