The 5000 P/C will fit on any of the 50/51/55 saws, the problem comes when switching top ends between the mentioned Husqvarna saws. Service Bulletins state there was a change made in the crankcase opening in late 1989, the older model saws had a smaller opening and you could not install the 51/55 top ends without modifications. From personal experience I can say that the change was actually made in either 1986 or 1987, I installed a 51 top end on a 1985 that required modification and on a 1987 that did not. Any way it goes you can make the top end fit with a Dremel and a few minutes work, you can use a 51/55 head gasket as a template to see how far you will need to grind the opening out.