I have a problem with #6 cylinder on three different engines possibly four that have been installed in my boat. The first engine was a Mercruiser 350 V8 and #6 hyperutetec piston top broke off and then I installed a second 350 engine and #6 sparkplug center electrode melted out. The third engine was a 305 and it only lasted less than 10 hours before it melted a hole in #6 piston. I set the timing at 8 BTDC used the correct marine sparkplugs ac mr43t - 350 and ac mr44t - 305 in the engines. I think it has to do with my Thunderbolt IV ignition - distributor and or the module that mounts on the port side manifold riser. It says 22a on the module and all I know is that I could see it advancing with a timing light. The second engine had a different carb on it but the first and third had the same Rodchester Qjet carb. All of the other pistons look fine but #6 is the only one with overheating signs. I feel that there must be something wrong with the ignition or the carb. Also the orginal engine blew up and the owner said he was told he had the wrong spark plugs in it but I believe it was probably #6 problem then again. When I installed the engines I reused the carb, ignition starter alternator, etc. so it's got to be something that is being reused on a different engine. Any ideas? Why predetonation on #6 only? Please respond.