Makes sense. I cleaned it out and inspected with a light in a dark room. I see no evidence of corrosion or a broken spring–though it could be broken in ways I can’t see. With a press I have closed it up to 6.728, so there is about 40 thou to go. I decided it would make sense to use a tube and press directly on the two halves of the spring housing, because I’m not confident that the cross pin that apparently holds the shaft in place could not be sheared. Am also wondering if there is a possibility of compressing it too much and buckling the spring (?).

It also seems that whenever there is a prop strike with one of these motors, there is a need for inspection of the shock setup.

Does anybody know if the rubber hubs in props continue to provide protection, or does the rubber harden with age and lock them up?

Not sure what you are saying here. Under what circumstances will the props "start to slip?"

Just to wind up this thread with a conclusion, It was not hard to compress the shock unit to 6 11/16 in. It seemed safer to press on the spring housing directly rather than through the shaft, so I used a 1/2" pipe nipple and a couple of grade 8 washers like this:

Will make sure there is some end play when I put it back together.

Appreciate all the expert guidance on this. Now, the only question in my mind is whether, in the event of a prop strike, the unit will expand back out again….