Coupler, Coupling, and Drive Saver Inspection
or "Don't Beat on Finely Machined Parts! "
How to kill a coupler/coupling:
- Beat on the machined faces while you're working on them
- Improperly seated key where the shaft fits in
- Snap off the set bolts while trying to break them loose
How to kill a drive saver:
- Hit stuff. It's what they're for.
If you bought a new shaft or did significant work to it, or if the shaft came out of the coupler really easily once you took out the setscrews, you need your coupler re-machined and/or faced.
If you are a machinist, face it by chucking it up in the lathe (with the shaft inserted) and micing the surface for square, re-facing it if necessary. Again, if you don’t understand, have a machine shop do this, it’s easy to mess up. If someone is selling you a new engine, coupler, or shaft, make sure the coupler gets faced or you won't get your engine aligned properly.
Check the key at both ends of the shaft for hammering, the neat line halfway up the key on both sides that indicates the shaft twists in the coupler or prop just a tiny bit every time it’s engaged. That means the key is loose, replace it. In fact, key stock is cheap, so replace the key if there’s and denting, gouges, or undue wear visible, or if you’re just not sure. When you put the key back in file it lightly to fit cleanly in the slots. When the coupler is put back on, it will need to be aligned, more on this later. Why not clean it up and paint it (only on the outside) while you have it out?
Coupling on engine –
If this is covered with ATF or oil, get a mechanic; you have a leaking rear main seal. Otherwise, just make sure that it is free of rust and debris before you try to align the engine. Do NOT sand it or gouge it, this is finely machined part and if you change the surface or create dips you will never get your engine aligned.
Drive Savers –
A drive saver is an optional part, a plastic or composite disk that fits between the engine coupling and the shaft coupler. It is designed to break if the prop catches on something solid, like a cable or a cargo net. As a bonus, a drive saver can sometimes work as a kind of cheat to get by some irresolvable alignment issues. Align the engine first as close as you can, then install these. There’s no way to align an engine with a drive saver in place. If you’re adding a new one, make sure your prop doesn’t hit the rudder now that it’s an inch or so further aft. You can get away with quite a lot of poor alignment, up to .012, as opposed to the .004 error margin without them, but they are not magic. They will not prevent shaft wear from a badly aligned strut or stern tube, and may actually make it worse in some cases.
Next we will look at the alignment
If you haven't checked your stern tube and stuffing box , check it out now
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