On Sun, 30 Aug 2009 19:54:16 +0100, Calvin Sambrook wrote:

Having spent a "fun" weekend totally stripping down and rebuilding my 20
year old touring bike because the bottom bracket seized... and then I
noticed the wheel axels were stiff... and then I noticed the headset was
tight... I was wondering why grease turns into something resembling glue
after a couple of decades. The bearings were all sitting in quite "tacky"
grease about the consistency of impact adhesive.

The grease I was using to re-lube was Castrol LM, a lithium grease, which
comes in a tub with a cover over the grease with a hole in the middle. You
push the cover down to get some grease up through the hole. I had always
assumed that was simply to keep the unused grease clean but I saw that the
older smears around the top part of the tub (it's at least a decade old!)
looked very similar to the stuff on my bike, no longer flexible and very
tacky whereas the product under the cover was fine so I guess it's something
to do with reacting to air.

Anyone know what's going on?

At a guess, the oil evaporates from the soap. I bought quite a lot of
Campag. Record hubs and pedals in the '80s and always stripped and rebuilt
them as the Campag. grease was like cold lard.
Some of the MTB greases are quite good - more gel-based.
--
Peter.
The head of a pin will hold more angels if
it's been flattened with an angel-grinder.