2018-2021 Harley-Davidson Softail Slim FLSL: Milwaukee-Eight 107 Bobber-Style Softail

The 2018-2021 Harley-Davidson Softail Slim FLSL belongs to the first generation of Milwaukee-Eight Softails, the family that replaced Harley-Davidson’s previous Softail architecture and absorbed much of the Dyna brief in one major 2018 model-year reset. The Slim was the deliberately pared-back member of that family: low solo saddle, floorboards, wide bars, laced 16-inch wheels, chopped fenders, and a visual language aimed at postwar bob-jobs rather than touring equipment or boulevard excess.

Mechanically, the FLSL is important because it joined traditional Harley-Davidson styling to the counterbalanced Milwaukee-Eight 107 engine and the much stiffer 2018-up Softail chassis. It was not the fastest or most heavily equipped Softail, but it is one of the clearest expressions of Harley’s modern retro-cruiser thinking: a factory bobber silhouette with modern injection, ABS availability, a six-speed gearbox, and a hidden rear shock.

Best Known For: the Softail Slim FLSL is best known as the Milwaukee-Eight 107, 2018-up chassis version of Harley-Davidson’s low-slung factory bobber-style Softail Slim.

Quick Facts

The table below gives the essential reference points for identifying the FLSL within the Milwaukee-Eight Softail line. Figures reflect commonly published Harley-Davidson factory specifications for the model family and U.S.-market documentation where applicable.

The FLSL should not be confused with the earlier FLS Twin Cam Softail Slim or the FLSS Softail Slim S. All three share the Slim concept, but the FLSL is the one tied to the new-generation Softail frame and the four-valve Milwaukee-Eight engine.

Why the 2018-2021 Softail Slim FLSL Matters

The FLSL matters because it captured a very specific moment in Harley-Davidson history: the company was trying to modernize the dynamic behavior of its heavyweight cruisers without severing the visual connection to its most commercially valuable past. The Softail Slim was an especially good test of that strategy. Its styling asked for rigid-frame nostalgia, but its engineering relied on a hidden monoshock, a more rigid chassis, modern fuel injection, and a counterbalanced engine.

For buyers and collectors, the interest is not rarity in the prewar sense, nor racing pedigree, nor police or military service. The interest is in the model’s position as a factory-built modern bobber with genuine Harley-Davidson parts-bin legitimacy: FL-style floorboards, big-twin pulse, restrained trim, and the cleaner stance that separated it from the Heritage Classic and Deluxe.

It also represents the final chapter of the Softail Slim nameplate in the U.S. Milwaukee-Eight catalogue. That gives the FLSL a defined production window and a clear identity within a broad Softail family that can otherwise blur together for casual observers.

Historical Context and Development Background

For the 2018 model year, Harley-Davidson undertook one of the most consequential big-twin platform changes in its modern history. The previous Softail line, with its twin hidden shocks and Twin Cam B engine, was replaced by a new Softail chassis built around a stiffer frame, a hidden monoshock, and the counterbalanced Milwaukee-Eight engine. At the same time, the Dyna line disappeared, and several models that had previously occupied Dyna territory were reinterpreted on the Softail platform.

The Softail Slim entered this new generation with a different role from the Street Bob, Fat Bob, Fat Boy, Heritage Classic, and Deluxe. Where the Street Bob was a stripped FX-style custom and the Heritage Classic carried touring nostalgia, the Slim leaned into the low, clean, broad-shouldered FL bobber idea: solo seat, floorboards, modest bodywork, and laced wheels rather than cast-wheel muscle or bagger equipment.

The competitor landscape was also changing. Indian Motorcycle had re-established itself with heavyweight cruisers that traded heavily on American V-twin heritage, while Triumph, BMW, Moto Guzzi, and Japanese manufacturers were using retro design in very different ways. The Softail Slim was Harley-Davidson’s answer in its own language: not a retro exercise built around a small parallel twin, but a heavyweight big twin with traditional proportions and modern drivability.

There is no racing or military specification attached to the FLSL. Its visual references, however, owe much to postwar bobber culture: removing weight, lowering the profile, shortening fenders, and emphasizing the engine and wheels over accessory equipment. The important distinction is that the FLSL is not a military Harley-Davidson and not a period bob-job; it is a factory interpretation of that aesthetic.

