Harley-Davidson FLSL Softail Slim (2018-2021) Maintenance Schedule / Guide

This is the maintenance schedule and general guide to servicing the Harley-Davidson Softail Slim (FLSL), powered by the Milwaukee-Eight motor, made from 2018 to 2021, before being replaced by the quite similar but subtly different) Softail Standrd.

The Harley-Davidson FLSL Softail Slim is a laidback everyday bobber-style cruiser based on the updated 2018 Softail chassis. Like the previous Softail Slim, the 2018 Softail Slim has no exposed suspension (giving it a classic “hardtail” look), but the twin shocks of the previous Softail are replaced with a single shock. The 2018 Softail chassis is a huge improvement over the former, giving it more structural rigidity but also reducing weight, helping the Softail Slim be one of the easiest Harley-Davidsons to ride.

The core of the FLSL Softail Slim is the 107 cubic inch Milwaukee-Eight air/oil-cooled 45-degree V-twin. It’s the entry-level M8 engine, but still has a responsive, torquey pull that’s fun at every part of the rev range.

The maintenance schedule is similar to other motorcycles with Milwaukee-Eight motors from the Softail line (e.g. the Harley-Davidson FXLR Low Rider), but of course, some parts and maintenance specs are always different.

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Harley Davidson FLSL Softail Slim Service Intervals

Like other motorcycles with the Milwaukee-Eight engine, the Harley-Davidson FLSL Softail Slim has 5000 mile / 8000 km or annual service intervals. Harley-Davidson recommends you service it more often if it’s ridden under harsh conditions, e.g. if you ride it stop-start at low temperatures, or take your bike on dusty paths.

At every service, change the oil and filter, and look over the motorcycle for items needing lubrication, adjustment, or cleaning. Also, check wearable items (like hoses, tires, or floorboards if you’re an aggressive rider) to see if they’re still in good condition. (See the end for tire specs unique to the Softail Slim.) Also, note that the Softail Slim has spoked rims, so you need to attend to the condition and tension of the spokes.

As the Milwaukee-Eight motor (like most air-cooled Big Twin motors) has self-adjusting hydraulic valves, there is no strict valve service interval. However, Harley-Davidson does say if there’s valve noise, you should see a dealer — so learn to spot it. H-D also suggests you change the spark plugs every 30,000 miles / 48,000 km or two years.

As well as engine oil, you need to periodically replace both primary chaincase lubricant and transmission lubricant (though less frequently).

A number of items need to be done less frequently, like changing the brake fluid (The Softail Slim models all have a cable clutch). See the schedule for more details.

Maintenance Schedule for the Harley-Davidson FLSL Softail Slim

Below is the service schedule for the Harley-Davidson Softail Slim (FLSL). This is adapted from the manual but clarified to make it more legible and easy to follow. For example, we add notes into the table, put the most important things on top (e.g. changing the oil), and group things by frequency where possible.

Major Items Maintenance Schedule

This is the major items maintenance schedule for the Softail Slim (Milwaukee Eight). See below for the “inspection checklist” to follow at every service.

This schedule is based on regular usage of your bike. If you regularly ride your motorcycle less than 15 mi / 24 km, in temperatures between 60 F / 16 C, then change oil three times as often — every 1500 mi / 2400 km.


  • Follow the earlier of the distance-based or time-based intervals. For example, change the oil every 5000 mi / 8000 km or year, whichever comes first.
  • Keep repeating the schedule in the pattern shown.
mi x 100051015202530
km x 100081624324048Every
Inspection checklist (see below) – PerformYear (some items only)
Engine oil – Replace
Drain plug torque: 19-25 Nm / 14-21 lb-ft
See notes below
Oil filter – Replace
Hand-tighten with a rag
Air cleaner filter – Inspect
Clean as necessary in lukewarm water with mild detergent. Replace as necessary.
Inspect, clean, and replace more often if riding in severe conditions.
Spark plugs – Replace
Tighten to 10-15 Nm / 7-11 lb-ft)
2 years
Spoke tightness (front and rear) – CheckMore often if riding in severe conditions.
Only need to check every 15K miles / 24K km
Hand control switch housing screw torque – TightenYear
Master cylinder handlebar clamp screw torque — TightenYear
Front brake handlebar clamp screw torque – TightenYear
Steering head bearings — Adjust / Lubricate as necessary
Steering head bearings – Lubricate
Windshield (if fitted) bushings – Inspect
Primary chaincase lubricant – ReplaceMore often if riding in severe conditions.
Transmission lubricant – ReplaceMore often if riding in severe conditions.
Brake fluid – Replace (use DOT 4 fluid e.g. Castrol DOT 4)2 years, or if moisture is >3%
Front and rear axle nut torque – TightenYear
12 battery – Check battery, terminal torque, and clean connections. Lubricate with electrical contact lubricantYear
Front forks — Rebuild (seals, fluid)50,000 mi (80,000 km)
Fuel filter – Replace100000 mi (161000 km)
Harley-Davidson Softail Slim Maintenance Schedule

