King Engine Bearings MB1337XP0.25-4 XP Series Main Bearing Set Undersize - 0.25mm

The King Engine Bearings MB1337XP0.25-4 is an XP Series (Xtreme Performance) crankshaft main bearing set in 0.25mm undersize, designed for 4-cylinder engine applications requiring reground or undersized crankshaft journals. This set is engineered for high-performance street, road racing, and circle track builds where sustained high loads and elevated RPM are a constant demand.

King's XP Series bearings are built around a tri-metal copper/lead construction featuring a nickel barrier layer and a lead/tin/copper Babbitt overlay bonded to a high-strength steel backing via King's proprietary SecureBond adhesion process. This layered structure delivers exceptional stability and durability under extreme operating conditions, and results in a 24% increase in load capacity compared to conventional tri-metal race bearings.

The XP Series also incorporates King's pMax Black™ overlay — a hardened performance coating that significantly improves resistance to wear, surface fatigue, and oil starvation events without altering the bearing's finished wall dimensions.

Key Features:

• XP Series Tri-Metal Construction: Copper/lead lining with nickel barrier and hardened Babbitt overlay over steel backing
• pMax Black™ Coating: 24% harder than conventional tri-metal race bearings for superior load capacity and wear resistance
• SecureBond Adhesion: Proprietary process ensures superior bonding between bearing layers for long-term durability
• Bull's Eye Tolerance: Wall thickness tolerance held to an industry-leading ±0.0001 in. for precise, consistent fitment
• Narrowed Design: Mains are narrowed to accommodate large-radius crankshafts common in performance applications
• Enlarged, Chamfered Oil Holes: Promotes improved oiling and oil film formation across the journal
• 0.25mm Undersize: Designed for use with crankshafts ground 0.25mm undersize (.010 in.)
• 4-Cylinder Application: Set configured for inline 4-cylinder engines (4 bearing positions)

King Engine Bearings has manufactured engine bearings exclusively since 1960, applying motorsports-derived technology to every product in their lineup. The XP Series main bearings are the preferred choice of professional engine builders for endurance racing, high-boost street performance, and any application demanding consistent oil film and reliable bearing retention under load.