My 1991 R100GS recently turned 123,000 miles. I've done some preventative maintenance (rebuilt carbs, replaced cam timing chain and tensioner, replaced front and rear tranny bearings 20k ago (but didn't have shaft machined for circlip). The original driveshaft did make it to almost 122,000 miles, and finally crapped out in central Indiana on homeward trip from Montana. I shipped it off for repair this Sept. to a shop in Green Bank recommended by Oak a few years ago. Repair looks good and runs well.
Re the borrowed time issue .... let me set the stage. The heads have never been off this bike. Compression when measured warm 10k miles ago was approx 140psi on each cylinder. Valves tend to stay in adjustment, and I ck them every 5k miles. Burns no more oil now than ever. Still wants to pull hard from 65-70mph in 5th gear. I realize common wisdom is to pull the heads and have them rebuilt at or before 100,000 miles.
Question - is the valve train in jeopardy of a catastropic (sp?) failure, due to a valve seat breaking off the stem, due to age and wear, or do the valves on higher mileage airheads generally burn out at their seats (non-catastropic)?