Hi, I have a 1975 Mercury 850 outboard with a 15 spline prop shaft. Will a Honda 15 spline prop fit the Mercury? It looks like a good fit but when installed with the old propshaft thrurst washer, the prop nut will start but only thread on about halfway. Is it matter of going to a thinner thrust washer, Keep in mind there is no contact between the prop hub and the lower unit?