Can a DA BR-7900 rear caliper be mounted on the front, or will things blow to pieces? Any problem apart from needing a different length mounting bolt? Thanks.

(Brake shoes will have to be swapped for correct direction, obviously)

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Bikes: Raw Ti, 650b flatbar CX

It should work in theory, if you can find a longer bolt for the front fork. But make sure you can find one before taking anything to bits. I think the only thing making the front and rear caliper different is the length of the bolt. Best of luck!

On three frames I tried this and the thickness of the front of the fork crown was sufficient that there wasn't enough thread exposed behind it to screw a mounting bolt onto it. Even when I milled a nut down to the right length so it would fit snugly, it simply didn't have enough threads to hold onto. You may have different results with a different fork, but this was the same problem on one steel fork and two carbon ones.

Check the shimano tech docs, but I'm pretty sure they would be identical apart from the long bolt and associated washers. And brake pad direction, switch them around!