Even AFR was going through some rough times with seats 0.007" out.

Guide sizing is an engine dependant tolerance, and should be checked. No different than rod and main bearings.

If you're cutting seats, it's good practice to touch up the seat with a fine valve stone by hand to true it up and cut down any micro chatter marks etc.

I don't think OP is still on this site and I don't think he built his own engines from what I read. I bought AFR heads and their site says specifically that if these are used in a marine environment you need to adjust them to suit your needs. I also called them and told them I bought them for marine use and they said they were "good to go as is". They make it YOUR responsibility to make sure their product works for your application. We will never know what actually happened between OP and Merlin. We only get one side of the story here. I think it was pretty generous of them to send him a replacement set of heads. It is beyond my comprehension as to why he didn't have the new set checked before install?? "Fool me once, shame on you, Fool me twice, shame on me" You have to take responsibility for YOUR actions. From what I have read on this site, most of you that actually build engines for a living (and some others) either order heads bare or disassemble a complete head and check everything. In my case, I supplied all of the "Top End" stuff and my builder simply installed it. No checks other than what was necessary in his opinion. I know he checked p/v clearance for example. I'm pretty sure he built 2 automotive engines for me but he also told me to run 10w30. I think you guys building for marine use run 20w50 for the most part. With your tolerances, could you even run a 10w30? With the advancement in oil protection over the years, do you really have to run a 20w50? Did this guy hang valves because he didn't warm up the engine before stomping on it? Would he have had this problem if he ran a thinner oil?

Many merc engines specify 25w-40, some older a sae40, so there’s that on thin oil in marine :)

My go to is mobil 1 15w-50 and valvoline vr1 20w-50 conventional depending what motor i’m dealing with.

15w50 synthetic is what GM says you need to run on the Corvette engines at the track.

Not sure now, but 0w40 was not a Dexos approved oil. It's pretty hard for a multiviscosity oil spread so far apart. As the oil shears...it gets reduced to its base viscosity. Large timings chains on DOHC engines have a massive shear factor on oil.