Somewhere I read that the max substained RPM for long stroke engines was 85% of rated horsepower RPM. The 218 cu flathead Plymouth engine falls into the long stroke engine catagory. The horse power is rated at 3600 RPM which limits the top driving RPM to 3060 RPM. Does anyone know if the 85% rule is a true statement for long stroke engines and where does it come from? Somewhere someone has a manual for engine design which probably has this information and I would like to know more about the subject as I have been cruising mine up to 3400 RPM and just replaced all of the piston pin bushings and all of the bearings. The bearings all showed signs of destress and two cylinders had the top ring broken. It was suggested that I had run the engine at too high of an RPM range for long periods of time and maybe to much advance. The top half of all four main bearings showed more wear than the bottom halfs did, what gives here? The engine had less,than 8600 miles from total rebuild, but I did a 4600 mile trip in August which probably did the most damage as we cruised between 65 and 75 MPH for hours on end (3200 to 3400 RPM).
