I had to replace the exhaust valves every 4 seasons in my W2 small block. If I ran them any longer it would pull a stem out of the exhaust valve. And it didn't matter what brand of stainless valve it was.
4 seasons is not 6-7 runs on a new motor that failed.
Machinist could be wrong. Only way to know is check piston to valve clearance. .090 intake .100 exhaust. Throw a head on with some clay
Or better yet I use light checking spring at 10deg btdc and other valve at 10 degrees atdc, that is always where the closest clearance is at. If you don,t have those numbers, you have potential trouble. If you change cam timing you also change the clearance numbers and need to recheck. This motor has all aftermarket parts from different manufacturers. Rods, pistons,stroker crank,cam, how do you know what cha got for clearance if you do not check. Trying to figure out whether the head broke off the valve stem
Or the piston hit the valve head and broke it off, the damage would look the same. It's like what came first the chicken or the egg lol.
IMHO.
