mine had the same brass washers between the counter wieghts and the roller bearings. i measured the bore and it was about a 1- 5/16". the HDC-65-1 was a 11.13 venturi so i am opting to believe its a 26 based on those two things. the piston is the same size as my homelite string trimmers. i kinda wonder if there was a roller thrust bearing out there to replace the brass thrust washer. i think it would make the engine spin more freely. i have some amount of work ahead to get it finished and mounted. also i have a different muffler setup. mine isnt on the back of the engine its on the side of the engine. looks to be a fun project!