I took a look in the AMEE book and it stated that the Fox big frame .40 and .45 engines were first produced in 1975. Fox made a bushed and ball bearing version at the time. Unfortunately the book didn't detail any crankshaft changes or differences though. I know the bushed engine crankshaft is slightly different from the ball bearing version. But I don't remember them changing anything else over the years with the crankshaft.