ARP 156-5201 Main Cap Side Bolt Kit Ford 4.6L/5.4L Modular Early Cast Iron Block

Vehicle Fitment

• 1991-2005 Lincoln Town Car (4.6L SOHC 2V Romeo Cast Iron Block)
• 1992-2005 Ford Crown Victoria / Police Interceptor (4.6L SOHC 2V Romeo Cast Iron Block)
• 1992-2005 Mercury Grand Marquis (4.6L SOHC 2V Romeo Cast Iron Block)
• 1993-1998 Lincoln Mark VIII (4.6L DOHC 4V Romeo Cast Iron Block)
• 1994-1997 Ford Thunderbird (4.6L SOHC 2V Romeo Cast Iron Block)
• 1994-1997 Mercury Cougar (4.6L SOHC 2V Romeo Cast Iron Block)
• 1996-2004 Ford Mustang GT (4.6L SOHC 2V Romeo Cast Iron Block - Romeo-built years only; see Fitment Notes)
• 1996-2004 Ford Mustang Cobra / SVT (4.6L DOHC 4V Romeo Cast Iron Block)
• 2003-2004 Ford Mustang Mach 1 (4.6L DOHC 4V Romeo Cast Iron Block)
• Compatible with any performance build using an early Ford Modular 4.6L or 5.4L Romeo-casting cast iron block (M8 side bolt pattern)

Fitment Notes

• Designed for early cast iron M8 Romeo production engine blocks ONLY - side bolts only, not a complete main stud kit
• Does NOT fit Windsor cast iron blocks (M8 Windsor blocks use a different main cap side bolt pattern)
• Does NOT fit aluminum blocks (1996-2004 Mustang Cobra Teksid/aluminum or 2005+ aluminum blocks) - see ARP 156-5001 for early aluminum M8 blocks
• Does NOT fit late cast iron M9 blocks (1999-2004 SVT Lightning, 2002-2003 Harley-Davidson F-150) - see ARP 156-5202
• 1996-2004 Mustang GT engines may be Romeo or Windsor depending on model year and production date - verify block casting before ordering
• Not compatible with windage tray mounting applications

Features

• Made from 8740 chromoly steel, heat-treated in-house to 200,000 psi tensile strength - far superior to OEM side bolts
• Precision J-form threads rolled after heat treatment for up to 2,000% stronger thread fatigue life than conventionally manufactured bolts
• 12-point head design allows easy socket access in tight main cap locations
• Eliminates main cap walk and fretting - a common failure point in high-boost and high-RPM Modular engine builds
• Protects the block's cast iron thread engagement zones from the stress of repeated assembly and disassembly
• Hardened parallel-ground washers included for even load distribution and accurate torque readings

Specifications

• Part Number: 156-5201
• Material: 8740 Chromoly Steel
• Tensile Strength: 200,000 psi
• Thread Size: M8
• Head Style: 12-Point
• Main Bolt Style: 2-Bolt Side Cap
• Finish: Black Oxide
• Block Type: Early Cast Iron Romeo (M8 thread pattern)
• Works with Windage Tray: No
• Nuts Included: Yes (12-Point)
• Washers Included: Yes (Hardened Parallel-Ground)
• Quantity: Complete kit (sold as a set)

Included Components

• 1x Complete set of ARP M8 12-Point Main Cap Side Bolts
• 1x Set of 12-Point Nuts
• 1x Set of Hardened Parallel-Ground Washers

Product Description

The ARP 156-5201 is a Pro Series main cap side bolt kit designed specifically for early Ford Modular 4.6L and 5.4L engines built with the Romeo cast iron block using M8 main cap side bolt threads. These blocks were used from 1991 through approximately 2005 in vehicles including the Lincoln Town Car, Crown Victoria, Grand Marquis, Thunderbird, and Mustang GT/Cobra. The factory main cap side bolts on these engines are torque-to-yield hardware - they stretch upon installation and must be discarded during any rebuild or performance assembly.

This ARP kit replaces them with 8740 chromoly bolts rated at 200,000 psi, with precision J-form threads rolled after heat treatment for exceptional fatigue life. Eliminating main cap walk and fretting is critical in any high-power Modular build, as movement at the main caps can quickly damage block threads and compromise bearing clearances. This kit covers the side bolts only - verify your block casting is Romeo (M8) before ordering, as Windsor, aluminum, and late cast iron M9 blocks require different ARP part numbers.