3/8" Internal Nut Wrench with 1/2" square drive. This wrench is designed specifically to install and remove the cylinder base nuts on Lycoming Narrow Decks (all models), IO-390, and IO-580 engines. Superior quality product made of drop-forged 4130 steel and hardened to 48 Rockwell.

Tip: If your engine was manufactured before 1960, then it almost certainly will be Narrow Deck, and these type of wrenches will work for you. This is also true if you have a Lycoming IO-390, or IO-580 (any year). A quick way to visually determine the type of wrench need is to look at the nuts securing the base of the cylinder. If they are round in shape and have an internal spline, then this is the wrench for you. It they look more like standard hex nuts, then your engine is a Wide Deck, and you will need one of our 3882 series wrenches.