Back in the 70s K81s were what was fitted and everyone stuck by them, but it also depend on where the tyres were made if they were made in England they had a softer compound, handled pretty good but wore out quicker, if they were made in Spain they were a bit harder compound and lasted longer but not as good grip, then they were made in Japan, they lasted as long as the Spain made ones and handled a bit better, but they are a old design now and need to get with modren tyre, the Avon Roadriders are a good tyre, they last as long as the made in Japan K81s but handle a lot better and grip really well in the wet and handle great in the dry, I push them a lot harder than the old design of the 70s, I ever run them on my 2013 Thruxton and can get them right over to the edge of the tyre without losing it and I do the same with them on my Norton, I can't do that with the old K81s without some grip lost and the front tyre wears a bit funny after they been on for some time, no problems with the Roadriders.
Ashley