Re: Mercury Six Cylinder Lower Unit Interchange
It seals against the inner diameter of the crankshaft when the driveshaft is inserted, in order to keep water out of the splines. Especially if you're running in salt water, if you don't have this O-ring the crank and driveshaft will rust together and you'll need small incendiary devices to pry them apart!
Later models did away with the O-ring groove and instead have a stainless-steel carrier assy pressed into the end of the crankshaft. This assy holds an O-ring which seals against the smooth body of the driveshaft to accomplish the same thing as an O-ring in driveshaft groove.
I imagine the O-ring in the carrier stays in there better and is less likely to deteriorate. Probably does a better job of sealing in the long run.
At any rate, you could always pull your powerhead and press the later-style O-ring carrier assy in place. Or just grind an O-ring groove in the later-style driveshaft, a lot less effort unless you already have the powerhead off.
When I rebuild my old 1350 and installed a later-style lower unit at the same time, I pressed the assy into the crankshaft and it worked great.
HTH.................ed
