These valves have become the standard upon which all marine
heads are constructed. If someone is telling you they have a valve just as
good or better than a Manley Inconel valve "Beware". There's a reason
they are the choice of real marine engine builders and cylinder head
manufacturers, no other valve has the ability to make the claim of having
over 1,000 times the fatigue strength of other competitive valves.
Note: This one part in your engine quite possibly takes the most strain
of any given part in your engine.