Engine and Drivetrain

The FLSL uses the Milwaukee-Eight 107, Harley-Davidson’s 45-degree big-twin introduced before the 2018 Softail range and adapted here with counterbalancing for solid mounting in the chassis. The engine’s major departure from the Twin Cam era is its four-valve cylinder-head architecture, using two intake and two exhaust valves per cylinder, with a single camshaft and hydraulic lifters. The layout preserved the appearance and cadence expected of a Harley big twin while improving breathing, thermal management, and refinement.

Fuel metering is by Electronic Sequential Port Fuel Injection, and ignition is fully electronic. The Softail application uses a counterbalanced Milwaukee-Eight engine rather than the rubber-mounted installation associated with touring models. That distinction is central to the FLSL’s feel: the engine is physically tied to the frame, but internal balancing reduces the shaking that would otherwise make such an installation unpleasant.

Drive runs through the primary drive and clutch to Harley-Davidson’s 6-speed Cruise Drive gearbox, then to a belt final drive. The FLSL’s drivetrain is built for low- and mid-range torque rather than high-rpm theatrics, and Harley-Davidson’s own published torque figure for the 107-equipped Softail Slim is commonly listed at 110 lb-ft at 3000 rpm. Factory horsepower was not consistently published by Harley-Davidson for U.S.-market models, and dyno figures vary with test method, exhaust, intake, tune, and correction standard.

Engine and Drivetrain Specifications

The following specifications are useful when verifying a stock motorcycle, comparing it with related Softails, or assessing whether a modified machine still retains its original mechanical identity.

The engine is one of the strongest reasons the FLSL is a distinct model rather than simply a facelifted earlier Slim. The Milwaukee-Eight makes the bike smoother, more flexible, and mechanically more modern than the Twin Cam FLS while retaining the low-speed cadence and visual mass that buyers expect from a Harley big twin.

Chassis, Suspension, and Braking

The 2018-up Softail frame is the defining chassis feature of the FLSL. Harley-Davidson replaced the earlier hidden twin-shock Softail arrangement with a new frame and a concealed under-seat monoshock, giving the motorcycle the visual line of a rigid-frame custom while substantially improving stiffness and suspension control compared with the outgoing platform.

At the front, the Slim uses a telescopic fork from the new Softail family. The model’s visual balance depends heavily on its 16-inch laced wheels and fat-sidewall tires, which give the bike its stocky stance and soften the otherwise low, spare silhouette. Braking is by single discs front and rear, appropriate to the model’s traditional cruiser brief rather than a performance-cruiser mission.

ABS availability depended on market and equipment. Buyers assessing a specific example should verify the presence of ABS by inspection and documentation rather than assuming all FLSL machines are equipped alike.

Chassis and Equipment

These details are the most useful factory-level chassis and equipment points for distinguishing a stock FLSL from a modified example.

The chassis made the FLSL a better motorcycle than its deliberately old-fashioned appearance suggested. Its low seat and floorboards still imposed cruiser limitations, but the 2018-up frame gave the Slim a more coherent, less hinged feel than earlier Softails when ridden at a brisk but realistic road pace.

Riding Experience and Mechanical Character

The FLSL starts like a modern Harley-Davidson, not a carbureted shovel-era custom. With the fob present, the rider uses the handlebar controls and electric starter; there is no choke ritual, no tickler, and no hand-advance drama. The idle settles into the familiar big-twin cadence, but the Milwaukee-Eight is smoother and less mechanically agricultural than a Twin Cam B or Evolution Softail.

The rider triangle is classic Slim: low saddle, wide bar, floorboards, and a relaxed reach that suits back-road cruising more than sport riding. The floorboards are part of the model’s personality. They give the bike an FL flavor absent from narrower peg-equipped Softails, but they also define the limits of lean angle when the rider begins to use the chassis with intent.

