I recently picked up an '85 (I think) Rocky Mountain Giro road bike which I've taken apart. When I went to install a different BB, I found that the diameter of this shell was slightly bigger than normal. I've never seen this before. Has anyone had a similar experience?

If by some chance that the frame was built in Italy it might have an Italian threaded bottom bracket shell. What is the width of the bottom bracket shell? If it is 70 mm that would be a clue

Maybe it had a Ashtabula one piece crank in it originally. Those are way bigger shells IIRC. But it's been over a dozen years since I had anything with a Ashtabula crank?

What did you take out of it? Or was it with no cranks?

Looks like it's an Italian BB shell on a Canadian frame, and the dropouts are Campy, guess I just got confused. That happens a lot.

Some mountain bike BB shells are 73mm vs the 70mm IT or 68mm BSA

What is the inside diameter of the shell? Are there any markings on the bottom bracket cups or cartridge that would indicate thread spec? Italian thread is 36mm x 24tpi; English thread is slightly smaller at 1.37" (34.8mm) x 24tpi.

And some Japanese built, English thread frames have 70mm shells, and some Italian made, Italian thread frames have 65mm shells. Bottom line: shell width is an unreliable indicator of thread spec.

Good reason to pick up an inexpensive Vernier caliper for the home mechanic.

When you removed the old bottom bracket was the cup right hand threaded on the drive side? If so, along with the shell being too big for the replacement cups you would have a good indication that the shell is Italian threaded

The cynic in me says there can't be any non-standard BBs. If you took a bear's claw and stuck it through a piece of metal, it would surely be within tolerance for at least one of the many BB standards!