Yep that should be it. The best solution is to try and get hold of an original P7-R OFF-ROAD crank. But if you've rad the instructions to start with you'd have seen that this mill only needs 15 minutes of breaking in. The hardened pins have become known to loosen when the engines are run in for too long at a rich seting. The extra stess from trying to compress the denser mixture in the chamber then the crankcase just hammers the Big end pin and that will loosen it allowing the scoring of the back plate. This will cause swarf to fall off and enter the bearing and also g throung the chamber, both wearing away the bearing and the liner face. Basically you broke the engine in for too long.