Throttle response is shaped by the Milwaukee-Eight’s torque rather than rev-hungry acceleration. The 107 pulls cleanly from low rpm, gathers pace with a deep push from the crankshaft, and rarely asks to be wrung out. The gearbox has the familiar Harley long-throw precision: positive, heavy enough to feel mechanical, and happiest when shifted with deliberate foot pressure rather than a light sportbike tap.

At low speeds, the Slim’s low center of gravity and broad bar make it less intimidating than its running-order weight suggests. On the open road, the fat 16-inch tires and long wheelbase give stability and a measured steering response. The single front disc requires realistic expectations; it is adequate for the motorcycle’s mission when maintained properly, but the FLSL is not a Fat Bob and not intended to be ridden like one.

Identification and Originality

The most important identifier is the factory model code: FLSL. On a genuine 2018-2021 Softail Slim Milwaukee-Eight, the documentation, VIN record, and motorcycle configuration should align with that model identity. Do not rely on tanks, fenders, wheels, or seats alone, because late-model Harley-Davidsons are frequently customized with take-off parts from other Softails.

A correct FLSL should present as a low, bobber-style Softail with the Milwaukee-Eight 107 engine, laced 16-inch wheels, solo saddle, floorboards, chopped fenders, and the 2018-up Softail frame with hidden rear monoshock. The engine should be the counterbalanced Milwaukee-Eight installation used in the solid-mounted Softail chassis, not the earlier Twin Cam B engine from the FLS or FLSS generation.

Originality concerns on these motorcycles are different from those on prewar or Panhead-era Harleys. The usual issues are not hand-painted pinstripes or casting-number minutiae, but exhaust systems, air cleaners, ECU calibrations, bars, lighting, seats, turn-signal relocation kits, side-mount plates, and suspension changes. A stock exhaust, original air box, factory calibration history, owner’s manual, key fobs, security information, service records, and take-off parts can matter to a careful buyer.

Inspect the frame VIN and engine identification surfaces for consistency with the title and service records, but avoid unsupported decoding claims unless using factory or official registration information. Any evidence of tampering, replacement cases, salvage branding, or mismatched paperwork should be treated seriously. Because the FLSL is modern enough to have electronic modules and emissions-related equipment, originality includes software and compliance parts as much as paint and chrome.

Model Code and Variant Breakdown

The FLSL is best understood alongside the immediately related Slim models. The table below separates the Milwaukee-Eight Softail Slim from the Twin Cam Slim and Slim S, which are often cross-shopped and sometimes confused in listings.

Within the FLSL years, equipment could vary by market, color, security and ABS specification, and accessory installation. Those differences do not create a separate model code in the same way as FLS, FLSS, and FLSL.

Performance and Dimensional Specifications

Harley-Davidson commonly listed the Milwaukee-Eight 107 Softail Slim with 110 lb-ft of torque at 3000 rpm, a 6-speed Cruise Drive transmission, a 5.0-gallon fuel tank, and a running-order weight of 671 lb. Published factory documentation emphasized torque rather than horsepower, and Harley-Davidson did not consistently publish U.S.-market horsepower figures for this model. Independent dynamometer figures should be treated as test results, not factory specifications.

Factory dimensional figures for the FLSL are generally listed around a 64.2-inch wheelbase and a low laden seat height, with 16-inch front and rear laced wheels. Exact detail can vary by market literature and model-year specification sheets, so a buyer should use the owner’s manual, factory service information, and the VIN-specific equipment record when precision matters for restoration or judging.

In real-world terms, the FLSL’s performance is torque-led. It is brisk enough for modern traffic and relaxed highway cruising, but it was never positioned as the aggressive performance member of the Softail line. Riders looking for sharper brakes, inverted-fork attitude, or harder cornering usually gravitate toward the Fat Bob or later performance-oriented Softail variants.

Compared With Related Harley-Davidson Softails

FLSL Softail Slim vs. FLS Twin Cam Softail Slim

The FLSL is smoother, more refined, and built around a much stiffer chassis than the 2012-2017 FLS. The earlier bike has the appeal of the first modern Slim and the Twin Cam B character, but the FLSL is dynamically the more modern motorcycle. Collectors who value the first appearance of the Slim name may like the FLS; riders who want the best version of the concept from the factory often prefer the FLSL.

