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Years ago I bought a set of SLX M675 disk brakes. At some point I changed the callipers to the newer M7100 and continued to use the old levers without issue.
I've just picked up a second hand pair of M7100 levers to change to and notice that they didn't come with flare nuts (looking and Shimano's exploded diagram, these aren't shown so they possibly come with the hose).
Can I simply use the flare nuts I'm using with the M765 levers? A quick Google shows spares packs with what looks like 2 lengths of flare nuts. My hose is BH90-SM.
Check the exploded diagram for your lever model on Shimano TechDocs
I’ve just picked up a second hand pair of M7100 levers to change to and notice that they didn’t come with flare nuts
I got caught out by that when putting my Shigura set up together. Pretty sure my BL-M8100 levers used the black ones. I suspect the extra length in that picture could be important but if you only have the silver ones I’d give them a gentle try making sure the thread matched and they tightened up before bottoming out
As long as they reach the correct torque to compress the olive before bottoming out you can swap those ones.
Some new road ones use a different thread size.
The old flare nuts seem to be fine (the M7100s are later versions of the SLX M765).
Out of interest, I've always just tightened them up firmly with the spanner (I don't have a crow's foot attachment for my torque wrench so there's an element of guesswork). Should I be looking to bottom them out?
Hell no, sounds like what you have always done works fine so I’d stick with that