The MMX Performance 90mm billet coolant expansion cap for BMWs has been designed from the ground up in the UK, from a solid piece of billet aluminium.
Each cap was designed to directly replace the OE cap, not merely stuck in place. Fitment is for models with the matching OE (17 11 7 639 021) part number.
Each MMX cap starts life as a solid billet of 6061 aerospace-grade aluminium. It's meticulously machined from this billet with the latest CNC 5-axis technology, before being polished, then gloss-anodised to stay beautiful year after year.

All MMX coolant caps arrive pre-sealed and pre-installed with an OE spec valve and internal spring. No need to remove the valve assembly from your OE cap. A true plug and play solution.

It has been designed to instantly give your BMW an under-bonnet boost in the aesthetic department, finished in a deep gloss black anodised finish.

Never again will you worry whether you tightened your expansion cap fully, as the cap contains complete OE replacement internals. The carefully considered design gives ample surface grip area for your fingers, making installation and filling your coolant that little bit easier.

It joins an existing range of engine dress-up products for a complete under-bonnet transformation to your engine bay.

Feature heavy:
¡¤ Designed and engineered in the UK

¡¤ Crafted from 6061 aerospace-grade aluminium

¡¤ Full OE cap replacement

¡¤ Sophisticated internal springing and bleed off

¡¤ Anodised in MMX signature gloss black

¡¤ Joins existing range of Yahua Billet caps

¡¤ Pre-installed factory spec Viton O-Ring

Please check the underside of your factory expansion cap for the OE part number. It¡¯s a larger cap, measuring roughly 90mm in diameter. The Yahua Billet coolant expansion cap for BMW fits OE part number 17 11 7 639 021.

Please note if the part is under excessive heat for prolonged periods of time, it can affect the overall finish, which is unavoidable with anodised aluminium products.