I recently purchased a new set of rotors & drive flanges (for 7.5” brakes), I have tapped the drive flanges on lightly but they won’t seat nicely on the face of the rotor.
It would appear that the base of the wheel stud (ie the non-threaded section) has an interference fit with the holes in the drive flange preventing the drive flange from seating parallel, is this normal & do you need to seat these with a heavy tap into place or do you think I might have to relieve the holes in the back of the drive flange?

Sounds dodgy, I've never seen this problem in the past. Got a pic?

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

does sound weird, they're normally a nice neat drop on fit - how do they compare with your old ones?

are the old rotors and flanges a neat fit?
do the old flanges sit nicely over the new rotors?
do the new flanges sit nicely over the old rotors?

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did I tell you that I won a trophy?

In the last 12 months or so i have put together a few disc brake kits for people and all of them have had this problem.

I don't really like it, but with some persuasion, tightening the screws, some more persuasion etc you should be able to get it all seated. You can tell as soon as you place the flange on the top of the wheel studs that they don't line up very well.

I have changed from 8.4's to 7.5's so can't make the comparision

As mini_mad_matt mentions, it's obvious that something is not right when the flange first goes on over the wheels studs, it's as if all the studs have a slighty smaller PCD that the holes on the drive flange.

Problem slightly exagerated, ie the drive flange is only just pressed on. It will go on further with a light tap, but still sits 1-2mm off the face on a similar angle

That's pretty ordinary. The load from the drive train and from the disc is transmitted through the interference fit of the studs onto the disc. If the holes are oversized it will start to flog, and it will also transmit load unequally onto the studs.

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SooperDooperMiniCooperExpertEngineering

All garage work involves equal measures of enthusiasm, ingenuity and a fair degree of irresponsibility.

yeah, what mick said, if the head of the wheel nut is tapered, it will sit the stud in the centre of the hole, so there is some play if the nuts are not tensioned evenly and tight.

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I'd be getting a vernier caliper and measuring the PCD of those studs. They should be 101.6mm (4"). If much more or less, they can break in service as the nuts will force them to 4" PCD at the wheel.
Also, measure the PCD of the holes in the drive flange.
Where did you buy them? It is unusual for the rotors to be dodgy (even Chinese made RDA ones I once had were OK).

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

PCD of the drive flange measures 101.6, & between 101.2 & 101.4 for the studs on the rotor.
Looks like the issue is the base of the studs, the non threaded section measures 9.9mm & the tread at 9.44mm causing the interference.

Doc, they came with a job lot of new & used parts I bought recently, they did however MiniS..... part number stickers on the wrapping. The edge of the rotor is stamped BDC.

That PCD is not far out enough to cause any problems except fitment, I would drill the drive flanges just enough to clear.
Can do them accurately here on the mill for nix if you care to drop in.

[edit] Or you can spend 5 mins with a round file, there is hardly anything to come out.

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DrMini- 1970 wasaMatic 1360, Mk1S crank, 86.6HP (ATW) =~125 @ crank, 45 Dellorto (38 chokes), RE282 sprint cam, 1.5 rockers, 11.0:1 C/R.

Doc, we need someone exactly like you down here in Mexico, have you considered moving down here, please, :-/ :-/ :-/

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Doc, thanks for the offer, will try the file first.
cheers, josh