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HollowTech road BB is 68 mm wide and fits a road crank without spacers. Hollow tech mountain BB uses up to 3 x 2.5 mm spacers. Hence crank is designed to be used with 75.5 mm wide bottom bracket including the spacers.

Question is if there are issues using a crank with 7.5 mm spacers in a 68 mm (road) bottom bracket? Does it affect load on bearings? Does it affect tightening of the crank?

Use case is quick change from 2x compact road crankset to 1x MTB and back without need to touch to bottom bracket. Front derailleur is already known to work without problems in both combinations.

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    Use case is quick change from 2x compact road crankset to 1x mtb and back without need to touch to bottom bracket. Front derailleur is already known to work without problems in both combinations. – mkpaa Jun 19 '18 at 0:36
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My guess is that thus setup could work. There should be no problems with the bearings, as moving the cranks slightly outboard is equivalent to moving the riders shoes slightly outboard in terms of forces and leverage.

However: are crank spindle spacers available? I guess plain washers with the correct inner diameter could be used. They would have contact only the bearing inner races though.

You;d also have to check what chainline you get with the MTB crank.

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