| Spec Parameter | Data Value |
|---|---|
| Production Years | 2018 – 2021 |
| Engine Type | Milwaukee-Eight 107 V-Twin |
| Oil Capacity | 4.3 L (4.5 US qt) |
| Oil Grade | SAE 20W50 |
| Front Tire | MT90B16 72H |
| Rear Tire | 150/80B16 77H |
| Spark Plugs | HD-6R10 |
| Battery OEM | 66000207 |
Engineering Highlight: The Rebirth of the Mono-Shock Chassis
The 2018 introduction of the FLSL Softail Slim marked a radical departure from Harley-Davidson’s traditional design philosophy. While it retained the ‘bobber’ aesthetic of the post-WWII era, the engineering under the skin was entirely modern. The 107 CUI Milwaukee-Eight engine was integrated into a completely redesigned Softail frame that utilized a hidden under-seat mono-shock. This innovation allowed for a 34% increase in stiffness and a significant weight reduction of approximately 35 pounds compared to the previous generation, effectively merging the raw agility of the defunct Dyna line with the classic lines of the Softail.
Technical Performance & Handling
The Milwaukee-Eight 107 engine provides a refined power delivery, characterized by dual counter-balancers that eliminate primary vibrations while retaining the visceral ‘thumping’ character owners expect. With a 107-cubic inch displacement, the FLSL offers linear torque across the rev range, making it exceptionally capable in both urban environments and highway cruising. The reduced lean angle, a byproduct of its low-slung profile, is compensated for by the improved suspension geometry which provides more precise feedback than its predecessors.
Maintenance Guide & Essential Specs
To maintain the high-performance standards of the FLSL, rigorous adherence to the service schedule is mandatory. Below are the critical maintenance specifications for the 2018-2021 models:
- Lubrication: The system requires 4.3 liters of SAE 20W50. We recommend synthetic fluids for better thermal stability in the air/oil-cooled Milwaukee-Eight engine.
- Tire Maintenance: Proper inflation is critical for handling. The front MT90B16 72H should be maintained at 2.48 bar (36 psi), and the rear 150/80B16 77H at 2.76 bar (40 psi).
- Electrical: The model uses the high-cranking 66000207 battery. Ensure terminals are clean and torqued to prevent vibration-induced starting issues.
- Ignition: Use HD-6R10 spark plugs. Check gaps every 5,000 miles to ensure optimal combustion efficiency.
MotoSpecsLab Editorial Verdict
The Softail Slim (FLSL) 107 is an engineering masterclass in balancing heritage with modern performance. It is a ‘rider’s bike’ that demands respect for its technical simplicity and rewards the owner with a pure mechanical connection. For those looking to maintain the value of this modern classic, following the 107 CUI specific maintenance protocols is non-negotiable.
Frequently Asked Questions
What is the oil capacity for the FLSL 107?
The total oil capacity is 4.3 Liters (4.5 US Quarts) using SAE 20W50 weight oil.
What are the factory tire pressures?
The front tire requires 36 psi (2.48 bar) and the rear requires 40 psi (2.76 bar).
Which spark plugs are recommended?
The factory-recommended spark plugs are the HD-6R10 models.