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Nurl, as I recall is a verb. Meaning to cut with fluting or ridges. Therefore "nurlizing" is the realizing of that process.

Knurl, is a noun, a single ridge or groove.

What are the advantages of "Nurlizing"?

1. Increases piston diameter to exact predetermined size, quickly, accuratly and permanently.

2. The interrupted surface insures adequate lubrication.
2.(a) The valleys act as oil pockets.
2.(b) Oil carried or trapped in these pockets acts as a cooling medium.
2.(c) The valleys act as traps for detached particles of metal or foreign substances which would ordinarily break through the oil film.

3. Permits closer fit without danger of scuffing or scoring.
3.(a) The interrupted surface provides a series of expansion jounts permitting surface metal to expand sideways under heat, rather than bulge and break through the protective oil film.

4. Eliminates piston slap, stabilizes pistons, and greatly increases ring life and efficency.

Nurlizing, is only done on a piston on the two thrust faces of the piston skirt.

These same principles can also apply to valve guides.