MMR Performance Aluminium 90mm Coolant Expansion/Header Tank Cap for BMW and MINI with OE 17 11 7 639 021 / 17117639021

Elevate your engine bay with this premium alloy radiator header tank lid from MMR Performance. Precision‑machined in the UK from a single piece of aluminium for a robust, high‑quality finish, this 90 mm reservoir top is a direct‑fit replacement for the factory unit on selected BMW and MINI applications using the correct OE reference.

Unlike flimsy stick‑on covers, this is a complete replacement reservoir cap that installs in place of the standard item for an authentic, OEM‑style fit and a cleaner performance look.

Verified compatibility: vehicles fitted with OE reference 17 11 7 639 021 (also written 17117639021). Please cross‑check your existing tank lid before ordering.

• Solid aluminium construction for long‑term confidence and a premium feel
• Direct fit on cars using OE 17 11 7 639 021 — no adapters, no covers
• 90 mm diameter design for the correct seating on the expansion system
• UK‑designed quality from MMR Performance with a tidy, motorsport‑inspired finish

We run this part on our own builds and it’s been a favourite with our customers for its superb fit, reassuring quality and slick engine‑bay upgrade. If you’re searching for a radiator reservoir cap, expansion tank lid, engine coolant bottle top, water tank cover, alloy header tank cap, performance reservoir lid or 90 mm coolant tank cap for BMW/MINI models using the above OE reference, this is the solution.

Part number (OE reference): 17 11 7 639 021 / 17117639021

Need it installed? Book professional fitting with MJ Modifications in Warwickshire. Request a quote: info@mjmodifications.co.uk.