The MMX Performance BMW Billet Coolant Expansion Cap is a precision-engineered replacement designed to upgrade both the reliability and appearance of your BMW’s cooling system. Developed as a true OE-style replacement—not a cosmetic cover—this cap directly replaces the factory plastic unit while retaining full factory functionality.
Designed and engineered in the UK, each MMX billet coolant expansion cap is CNC-machined from a solid block of 6061 aerospace-grade aluminum, delivering exceptional strength, heat resistance, and long-term durability. The cap is finished in a deep gloss black anodizing that enhances under-bonnet aesthetics while providing excellent corrosion protection over years of use.
Unlike decorative alternatives, the BMW coolant expansion cap features pre-installed OE-spec internals, including the factory-style pressure valve, internal spring, and Viton O-ring seal. This ensures correct pressure regulation and reliable sealing without the need to transfer components from your original cap—making installation truly plug-and-play.
The thoughtfully machined exterior of the MMX Performance BMW Billet Coolant Expansion Cap provides improved grip, allowing for confident tightening and removal during servicing. Once installed, it eliminates the uncertainty associated with aging plastic caps while delivering a clean OEM+ appearance that complements other billet engine bay upgrades.
Available in 60mm and 90mm fitments, this billet coolant expansion cap replaces BMW OE part numbers 17117639024 (60mm) and 17117639021 (90mm). Please verify the underside of your factory cap to confirm correct sizing prior to purchase. Caps are sold individually.
An ideal upgrade for enthusiasts seeking durability, functionality, and refined aesthetics, the MMX billet coolant expansion cap is the perfect finishing touch for a properly detailed engine bay.
Key Features & Benefits:
CNC-machined 6061 billet aluminum construction
Direct OE replacement with full factory-style internals
Pre-installed pressure valve, spring, and Viton O-ring
Gloss black anodized finish for long-term durability
Improved grip for easier installation and removal
Available in 60mm and 90mm BMW fitments