| Component | Specification |
|---|---|
| Engine | Milwaukee-Eight™ 117 V-Twin |
| Oil Type | 20W50 HD Genuine 360 / SYN3 (4.7 L) |
| Front Tire | 160/60R18 70V (36 psi) |
| Rear Tire | 240/40R18 79V (42 psi) |
| Battery | 66000207 (65989-97E) |
| Spark Plug | HD-6R10 (Gap: 0.76–0.89 mm) |
Engineering Highlight: While the Fat Boy is legendary for its ‘Steamroller’ aesthetic, the 2025 FLFB/25 marks a significant engineering milestone as the first standard production Fat Boy to fully integrate the 117 cubic inch Milwaukee-Eight powertrain as the base configuration. This shift necessitates a refined approach to thermal management, utilizing a high-performance oil cooling system to manage the increased caloric output of the 1923cc displacement.
Powertrain & Lubrication Analysis
The Milwaukee-Eight 117 is a precision-engineered beast that demands strict adherence to lubrication standards. At MotoSpecsLab, we emphasize the use of Harley-Davidson Genuine 360 or Screamin’ Eagle SYN3. With a total capacity of 4.7 liters, the system is designed to provide high-pressure lubrication to the rocker arms and hydraulic lifters while maintaining thermal stability during low-speed cruising—a common challenge for large-displacement air/oil-cooled V-twins.
Chassis & Tire Dynamics
The defining characteristic of the FLFB/25 is its massive 240mm rear tire. To maintain the intended handling geometry, tire pressures must be monitored with surgical precision. Running the 240/40R18 rear lower than the recommended 2.90 bar (42 psi) can lead to ‘heavy’ turn-in and premature cupping of the tread. Conversely, the 160mm front tire requires 2.48 bar (36 psi) to ensure the Lakester cast aluminum wheels track straight under heavy braking loads.
Electrical & Ignition Maintenance
The 2025 model utilizes the high-output 66000207 (65989-97E) battery to provide the high cold-cranking amps (CCA) required to turn over the high-compression 117 engine. For ignition, the HD-6R10 plugs are calibrated for the specific combustion chamber pressure of this variant. Technicians must ensure a precise gap of 0.76–0.89 mm to prevent misfires and ensure optimal fuel atomization under the Heavy Breather intake’s high-flow conditions.
Maintenance FAQ
How often should I check the spark plug gap?
While HD-6R10 plugs are durable, MotoSpecsLab recommends inspecting the 0.76–0.89 mm gap every 8,000 km, especially on 117 engines that see frequent high-torque demand.
Can I use automotive 20W50 oil?
No. Motorcycle-specific oil like SYN3 contains friction modifiers and shear-stable polymers necessary for the shared primary/transmission environment (if applicable) and high-heat air-cooled cylinders.