This tools expands the spark gap of any massive electrode aviation plug safely and efficently -- with the key word being safely... In the past when you over-gapped a plug, and wanted to expand it back out, you would resort to using a screwdriver, dental pick, or whatever else was on hand. But this was almost a sure way to do damage to the plug. Because you would invariably push the screwdriver between the electrodes, which would cause pressure to the center electrode and risk cracking its ceramic casing. The end result would be either reduced performance or total spark plug failure. No so with this tool. Simply hook it under the outer electrode and apply a bit of downward pressure to gently and safely expand the gap without any contact to the center electrode. 5.6" long, .375" diameter. Made of Nickel-Cadmium plated tool steel in the USA