It it safe to grind an 8.4" disc down to use with 7.5" calipers? I am wanting to go back to 10" wheels but don't want to have to buy 7.5" rotors and drive flanges as cash is tight at the moment.

It would be safe to do so, but won't work / fit with Cooper S calipers. You will need to use Cooper S Rotor and Drive Flange.

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I've wondered this as well. Is it the offset of the disc which means the calipers cannot be swapped?

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SooperDooperMiniCooperExpertEngineering

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Mainly the position of the caliper bolted to the hub . If you make the disc smaller then the caliper will have to be shifted in 1/2 of the overall diameter the disc was reduced or change the caliper

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No, not the offset, it's the diameter of the (bad memory) the tubualar part of the Disc Rotor. The Cooper S Calipers sit in closer to the centre of the hub.

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The discs (8.4 vs. 7.5) sit at different 'depths'. The 7.5 disc sits much closer to the inside of the car compared to the 8.4.

I looked at doing this a while ago. I'm sure it would be possible if you put enough time into it and modified everything but at the moment you can buy a full set of new 7.5's locally for under $1000. If I had the facilities to mod everything myself I probably would but having to nut things out, take it to a machinist, try to convey what you actually want done, hope he gets it right and then pay for it it's probably not worth it...