What is a 6-bolt engine?

A 6-bolt engine is one that has a 6-bolt flywheel - that is, there are 6 bolts holding the flywheel to the crankshaft. A 7-bolt motor has 7 bolts there.

Along with the flywheel change, there are many other internal changes between the two engines. The crankshafts are different sizes, as are the rods and crankshaft seals. 6-bolt motors have 'big' rods, while 7-bolt engines have 'small' rods. The journal and bearing widths different, although the bearing diameters are the same.

There may also be other changes between the two engines, and the parts are not interchangeable.