In article , says...

I was b being sarcastic for humourous purposes. Of course tthey do but i do
wonder if they make the right guesses about which bits fail.

For this sort of thing, you can estimate a mean time between failure
(which might be wildly off for purpose built aerospace hardware, but you
do the best you can). Plus you can ask the question "how critical is
this unit?" to come up with a ranking of "most important" to "least
important" equipment. Combine that, and other, data and you can come up
with a "wish list" of spares you'd like to have on board ISS. Since up-
mass to ISS is limited, some things lower down on the wish list have
fewer spares. For non-critical equipment (i.e. an experiment), there
may be no spares on ISS.

Some equipment may be too expensive, large, and etc. to make spares in
the first place. Things like the hulls of the pressurized modules would
fall into this category. Get a hole in something like an MPLM and
you've either got to patch it, or shut all the hatches to it and figure
out where to go from there.

How are the rotary race rings and bearings doing these days?

No idea. I've seen nothing about them in the on-line "space media"
since the EVA to clean and lube them.

Jeff
--
"the perennial claim that hypersonic airbreathing propulsion would
magically make space launch cheaper is nonsense -- LOX is much cheaper
than advanced airbreathing engines, and so are the tanks to put it in
and the extra thrust to carry it." - Henry Spencer