The gap is 0.060 inches, and the plug is an AC type 41-940. The plug gap is the space between the metal tip of the center electrode and the electrode attached to the side of the metal part that screws into the engine. It is checked with a spark plug gap gauge and there are a few different varieties available. They normally have instructions on how to use them. Probably the best to use is the one that is tapered numbered surface, and you start it into the gap at the thinner area, and you slide the plug gap onto the thicker part with out forcing it and you read the number on the gauge that is at the spot where the gap is just a little snug with out forcing it.