The last time I had the KZ head off.... maybe 4500-5000 miles ago.... I plastigauge checked my cam bearings and they were all still well within specs.
Even after I replaced that galled intake camshaft, you might recall it was the lobe and shim that was screwed up, not the bearing journal, but the clearances were great even with that other used camshaft!!
That would be at least 35,000-37,000 miles on those cam bearings then... plus the 4-5 thousand miles since .
Since I bought this bike NEW and took ownership in February or March of 1977....I have always been religious about oil and filter changes over the years, picking 2000 to 2500 miles as my oil and filter change targets.
I really think that helped.
But even at that.... 40,000-50,000 miles is not really that long for babbitt style bearings with proper maintenance.
2016 Triumph T120 Bonneville
Far North East Metro Denver Colorado
In my case there was a oil pressure problem caused by the welding. I corrected it but too late. The valve train doesn't sing like they did and oil pressure is down a little. The bearings are almost $10 a piece now. The cams are still in spec and I just ordered a whole set of bearings for $30 shipped. Came out of a KZ 1100 with 22,000 miles on it. www.z1enterprises.com/cam-bearing-z1-kz900-1000-2.html
I changed mine in 13 and god only knows how many miles were on them. The ones shown in the clip are them. I should be able to make up a good set with the new ones. I'll consider myself lucky if this is all it is.
I was in a hurry to get to my daughter's place so we could drive to Stockton pick up my granddaughter and drive back in her car the next morning at 4 AM. About 800 miles. I didn't want her to drive all that way alone. I'm going there tomorrow to fix a bad oil leak before the next trip a few weeks from now. Her man is an electrical engineer, not a mechanic. He works his ass off for his family and this is my contribution to the her and the family.