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On my Kross Level 10.0 MTB (2x10) used mostly on dirt roads and pavement I find I never use 1-10 and seldom go below 15th but I need more top end speed. Wondering if the Ultegra would be a straight swap out?

Thanks!

Unfortunately no, the Ultegra crankset is not compatible, nor is any other road Shimano crank designed for a Hollowtech II bottom bracket (which is what you bike has).

The issue is that there are MTB and Road versions of Hollowtech BBs. MTB ones fit in 68 or 73mm shells, using 2 2.5mm spacers on 68mm ones. Road BBs only fit in 68mm shells. MTB BBs are therefore 5mm wider than road ones. Road crank axles are 5mm shorter than MTB cranks, and are only long enough for road BBs.

If you have a 68mm BB shell you could put a road BB in it and fit a road crank but the chainline would be way off.

You could look for larger aftermarket chainrings in the Shimano asymmetric 4-bolt pattern, but you have to deal with chainring clearance issues Andrew Henle described in his answer.

Probably not.

Per How do I calculate the diameter of a chainring from the number of teeth? the difference in diameter of the 46-tooth chainring over the original 36-tooth chainring is going to be about 10 * 12.7 mm / 3.14159 or about 40 mm, so the difference in radius will be 20 mm, or about 0.8 inches.

That means your front derailleur would need to be quite a bit higher than it is now, so much so that the hanger probably won't be high enough, assuming a braze-on attachment.

So it probably won't work because of that, but you can measure how much space you have available to move the front derailleur higher to get a better idea, especially if you can measure how much space there is between the derailleur and the current crankset.

If, for example, there's 7 mm between the front derailleur and the current chainring, and there's only 5 mm of space left in the braze-on's slot to move the front derailleur higher, there won't be enough space for a chainring that has a 20 mm larger radius.

Also, the larger diameter of the inner chain ring might wind up hitting the chainstay. How much space is there?

All that does assume an identical chainline, too. If the R8000 crank places the chainrings farther out or closer in, that has to be accounted for, too. That can be really important for clearing the chainstay.