Cletis if you have a shop (drill press vise etc ) and a lil time the heli-coil idea works well i use them were there's not a lot of meat around the offending stripped out threaded hole. and threaded inserts were there is more meat around the stripped out threaded hole. one place on the web to get the heli-coils is.
www.mcmaster.com cat#112 pg3096-3099 and threded inserts on pg 3100-3104. as far as useing car head on a boat i'm not sure of the differents between them the 305 to 350 i think there here spring and valve sixe differents .Later Earl
