Bicycle Mechanics - How frequent do Shimano bb's go bad

Last fall I suspected something might be wrong but the LBS owner said "those bottom brackets" rarely go bad. Yeah, should have listened to my intuition. Today I was on a 100 mile ride and had to cut it short because of what felt like the bb going bad. A fair amount of "clunking" or grinding feeling coming through the crank arms and pedals. Got it home, cleaned off the bike and slowly turned the cranks on the workstand and something definitely feels right. The bike is a 97 Bianchi Trofeo and 6-7 years ago the bike shop replaced the original Miche bottom bracket with a sealed Shimano unit.

So, how often do they need to be replaced?

As with all mechanical things, it depends. It depends on what kind of riding you do, like whether you ride in the rain or not, or whether you have the correct bike washing technique. Nevertheless, Shimano sealed cartridge-type bottom brackets (I'm assuming this is a square taper) are pretty hard to kill, especially the good ones that are Ultegra/XT level or above. Your LBS owner was right in that generally speaking, those Shimano sealed cartridge BBs rarely go bad as I have a Dura Ace unit that's seen better than 10k and still spins smoothly.

slowly turned the cranks on the workstand and something definitely feels right...

What do you mean by "feels right"?

I assume the bottom bracket is smooth, yet you have some clunking and grinding? Perhaps it's the pedals and not the BB. Are the crank arms tight?

What do you mean by "feels right"?

I assume the bottom bracket is smooth, yet you have some clunking and grinding? Perhaps it's the pedals and not the BB. Are the crank arms tight?

Sorry, meant does not feel right. The bottom bracket is not smooth. With my hand spinning the cranks it feels rough at both the pedal and on the crank arm itself. Crank arms are snug.

Today was the first ride I've done in rain. Usually only on dry pavement because I know if it rains here it'll be sunny again in a day or two.

Nothing lasts forever but Shimano square taper and even Octalink bottom brackets are very durable. I've gotten well over 25,000 miles on both types with no problems. That said, nothing last forever and it might indeed be the bb bearings are failing. BTW, did you remove the chain from the chainrings while you were testing the crank? You have to be sure you are checking only the bb bearings, not them plus the chain plus the rear wheel, etc.

That said, nothing last forever and it might indeed be the bb bearings are failing. BTW, did you remove the chain from the chainrings while you were testing the crank? You have to be sure you are checking only the bb bearings, not them plus the chain plus the rear wheel, etc.

Just pulled the cranks. There is no side to side or up and down play in the spindle. Spinning the axle I can feel a slight catch in the bearings. Time to put it back together and swap pedals and see if I still feel it. If it's still there it's off to the shop for another bb.