I'm thinking of replacing the old three piece BB on one of my bikes
with a Shimano cartridge bearing and I see that Shimano markets a
UN-26 and UN-55 cartridge bottom bracket. I believe that the UN-26
has a solid spindle and a plastic bushing on the non-drive side while
there is reference to a hollow spindle and steel bushing on the UN-55.
Both are made for the 69 and 73 mm frames and in roughly the same
spindle lengths.

In normal use is there a practical difference? Disregarding the weight
difference, if any, is there a noticeable difference in service life?

The UN-26 seems to be the cheaper version and is quite common in shops
here while the UN-55 is somewhat rare, probably due to price. But if
the UN-55 lasts (for example) five years while the UN-26 only lasts
two years it is probably worth searching out the higher priced
version.
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Cheers,

John B.