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JCW GarageInterested in John Cooper Works (JCW) parts for your 1st Generation MINI? This is where JCW upgrades and accessories for the Cooper (R50), Cabrio (R52), and Cooper S (R53) MINIs are discussed.
Looks like a coupling nut. Designed to put in a bolt from both ends. Take one of the bolts (or one the same size) to a good hardware store and ask for a coupling nut for bolts or "all thread". They should know what you are asking about.
OR, just buy a new intercooler that is clean and may cool and move more air than the 10 year old intercooler you are playing with now.