Is this grooving normal (from the 911 technical forum)
 

I posted this in the other forum but wanted to see if there are ideas here about the origins of these marks. Made at the factory or made by a previous owner or his mechanic?



http://forums.pelicanparts.com/uploa...1325441513.jpg

My heads are off, all 6 of them have the same die grinder chatter on them. It was factory.

I've seen a number of die grinder marks here and there, looks like the grinder operator was looking at something else and waving the tool around from muscle memory or was getting paid by the piece. Maybe the marks you have are vortex generators producing more flow, probably not.

Carbide Marks
 

They are from the factory when they blend the seat to the port, they vary in detail but thats production work by hand.

Mike Bruns

Thanks for the info. Next question: when you tear down your motor, do you have the machinist smooth them out or do you leave them as is?