Re: 1971 135hp Mercury outboard lower unit
The center splined piece is bonded to the rubber and this whole assy is pressed into the prop with special tools. It's designed to 'give' when striking an underwater object, in order to preserve the drivetrain. If the rubber gets old/deteriorated and slips, it can allow the prop to wobble or move a bit and cause damage. Combine this with the wrong thrust washer and the potential for damage is high.
Very important to use the washer that's specified for the particular prop. Later-style props will use a different washer than the older ones. The newer-style uses a flat washer, whereas the older-style prop will use a thrust hub with a shoulder that fits into the prop body.
And there's no question that a later-style inline lower unit will fit your motor. I had a '79 Inline Six lower unit on my 1350; it looked good and worked great. '79 and newer had the re-worked gearcase with swept-back skeg and additional design features to prevent blowout at higher speeds. An excellent upgrade on an older motor.
One caveat: if your old lower unit's driveshaft uses an O-ring at the splined end, and the replacement doesn't have the O-ring groove, you'll want to grind a groove. This can be done easily with a Dremel or any old thin cutoff/grinding wheel mounted on an arbor in your portable drill motor. This O-ring seal is critical to keeping water out of the crankshaft splines (which will cause nasty rust). Later models use a special O-ring carrier assy pressed into the end of the crankshaft, and the O-ring just stays up there in the carrier. Thus the lack of a groove. Not a big deal but something to be aware of.
HTH..........ed
