V-Twin Mfg Billet Rear Master Cylinder Kelsey Hayes 5/8 Bore for 1979-1984 Harley-Davidson FL FX XL

V-Twin Mfg Billet Rear Master Cylinder Kelsey Hayes 5/8 Bore for 1979-1984 Harley-Davidson FL FX XL

For riders upgrading to aftermarket multi-piston calipers on a Shovelhead build, pedal feel lives or dies with the master cylinder behind it. This V-Twin Mfg reproduction delivers the precise, consistent hydraulic response that makes a high-performance brake setup feel like it belongs — not like an afterthought bolted to a vintage frame.

Key Features

• Reproduction of the original Kelsey Hayes design machined from billet aluminum for tighter tolerances and longer service life than cast alternatives
• 5/8″ bore delivers the hydraulic volume required to properly actuate multi-piston aftermarket calipers without a spongy pedal feel
• 1/8-27 NPT inlet is compatible with standard hard-line brake fittings
• Correct 5/16″ mounting holes on 2.25″ centers bolt directly to OEM bracket spacing without modification
• Replaces OEM part numbers 42453-80A and 42454-80A

Technical Specifications

• Bore Size: 5/8″
• Inlet Size: 1/8-27 NPT
• Mounting Hole Diameter: 5/16″
• Mounting Hole Spacing: 2.25″ on center
• Length: 4″
• Width: 1.5″
• Height: 3.125″
• Construction: Chrome-plated billet aluminum
• OEM Cross-Reference: 42453-80A, 42454-80A

Fitment Notes

• All 1979 fitments apply to late production only; early 1979 models using a Wagner-style offset mount are not compatible
• 5/8″ bore is required for multi-piston aftermarket calipers; use a 3/4″ bore unit for stock single-piston calipers
• Review the Fitment tab for the complete year and model breakdown