have an alpha one outdrive s# 1623 8950 -01 on a mercruiser 3.0,, when I pulled my l/u I only found half an o-ring on the shaft, I've herd it's not neccesary to replace the o-ring, but I do wish to, any one know what size I need? My loacal mech did not, and had nothign in stock.. I plan to go...