There was a lot of things left unsaid in the original post. Was it both the intakes and exhausts or just one side? What guides were used, were they installed correctly and most importantly....were the guides honed for the correct ID?? Were the guides thoroughly cleaned? Was the engine overheated? Etc.
Valves will gall and scuff up badly if the guide ID is not correct and/or over heated. The guide should be honed....not reamed. Honing allows for precise control of the ID and...If nothing else.... honing leaves a nice finish which will hold oil.
As somebody mentioned...the coating is very thin (like .0001 or so)....you are not going to change the OD of the stem if .0001 goes away.
Having to mill the head .060 is over the top IMO....I have seen rusted seats clean up with minimal fuss. I don't know if people understand how much .060..(or 1.5mm) is off the head??? Especially if it had already been milled! If you sink the valves that deep you absolutely kill flow especially on the exhaust side and your stem height is wrong on both sides.
The main point is there are many, many variables that were/are unaccounted for and I don't know if people understand that in a rush to judgement. I'll leave it at that and say if you want the longest lasting guide in a 750 head use cast iron. And make all operations and dimensions are correct.
