Yup. I was about to dig those out. They tell a tale. From memory that one had about 40K km but it is very dependent on how the bike is used and the environment it is used in.
One of the common mistakes people make is if they pull the swingarm early for a check they will find the grease the bearing came with from the factory and maybe a small smear of something else. This is entirely inadequate because of the nature of the seals on the bearings.
The bearings Guzzi use are a very common taper roller bearing. The thing is though they are an RS, (Rubber seal) type bearing that has said seal around the perimeter of the outer edge of the wide end of the cone. This, when installed in the frame, is designed seal out water and grime and seal in the grease.
The problem is that it doesn't do it very well and water can and will get past that seal. If the bearing isn't stuffed full
of grease then the water will get in and, as can be seen from Paul's picture above, rust will begin to form.
Other things of note:
If you look closely at Paul's second picture above you can see the second seal that is meant to keep the grease in and water out. It sits on a register in the frame at the 'Narrow' end of the cone of the bearing. If you look carefully you can see that the 'Flat' side of the seal which I would always consider the 'Outer' side or 'Dry' side in fact faces inward
towards the bearing and grease! It has always been thus and in my opinion is utterly wrong! The whole way a seal performs its function is turned on its head by this fitment so I suspect it's a mistake that has never been rectified by the factory! That being the case I always remove the seals and replace them the other way round when performing the greasing task. It helps if, after cleaning the register thoroughly, you glue the seal in with Stag or similar to prevent especially the right one being pushed of its register when you push the swingarm spindle back through.
Now the bearings themselves are, as I have said, a very common 'Free Grandfather clock with every half dozen' type bearing. The issue is their seal on the wide end of the cone. Now others may have more luck but I have searched high and low for an easily available aftermarket replacement but have failed to find one. Oh yes, there are plenty of similar bearings with a seal. The problem is all the ones I've found the seal sits proud of the face of the bearing and if you try and use these they prevent the swingarm slipping over the bearings and frame! The good news is the bearings from Guzzi are not outrageously priced.
When packing the bearings don't stint on the grease. Use a 'Waterproof' marine type grease and pack it in bigly and fatly! The idea is to leave as little air as possible within the bearing cavity to be displaced by water that may leak past the seal. Once you sqodge, (A technical term!
) the bearing into the race in the frame you can wipe off excess that is forced out but don't stint on the grease in the cavity.
Finally, do this early! The earlier you get to it the less chance there is of your bearings ending up looking like the ones in the pictures Mark has posted above. Removing the outer races from the frame is an absolute bastard of a job best avoided if possible. If you simply repeat the greasing process every four or five years, (Or 50,000km.) they will probably last forever. The same is true of the bearings in the shock linkage, although I'd do them more often. Obviously if you live in Atlantis or one of those awful places where it rains 24/7, (Hello Seattle!) it behoves you to be more diligent with the greasing and do it more often.