I'm pretty sure its a .40 or .45 but I know very little else about it. I'd like to know the brand if anyone recognizes it. From what I can tell, it looks to be an ABN sleeved engine - No rings or oil grooves. Bore is .835" and stroke .716" (approx.) It has 1 ball bearing in it, and the carb is about 7.2mm. Square flange going into engine block. Backplate held on by 6 screws, which acts as one of the internal ports.