The coupler is a rubber filled cup that mounts to the flywheel end of the
engine and has a splined socket to mate with the shaft that goes to the
lover unit.

It will not be possible to replace the coupler without pulling the engine.
(Pulling the engine itself is about a 15 minute job if you have a overhead
hoist.) The first step is to remove the outdrive. There is a bell housing
around the flywheel that will prevent you from getting access to the nuts
that secure the coupler to the end of the crankshaft. You must pull the
engine, remove the bell housing and then remove the nuts holding the
coupler. The nuts are usually stuck and are easily rounded off, so to save
yourself a lot of trouble I'd suggest that you purchase a Snap-On 6 point
long handle boxed end wrench for the job, it will be expensive but the
hassle is saves is worth the cost of the wrench many times over.

With a 1973 boat it would be common for the wood where the engine mounts to
be soft and the lag screws holding the motor mounts may have worked loose.
The reason the coupler has "gone bad" is that with a boat that age things
may have settled and are no longer in proper alignment. There is a gimble
bearing in the transom gimble mount that the drive shaft passes through and
that shaft goes into the coupler. If either the transom has sagged or the
motor bunks are soft then the shaft, gimble bearing and coupler will not be
in proper alignment. This puts a lot of pressure and wear on the coupler
until it gives up.

A somewhat expensive alignment shaft tool must be used to adjust the motor
mounts to align the coupler and the gimble bearing.

Depending on your skill level, the cost of the wrench and trying to borrow
an alignment bar, plus an overhead hoist, you may find it more economical to
hire it done by a marine mechanic. Or you can certainly do the job but
purchasing a service manual would be wise.

Don Dando