be careful and make sure to get THE RITE piston and rings from Briggs and stratton, I'm not SURE but would be VERY willing to bet that, there are no aftermarket parts for the 10CI lower end.. YOU CAN NOT use 5 horse crank and rod OR piston in a 4 horse, unless you change EVERYTHING and MAKE IT a 5 horse.. The wrist pin to piston top is a different hight than the 5 horse, so none of the 4 to 5 horse parts will interchange, then actually work...