Was topping up and forgot to pt the cap back on, I know, stupid.

Need to get the right size from the mech, but he wants to know what size before getting it in.

It's a standard 76 clubbie, any clues?

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Clocked a faster hill climb than a WRX in a 998cc clubman!

As far as i Know there are only two diameters - you need the larger one

13psi

Most minis had a "long neck" as standard but many have been changed to the more common short neck over the years. If you're ordering by phone you'll need to measure from the top of the filler down to the lip the cap seals against inside the neck and get your mechanic to measure the cap.

Got it, thanks

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Clocked a faster hill climb than a WRX in a 998cc clubman!

It's not that simple-
there are 2 different lengths, the Oz radiators have a longer neck than the Pommy ones.
For Australian made CPC caps-
Oz (long) p/no is 517/13
Pommy (short) p/no is 508/13, fits the UK 2 core and 3 core.

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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.

Thats the rule of thumb... but... I sent a Mini K radiator away once to have a new core fitted and it came back all shiney and new looking - with a short neck filler

Well there's always the exception...
If you start blowing welch plugs or hoses out, you have the long where you need the short.

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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.