Is the Truvativ Noir GXP cank and BB cups compatible with Shimano Hollowtech II?

I measured both to have 24 mm diameter spindle so looks like they are but anyone mixed both e.g. (Shimano BB cups with the Truvativ crank)?

No, they are not compatible AFAIK.

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my plastic bikes, 29ers & stable of old steel

Truvativ/SRAM tapers to 22mm on the NDS.

hmmm, I didn't spot that, 24 to 22 mm should be quite noticeable, a 1 mm step ; best photo I could find on t'web is this:


and another, since we all like pictures, right?


.... and so my confusion continues ....

maybe it is the "lands" on the spindle that are in different places for the two BBs?

why would you be confused? you have been given the correct answer. They are not compatible.

.... because if they are the same spindle diameter, how come they don't fit?

think you are missing the point, they are only the same size on the drive side, the NDS is 22mm on GXP.
http://www.bikyle.com/bottom_brackets.htm

The NDS bearing contacts the splined portion of the spindle, not the smooth section. You can actually see the step in the pictures you've provided on the splines.

As stated repeatedly above, they definitely are not compatible.

ok, sorry if I appear to be non trusting but it is my nature to question whenever I see something but am told something else so I like to understand.

a close look at the chainset indeed showed polishing on the splined section where the ND bearing made contact, not the way I would have designed it but ......

I wonder if making a shim form a stip of 1 mm thick aluminium would work???

GXP BB is like $20-$30...

Hope make an adaptor so that you can use Truvativ / SRAM GXP cranks with their own BB, maybe one of those would fit a Shimano BB as well?

They're incompatible, and that's that. Just buy a new BB, they're cheap. Don't futz around with shims or adapters, get the parts that work.