As the picture says, is this hard rubber part replaceable and what is the part number and where can I get one. Is is loose in the tube and rattles and does not seal well.

Do a Google search on "soften old hard rubber" and you'll find a lot of hits showing methods to soften old brittle rubber items on motor bikes, cars, etc. Basically it involves the use of a mixture that includes wintergreen oil.

To answer your question, I'm not aware that the rubber part is available separately.

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ex-NSW Police 1970 MK II Cooper S
VMCI #43

Methyl salicylate which is the smelly stuff in sports rubs ie deep heat, metsal etc

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1970 Mk2 Cooper S under restoration

you can try fitting a spring to it to hold it down, just a bit fiddly getting the spring off every time you check your oil

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No offence intended here but--> anyone writing a book about minis 30 years ago may not have experienced such worn or stuffed-with components as we are finding these days.

You should put your heart & soul into everything you do.

Maybe try soaking it in alcohol? It may cause it to swell...
And give you an excuse to open a bottle of scotch...

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67 Deluxe
Dry suspension
1293cc, 11:1 comp, RE 282 camshaft, 45 Weber, extractors etc
4 synchro close ratio gbox
Cooper S brakes, no booster
Stealth box, as Doc says "goes harder and idles!"
RH tank cos I needed it crossing the Nullarbor etc..