Originally Posted by tekenaar
This is decidedly "finish" head work, so "after".
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True Otto, I always have the guides and hardened exhaust seats installed first, and of course the seat diameters are cut to the size of the new valves before porting begins. But I tend to hold off on all other machine work until after the porting is complete. For one thing, there will be a lot of cast iron chips and dust through out the head after porting, so it needs to be hot-tanked again. Secondly, there's always a chance you will slip with the die grinder during the porting or polishing steps and will hit the valve seats, so this may need to be 'touched up' by the machinist after porting. And third, when you flip the head around on your benchtop during the porting process 50 or 60 times, you will undoubtedly scratch the head's deck surface, so the final head surface milling should be done after the porting is completed.
HTH,
Bob