Hi,
What is the minimum thickness allowed on the disc's on Cooper S front brakes?
That's the 7.5 inch ones.
Mine are at about 6mm.

I thought it was about 9mm. Anyone else?

8mm (5/16") unless marked otherwise. They are 3/8" thick when new.
Some newer ones are marked 8.5 limit.

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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.

My old Scientic Publications manual has the following :


from the diagram

2.292 - 2.078 = 0.214

Using 1 inch = 25.4 mm

0.214 = 5.4356 mm

So if "about 6mm" I'd be looking to replace...

That looks like a Cooper 997/998 rotor, they were only 6.35m (1/4") thick when NEW.
I would never run S ones below 8.0mm machining, or 7mm wear limit.

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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.

Hey Doc,

I've never seen anyone use a brake rotor to destruction. Though it sounds like you have, what's the results of using them below the thickness? I always assumed they wouldtend to distort a little to easily by then.

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SooperDooperMiniCooperExpertEngineering

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

Checked the book and is dealing with both Cooper & S - and makes no distinction between them....and no identification in diagrams ..... so very possibly true

None of the other "books of words" give any specs....

Remember seeing someone at Minifest 91 trying to sell some old 7" rotors that were so thin you could almost see through them.... he refused not sell them - so someone bought them in the interests of safety to stop some unsuspecting person using them...

One problem with indersize rotors is the pads can fall out on da road...
As my mate with the Fiat 125 found out.
Other problem is they fade sooner as there is less thermal mass.

The general rule with most small rotors I've found is the machining limit is around 1.5mm less than new thickness.
Obviously you can then wear a bit off them with the pads- but once they need it done again they are stuffed. It's illegal for brake shops to machine them again if worn below the limit.

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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.

Oh so only the obvious stuff like you mentioned, I thought maybe they failed in some other way but I couldn't guess what.....

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SooperDooperMiniCooperExpertEngineering

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

On my old ford escort, I used to get brake fade fairly easily. The discs where the only thing that hadn't been replaced, I ditched them and the extra meat in the new rotors made a big difference.

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One should never skimp on the zip ties.

I have rotors that are on the 8mm limit sitting on my shelf. They are mighty weak looking compared to the new rotors I have on my car.

Rotors are relatively cheap, why would you risk it?

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Lillee - 1969 Morris Mini K

Thanks for the info guys.
I will fit some new rotors.
Are standard rotors all the same,
Or are there some inferior ones out there?

I've found even the RDA ones (made in China) are OK, mine stop fine with EBC Greenstuff pads. For road use any brand is fine.
If you are going racing it might pay to pay more $$ and buy crossdrilled or slotted,.

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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.

OK, stupid question: why are they still on your shelf then? Why not chuck em?

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"Life is not like a box of chocolates... it's more like a jar of jalapenos. Whatever you do today, may burn your arse tomorrow."

Nobody likes a hoarder