You asked why I made the comment and I'm sorry you took that personally. It was just friendly advice and you're certainly welcome to take it as you like.

yes they do.and i will add that they are the best fitting girdles i have ever installed.

They sure look like the Dart ones, that cost 100+ more dollars.

To sum it up, just buy the least expensive valve train components that you are sure will not break for the extended time and RPM range you expect to run. Mercury has this figured out, many of us DIY engine builders are just guessing. You can increase you odds of valve train success if you use the biggest diameter and heaviest wall PR you can fit in your heads.

I don`t see any Mercury engines with those and they run just fine for hours and hours

Just what do stud girdles do , is it merely to keep the deflection under control or is there another purpose as well?
if so , why aren't the studs large enough in the first place?

the valve springs on 500&525 engines might last longer if merc used them.the bean counters at mercury marine don,t want to spend the xtra money for them,all they care about is getting past the warranty period.

They support the stud from the outboard end resulting in a much stiffer setup. Why not larger studs? Packaging for one, necessity for another. These originally being car engines, we push them harder than they were ever meant to be pushed.

Better question- why not shaft rockers?

not sure what you mean by outboard end,the way i see it they are all tied togeather.