FLSL Softail Slim vs. FLSS Softail Slim S

The FLSS is attractive because of its factory 110 cu in Twin Cam engine and darker, more aggressive trim. It is the more obviously special variant within the pre-2018 Slim family. The FLSL counters with the new chassis, Milwaukee-Eight refinement, and a cleaner mechanical future for riders planning regular use.

FLSL Softail Slim vs. FLDE Deluxe

The Deluxe shares the 2018-up Softail generation but takes the nostalgia in a different direction: chrome, whitewall flavor, fender trim, and a more dressed visual identity. The Slim is visually leaner and less decorative. For buyers, the choice is usually between show-era flash and bobber restraint.

FLSL Softail Slim vs. FXBB Street Bob

The Street Bob is the stripped FX-style Softail, lighter in attitude and fitted with mid-control ergonomics in many markets rather than the Slim’s FL-style floorboards. The Slim feels lower, broader, and more traditional. The Street Bob is often the better blank canvas for a club-style custom; the Slim is the cleaner factory bobber statement.

FLSL Softail Slim vs. FLHC Heritage Classic

The Heritage Classic adds windshield, bags, and touring utility, while the Slim deliberately avoids that equipment. Mechanically they belong to the same broad generation, but the buying logic is different. The Heritage is for riders who want distance capability in vintage dress; the Slim is for riders who want the look and feel of a pared-back big twin.

Restoration and Ownership Notes

Because the FLSL is a late-model Harley-Davidson, ownership is supported by strong parts availability, extensive dealer knowledge, independent specialist familiarity, and a large accessory market. That is a blessing and a problem. Many examples have been modified, and the abundance of bolt-on parts means a motorcycle can drift far from factory specification without looking obviously abused.

The Milwaukee-Eight 107 is a robust engine when serviced correctly and left within sensible tuning limits. Buyers should be more cautious with heavily modified examples than with stock machines, particularly where aftermarket exhausts, high-flow intakes, cams, big-bore kits, or unknown tuning devices have been installed. A well-documented Stage I machine is very different from a motorcycle carrying mismatched parts and no calibration history.

Inspect the primary drive, clutch action, compensator noise, oil leaks, brake-fluid service history, spoke condition, belt condition, wheel bearings, steering-head bearings, and rear shock adjustment mechanism. On early Milwaukee-Eight motorcycles generally, oiling-system updates and cam-chest work are sometimes discussed in the enthusiast community, especially on modified engines; the correct response is not panic, but careful inspection, service-record review, and verification of any claimed updates.

Restoration difficulty is moderate rather than severe. Returning a customized FLSL to stock can become expensive if the original exhaust, intake, seat, lighting, bars, and emissions equipment are missing. Paint and trim are less obscure than on older limited-production machines, but correct factory finishes, undamaged tins, and unmolested wiring still carry weight with serious buyers.

Buyer and Restoration Inspection Points

A good FLSL inspection should be more specific than a generic used-motorcycle walkaround. The motorcycle is modern enough that documentation, electronics, and modification history are as important as compression and cosmetics.

The best examples are not necessarily the lowest-mileage bikes. A carefully serviced FLSL with tasteful, documented accessories and all stock parts retained is often a better ownership proposition than a very low-mileage motorcycle with stale fluids, aged tires, missing fobs, and uncertain electronic modifications.

Collector and Market Relevance

The FLSL is a collector-relevant modern Harley, but not in the same way as a Knucklehead, EL, XR-750, or limited-production CVO model. Its importance lies in being the final Milwaukee-Eight expression of the Softail Slim idea and one of the most visually disciplined bikes in the 2018-up Softail family. It is a model that appeals to riders who want factory authenticity rather than a custom assembled from catalog parts.

Collectors typically value original paint, clean documents, uncut wiring, stock exhaust and intake parts, undamaged laced wheels, correct solo-seat presentation, and evidence that any tuning work was done properly. High-value modified examples usually have coherent, reversible changes rather than random accessory accumulation. The market generally rewards clarity: stock, documented, well-kept motorcycles are easier to understand and easier to sell.

