Indmar Engine Block Plug Brass engine block plug from Indmar Marine engines. This is a 1/4 plug with hex head. Replace your worn block plug(s) regularly to speed up engine block drain time, winterization and more. Please note some engines may have 2 block plugs while others have 1 block plug and 1 knock sensor. This part is commonly found on Indmar engines found in MasterCraft, Malibu, Supra and Moomba boats.