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I had to do the exactly same job several years back. I believe we have the same engine/gearbox (Universal 5432 & Hurth HBW 100). I replaced ours with a new gearbox and put in a new damper plate at the same time. The job is not difficult but does require some planning due to the short-coupling of the 37's engine and prop shaft, requiring lifting the back of the engine to clear the shaft flange and remove the gearbox. As best I remember, the steps went something like this:
1) Disconnect prop shaft flange from gearbox, slide shaft aft slightly. Disconnect exhaust hose from waterlift. Remove top nuts from aft motor mounts, loosen top nuts on forward motor mounts.
2) Lift aft end of engine enough to clear prop shaft when removing gearbox- as I remember this was about 5". (*see below)
3) Remove bell housing bolts and pull bell housing/gearbox combination off engine. You will have to slide it aft about 2" to get the gearbox spline out of the vibration damper on the engine. The 4 bolts that secure the gearbox to the bell housing are inside the bell housing.
4) Once the bell housing is off, you can swap the new gearbox for the old.
5) Before replacing the bell housing and gearbox, replace the vibration damper. It is bolted to the aft face of the engine flywheel. It was good that I planned to replace ours, as it had several broken springs.
6) Replace bell housing/gearbox- you may have to wiggle it a bit to get the gearbox spline started into the spline on the vibration damper. Once seated replace bolts.
7) Lower engine onto aft mounts, slide prop shaft forward to gearbox flange, and reconnect exhaust hose to waterlift. Realign engine, good alignment is very important to get max life out of your new gearbox.
8) Reconnect gearbox linkage being sure that when actuated by shift lever that the lever on the gearbox has the proper throw. The Hurth manual supplied with the new gearbox describes this.
9) Add oil to gearbox, go sailing. Recheck engine alignment after several hours of use.
* To lift the aft end of my engine I used a turnbuckle attached to the aft lift eye on the engine. I secured the top end of the turnbuckle with some chain to a hook. The hook was placed in the opening in the cockpit front made by temporarily removing the instrument panel. A jack under the engine may be another way of doing this.
Hopefully I did not make this sound tougher than it is. As I remember it took me about 4 hours.
Crealock 37 #151