I have taken the jib halyard winch off it's base, and also had a good close examination of the mast where it mounts. The only "interesting" thing I can find is that the base plate, which is really just a stand-off for the winch, was originally drilled incorrectly when it was made; all six main screw holes were drilled low by 1/2". They then plugged these holes and re-drilled them, with the new hole slightly cutting into the plugs. The wood has dried etc as it has aged, and the glue for the plugs has failed, so they basically all fell out. This may have had some impact on the ability of the big bronze screws to work vertically, but I doubt it was a huge impact.
Two of the screws were seriously bent (had to cut them out), these were obviously the ones that held on till the last. Otherwise the screws all appear to be in perfect order.
It really does just look like time, load, and endless "wiggling".
At this stage I will probably over-drill the existing holes, plug them with hardwood dowels epoxied in, and then re-drill and screw. I was mad keen on using stainless steel machine screw inserts, which would give a bigger load-bearing area and not be prone to working the wood as much, but can't find anything above an M8 and I need M10. Will keep looking for now though.
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