Chatting to Speedy Sheady today, he reminded me of something else to do with valve seat failure that may be worth a mention on this topic. When porting these heads, you must not take too much material away from the actual valve seat area as you compromise its seal against the head and this can DEFINATELY cause valve seat failure.

As standard, the seat throat is about 27mm IIRC. Its acceptable to extend to maybe 28/29mm, but after that you really are compromising the valve seat security.

Ive seen one fail recently at around 31.6mm, which means the seat lost 2.3mm of its material all round during the porting process! No wonder it fell out when it got hot!