[Tom Vaught]

Knurled is NOT a groove top to bottom.

Knurled is like a tiny Hills & Valleys surface finish that traps oil in the voids and because the valve is moving rapidly top to bottom the oil cannot escape.

With the head off of the engine and the valve out, a tool that looks like a deep hole tap, but without the interruptions that would cut a chip, is lubricated and then run down the valve guide hole. The tool is spiraled like a bolt, but does not cut threads, instead it displaces the metal and thereby makes the diameter of the hole smaller. It is then reamed to the proper size according to the diameter of the valve stem + a clearance. The grooves that are left in the guide will actually hold more lubrication than a plain guide. I have pressed in new guides that already had been knurled from the factory, I assume (yes, I know the rule of assume) to provide an extra degree of lubrication.

MORE OIL TRAPPED FOR LUBRICATION but a lot less surface area supporting the valve, especially in a high spring pressure application and you get a lot less durability out of the repair.

Tom V.