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Harley-Davidson Fat Boy (2018-2021) — Maintenance Specs

The 2018 Fat Boy (FLFB) was rebooted on the all-new Softail chassis with the Milwaukee-Eight 107 (114 in the FLFBS), solid cast-aluminum disc wheels, 240 mm rear tire and LED lighting. It shares the M8's 4.73 L oil-tank fill, 0.95 L transmission and 1.18 L primary chaincase with the rest of the 2018+ Softail line - a completely different fluid picture from the pre-2018 Twin Cam Softail Fat Boy (FLSTF), which took just 2.8 L of oil; this entry deliberately covers only the 2018-2021 FLFB. Note the owner's manual omits the M8's spark plug part number, publishing only the 10 mm size, 0.80-0.90 mm gap and 9.5-12.2 N·m torque. The 30,000-mile/two-year plug interval and DOT 4 brake fluid follow the standard H-D schedule.

Quick specs

Oil capacity (with filter) 4.73 Loil tank with filter (5.0 US qt); refill initially 3.78 L then top up 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Oil type Harley-Davidson H-D 360 (Screamin' Eagle SYN3 full synthetic acceptable) 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Oil viscosity SAE 20W-50 (H-D 360 preferred; SYN3 acceptable) 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Front tire pressure (solo) 36 psi248 kPa cold 68u00b0F/20u00b0C 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Rear tire pressure (solo) 42 psi290 kPa cold 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Spark plug gap 0.80-0.90 mmMilwaukee-Eight, 10 mm 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Fuel capacity 18.9 L5.0 US gal (FLFB) 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)

Maintenance schedule

Specification Value Source
Engine oil & filter 1,000 mi first service, then every 5,000 mi (8,000 km) 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Spark plug replacement every 2 years or 30,000 mi (48,000 km) 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Brake fluid change every 2 years 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)

Fluids & capacities

Specification Value Source
Oil capacity (with filter) 4.73 Loil tank with filter (5.0 US qt); refill initially 3.78 L then top up 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Oil type Harley-Davidson H-D 360 (Screamin' Eagle SYN3 full synthetic acceptable) 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Oil viscosity SAE 20W-50 (H-D 360 preferred; SYN3 acceptable) 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Final drive oil transmission 0.95 L (1.0 qt); primary chaincase 1.18 L (1.25 qt) Lapproximate 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Brake fluid type (DOT) DOT 4 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Fuel capacity 18.9 L5.0 US gal (FLFB) 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)

Torque specs

Specification Value Source
Spark plug 9.5-12.2 Nu00b7m84-108 in-lbs 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)

Electrical

Specification Value Source
Battery 12V, 17.5 Ah, 315 CCA, sealed maintenance-free 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Headlight bulb LED standard (halogen sealed-beam variants on some trims) 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Spark plug gap 0.80-0.90 mmMilwaukee-Eight, 10 mm 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)

Tires & wheels

Specification Value Source
Front tire size 160/60R18, Michelin Scorcher 11 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Rear tire size 240/40R18, Michelin Scorcher 11 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Front tire pressure (solo) 36 psi248 kPa cold 68u00b0F/20u00b0C 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)
Rear tire pressure (solo) 42 psi290 kPa cold 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal)

Official manual

Owner's manual on Harley-Davidson official site

Opens the manufacturer’s official website in a new tab.

Frequently asked questions

How much oil does a 2018+ Fat Boy take?
4.73 L (5.0 US qt) with the filter on the Milwaukee-Eight 107 - refill starts with 3.78 L and tops to level, per the official manual.
What is the Fat Boy tire pressure?
36 psi front / 42 psi rear (248/290 kPa) on the 18-inch fitment - note the higher rear figure vs most Harleys.
What oil does a Fat Boy use?
H-D 360 SAE 20W-50 or Screamin' Eagle SYN3.
What battery does an FLFB use?
A sealed 12V 17.5 Ah unit rated 315 CCA.

Sources

Value Source Checked
Oil capacity (with filter): 4.73 L 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal) (p. Specifications; Capacities Table 10) 2026-11-03
Oil type: Harley-Davidson H-D 360 (Screamin' Eagle SYN3 full synthetic acceptable) 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal) (p. Engine Lubrication, oil recommendation chart) 2026-11-03
Oil viscosity: SAE 20W-50 (H-D 360 preferred; SYN3 acceptable) 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal) (p. Specifications; Capacities Table 10) 2026-11-03
Final drive oil: transmission 0.95 L (1.0 qt); primary chaincase 1.18 L (1.25 qt) L 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal) (p. Specifications; Capacities Table 10) 2026-11-03
Brake fluid type (DOT): DOT 4 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal) (p. Specifications; Capacities Table 10) 2026-11-03
Fuel capacity: 18.9 L 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal) (p. Specifications; Capacities Table 10) 2026-11-03
Engine oil & filter: 1,000 mi first service, then every 5,000 mi (8,000 km) 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal) (p. Specifications; Capacities Table 10) 2026-11-03
Spark plug replacement: every 2 years or 30,000 mi (48,000 km) 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal) (p. Specifications; Capacities Table 10) 2026-11-03
Brake fluid change: every 2 years 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal) (p. Specifications; Capacities Table 10) 2026-11-03
Spark plug: 9.5-12.2 Nu00b7m 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal) (p. Specifications; Capacities Table 10) 2026-11-03
Battery: 12V, 17.5 Ah, 315 CCA, sealed maintenance-free 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal) (p. Specifications; Capacities Table 10) 2026-11-03
Headlight bulb: LED standard (halogen sealed-beam variants on some trims) 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal) (p. Specifications; Capacities Table 10) 2026-11-03
Spark plug gap: 0.80-0.90 mm 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal) (p. Specifications; Capacities Table 10) 2026-11-03
Front tire size: 160/60R18, Michelin Scorcher 11 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal) (p. Specifications; Capacities Table 10) 2026-11-03
Rear tire size: 240/40R18, Michelin Scorcher 11 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal) (p. Specifications; Capacities Table 10) 2026-11-03
Front tire pressure (solo): 36 psi 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal) (p. Specifications; Capacities Table 10) 2026-11-03
Rear tire pressure (solo): 42 psi 2019 Harley-Davidson Owner's Manual: Softail Models, 94000764 (H-D Service Information Portal) (p. Specifications; Capacities Table 10) 2026-11-03
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