Most manufacturers' maximum acceptable worn guild clearance is on the order of 3 to 3.5 thou.

Changing guides should be left to a shop with the proper equipment. The head is usually heated in an oven to about 400F and the guides are chilled in refrigerator. Upon removal from the oven the tech quickly taps out the old guides and taps in the new. Then the head is placed back in the turned off oven and allowed to cool very slowly.

Slight distortions invariably arise, so reaming is almost always necessary and final clearance should be about one thou.

Duke