his steel 1/4 NPT pipe plug has a hex recess. This plug is used to plug several holes in the FF1600 engine block. One (which is sometimes replaced by a drain petcock) plugs the water jacket on the exhaust side. Two plug the oil galleries on the front of the block, one plugs the main oil gallery on the cam side of the block, and one plugs the main oil gallery on the rear of the block. Sold individually.