Exact production numbers for the FLSL are not consistently documented in public factory sources. That means rarity claims should be treated cautiously. The stronger argument for the model is not numerical scarcity, but defined identity: a short-run Milwaukee-Eight Softail Slim with a clear mechanical break from the Twin Cam era.

Cultural Relevance

The Softail Slim FLSL sits within the long Harley-Davidson tradition of making a motorcycle look older than it mechanically is. Its cultural reference point is the postwar bob-job, the stripped big twin built after removing unnecessary bodywork rather than adding touring equipment. Harley-Davidson turned that instinct into a production motorcycle: clean fenders, solo stance, visible engine mass, and a low profile without the compromises of a true rigid frame.

It has no factory racing legacy and no legitimate military-service identity, despite visual cues that some owners associate with utilitarian or wartime-inspired customs. Its cultural importance is instead tied to street custom culture, dealer-floor personalization, and the continuing demand for a heavyweight cruiser that looks simple without being primitive.

The FLSL also marks a point where Harley-Davidson’s cruiser line became more integrated. The Dyna versus Softail divide that shaped generations of enthusiast debate had effectively ended, and models like the Slim had to justify themselves by character, stance, and mechanical clarity rather than by old platform loyalties.

FAQs

What years was the Harley-Davidson Softail Slim FLSL produced?

The Milwaukee-Eight Softail Slim FLSL was produced for the 2018-2021 model years. Earlier Softail Slim models used different model codes and Twin Cam engines.

What engine is in the 2018-2021 Softail Slim FLSL?

The FLSL uses the Milwaukee-Eight 107, a 107 cu in / 1746 cc air/oil-cooled 45-degree V-twin with four valves per cylinder and electronic fuel injection. In the Softail chassis it is a counterbalanced, solid-mounted installation.

Is the FLSL the same as the older FLS Softail Slim?

No. The FLSL is the 2018-up Milwaukee-Eight version built on the new Softail frame with a hidden monoshock. The earlier FLS, sold from 2012-2017, used the Twin Cam 103B engine and the previous Softail chassis.

Did the Softail Slim FLSL come with the 114 engine?

The FLSL Softail Slim is generally identified with the Milwaukee-Eight 107 engine. Other Softail models and some later or special configurations used larger Milwaukee-Eight engines, but the FLSL Slim’s standard identity is the 107 cu in version.

How much torque does the Softail Slim FLSL make?

Harley-Davidson commonly listed the Milwaukee-Eight 107 Softail Slim at 110 lb-ft of torque at 3000 rpm. Factory horsepower was not consistently published for U.S.-market versions, so dyno figures should not be treated as factory ratings.

What are the common issues to inspect on a used FLSL?

Look for undocumented engine modifications, poor tuning, aftermarket exhaust without proper calibration, primary-drive noise, neglected brake fluid, worn or damaged drive belt, corroded spokes, fork-seal leaks, missing stock parts, and inconsistent paperwork. On modified Milwaukee-Eight engines, service records and parts receipts matter.

Is the 2018-2021 Softail Slim FLSL collectible?

It is collectible as a defined modern Harley-Davidson variant rather than as a rare antique. The strongest examples are stock or reversibly modified, well documented, and complete with original parts. Its appeal comes from being the final Milwaukee-Eight Softail Slim and one of the cleanest factory bobber-style big twins of its period.

Collector Takeaway

The 2018-2021 Harley-Davidson Softail Slim FLSL is the motorcycle for buyers who understand that restraint can be a specification. It does not need saddlebags, a fairing, a 114 badge, or a club-style makeover to explain itself. Its value is in the combination of Milwaukee-Eight torque, the 2018-up Softail chassis, and a factory bobber silhouette that stayed unusually faithful to a simple idea.

As a collector machine, the FLSL rewards originality and documentation more than ornament. The motorcycles to keep are the clean, correctly identified examples with stock parts retained, unbutchered wiring, clear records, and modifications that can be reversed without a forensic restoration. In the long view of modern Harley-Davidson history, the Softail Slim FLSL matters because it was one of the last pure expressions of the low, stripped FL-style big twin before the market pushed ever harder toward larger engines, darker performance cruisers, and accessory-heavy touring identity.