Harley-Davidson SOFTAIL STREET BOB (107 CUI) – FXBB Service Guide: Oil Level & Tire Specs

Engineering Highlight: The 2018 introduction of the FXBB Street Bob represented a pivotal moment in Harley-Davidson’s engineering history. It was the first time the iconic ‘Street Bob’ nameplate migrated from the Dyna family to the redesigned Softail chassis. This move resulted in a frame that was 34% stiffer and significantly lighter, effectively merging the raw, bobber aesthetic with modern handling dynamics previously unattainable in the cruiser segment.

Technical Review: The Milwaukee-Eight Powerhouse

At the heart of the FXBB is the Milwaukee-Eight 107 V-Twin. Unlike the Twin Cam predecessors, the M8 features a four-valve cylinder head design and dual spark plugs, optimizing flame propagation and thermal efficiency. The counter-balanced engine provides a smooth idle while retaining the characteristic Harley pulse under load. From a technical perspective, the integration of the monoshock rear suspension hidden beneath the seat allows for a classic hardtail look while providing modern damping characteristics.

Maintenance Guide & Vital Fluid Specifications

Maintaining the FXBB requires precision, particularly regarding the oiling system of the Milwaukee-Eight engine. The 107 CUI powerplant utilizes a dry-sump system that demands high-quality 20W50 lubricant. We recommend H-D 360 or Syn3, with a total capacity of 4.7 Liters. Ensuring the oil level is checked when the engine is at operating temperature is critical for an accurate reading.

Ignition and Electrical System

The FXBB utilizes high-performance 10R12X spark plugs. To maintain optimal combustion and prevent misfires, the electrode gap must be precisely set between 0.76 and 0.84 mm. For the electrical system, the 66000174 (65989-97D) battery provides the necessary cold-cranking amps (CCA) to turn over the high-compression V-twin, even in adverse conditions.

Chassis and Contact Patch

Handling precision on the Street Bob is highly sensitive to tire condition and pressure. The factory-specified 100/90B19 front and 150/80B16 rear tires must be maintained at 2.5 bar (36 psi) and 2.8 bar (40 psi) respectively. Under-inflation leads to sluggish turn-in and increased heat buildup, which can compromise the structural integrity of the sidewall during heavy lean angles.

MotoSpecsLab Verdict

The FXBB Street Bob 107 is a masterclass in minimalist design backed by sophisticated engineering. For the owner-mechanic, it offers a relatively accessible maintenance platform, provided that the technician adheres strictly to the torque specs and fluid requirements outlined in this guide. It remains one of the most balanced Softails in the modern H-D lineup.