I run an auto repair shop, though we don't do machining type work, (we'd have the head sent out and new guides installed). I don't know of anyone that knurls valve guides or for that matter I haven't known anyone at least any professional that has knurled guides on any regularity since the 80's. If you think back to the old days... Lets say the 60's. Most people were doing good to get 100K out of their engines, and a 200K car was much more rare than today. Labor costs were lower. Engine oil was not nearly as good. In that time a person with a car with 120K and valve guide problem, it was probably very much worth while to knurl the guide to get another little bit out of the engine. Or even at the time of a rebuild, assuming the car probably wouldn't see another 100K put on it before it hit the wrecking yard anyway. Today it's a waste of time, and nothing more than a stop gap. Why because we put many more miles on our vehicles, per-year, and per-vehicle life. Engines last much longer if taken care of. My thoughts anyway
