Rich in cylinders 1&2

[Jac Mac]

Are you sure its not the radius under the hex of the screw in studs that is contacting the guide plates before the flat surface and acting like a taper to split the guide plates?

[Texasdoc]

Possible, but it looks like the bottom of the stud is pretty flat - I don't see a radius. These guides were used with these studs previously and they didn't break. The guide plates are all the way over to one side just to get the rockers aligned with the valves. Don't know if it matters or it is just coincidence that they are all the right side pieces of the guides that are breaking. It is the stud that goes into the intake runner and all the bad valves were intake valves.