So this is a unique situation I'm sure!

I am running Mustang GT calipers with 300M rotors. The clearance is very tight at the control arm where the ball joint is. So tight that to make them fit, the control arms were beaten with a BFH

Even so, the control arm grinds a groove into the back of the rotor.

It's about time for a brake job and I want to get the right spacing there. Any suggestions besides "trash it for a different brake kit"?

Relative to the centreline of the caliper (lengthwise, between the pads), is the rotor near the back (i.e. 'inboard') or is it fairly evenly centred. If it's 'inboard' (very possible given the description) then you can by a thin wheel spacer (3mm? 5mm?) and stick it behind the rotors and then use otherwise as is. Ideally you want the rotor to be fairly centred in the caliper. If it's already fairly centred, depending on the way the pad bracket/caliper mount is setup, if it mounts on outside (wheel side, not car side) of the knuckle then you could add a washer/spacer (3mm, for example) between the bracket and the knuckle and then add an equivalent spacer behind the rotor and it would shift everything out.

Hopefully that makes sense. I can try and draw some crude pictures later if not.

Thanks, Salomon!

What you described is just like I envisioned. I was hoping nobody would say that it's wrong or dangerous to do such a thing
I will take a peek at how it centers up and hopefully just the simple wheel spacer will take care of it.

No problem. I can't see any issue with this at all and I've seen it done before. Just make sure you are still getting adequate lugnut engagement with the spacer installed. Depending on what size you use I would imagine you may need longer wheel studs, but I doubt you would with only 3mm, just make sure to verify!

So here is what I'm dealing with. The rotor is very outboard (as pictured) before adding a wheel spacer behind the rotor.
I bought a 5mm wheel spacer and obviously it won't work with just the spacer as the rotor will interfere with the pad bracket (the red line).

The plan now is to add the 5mm wheel spacer and also 5mm washers (the green lines) and use longer bolts.
The lower green line is the outside of the adapter bracket that connects the pad bracket to the steering knuckle. Hopefully the adapter bracket won't then interfere with the caliper body!

Is it safe to shim this here and run longer bolts? I think it's similar to what you were suggesting, Salomon.

This is pretty much exactly what I was suggesting. I don't see it being an issue, especially under normally driving conditions. Spacing things out does increase leverage but it should be fairly negligible for such a small increase. Even for people who punish their cars I would say it's a non-issue.

Just make sure your caliper bracket bolts and wheel studs are long enough for proper engagement and you're golden. With regards to the wheel studs, the 5mm shouldn't be enough of a change to need longer studs, but you will probably want to buy 5mm caliper bracket bolts like you mentioned.

Cool, thanks again for easing my mind!

Lug nut thread engagement must be >= lug diameter, correct?

No worries. This ranks pretty much right at the very bottom on my scale of 'should I be worried about driving after doing this".

As for the thread engagement, you've got it. In our case you'll want at least 12mm of engagement as we run M12x1.5mm studs. Theoretically this would require 8 full turns (12mm/1.5 = 8) at minimum to get the necessary engagement. If you've got an open-ended lug then it's pretty obvious if you've got proper engagement but if not then just mark one side of your lug(s) and count the turns.

Going to need extended studs. Lugs were 7 to 8 turns before the spacer and now I'm down to 5

I also found that I have a hubcentric ring for the rotors. It doesn't meet them anymore. Can I just shimmy the ring out 5mm so that the spaced out rotor is contacting it? Or do I need to add another ring?

And I'm left with 3mm of hub sticking out for the rim centre. Is that enough?

Everything is good!
Thank you my friends!

Glad you got it sorted out. What did you do about the hubcentric ring?

I just shimmied it out most of the way, put the rotor on and then used a flathead to tap it back in line with the rotor. I can’t imagine how it could move on its own.

Once the rotor is tightened down, it no longer matters anyways...