Inspection checklist for the Harley-Davidson Softail Slim

Below is the regular inspection checklist for the FLSL Softail Slim. Do this checklist at every service, per the schedule above. Items on the right also need to be done annually.

Standard Inspection Checklist — Softail, Milwaukee EightAnnual
Drive belt and sprockets – Inspect for wear
Drive belt – Check / Adjust tension
See notes below on checking / adjusting belt tension
Rear sprocket isolators – Inspect at every tire change
Electrical equipment and switches – Inspect for correct function
Tire pressure and tread (front and rear) – Check
Brake and clutch controls – Lubricate
Brake pads and discs (front and rear) – Check wear and that the discs run true
Minimum pad thickness: 0.04 in / 1 mm
Brake fluid level (front and rear) – Inspect level (fluid drops as pads wear), and check moisture content. Replace if moisture over 3%. (Castrol DOT 4)
Clutch cable – Adjust and check for leaks, contact, or abrasion. Lubricate cable with Harley-Davidson lubricant
Fuel lines and fittings — Inspect for routing, condition, leaks, and abrasion
Floorboards — Check for wear
Oil cooler – Clean
Oil and brake lines – Inspect for wear / routing
Jiffy stand – Lubricate
Exhaust system, fasteners, and shields – Inspect
Fuel tank cap / hinge / latch – Lubricate
Component and system function – Road test
Standard Annual Service (Harley-Davidson Milwaukee Eight, Softail Slim)

Harley-Davidson FLSL Softail Slim Belt Maintenance

Below are more details on the belt maintenance of your FLSL Harley-Davidson Softail Slim. To check the belt tension on the Slim, follow the procedure below.

Firstly, you need a belt tension tool and a ruler (either metric or imperial, whatever you’re used to). You can use either the standard Harley-Davidson belt tension tool or a generic one from Amazon – they do the same thing (and they’re also usable on other cruisers), though one might be more reliable than the other…

Harley-Davidson Low Rider belt deflection test
  1. Put the motorcycle in neutral on its stand, with the rear wheel on the ground. There should be no luggage or load on the bike, either.
  2. Push up on the middle of the bottom half of the belt until the belt tension gauge shows 10 lb (4.5 kg) of pressure.
  3. Use the ruler to measure belt tension deflection. Some models have a deflection window — you can use that too.

Target belt tension for the Harley-Davidson FLSL (existing belt) with 10 lb force: 1/2-9/16 in / 12.7-14.2 mm

If the belt tension is out of spec, you can either adjust it yourself if you know how, or take it to a dealer.

Oil Change on the Harley-Davidson Softail Slim (FLSL)

There are three fluids that need to be changed on the FLSL, as with most Harley-Davidsons, though you only need to change the engine oil at every service.

For all of them, you can use the same oil — Castrol V-Twin 20W50, a popular oil for Harley-Davidson big twins, or Mobil 1 V-twin 20W-50. You can also use Screamin’ Eagle Syn3 Synthetic 20W50, Harley-Davidson’s own in-house high-quality synthetic, which you can get from dealers.

For the Milwaukee Eight motor you need 4 quarts / 3.8L of oil (usually just one big bottle — or a bit shy). This is for a complete change, including the filter.

Here’s a pretty decent video going over the process for the oil change on the FLSL (he recommends different weight oils).

Softail Slim — M8 — oil / fluid change

To change the oil (in all locations) you need

  • A wrench with a 5/8 inch (16 mm works) socket
  • The right nuts to open the various drain plugs
  • An oil filter socket (access is hard — it’s behind the oil cooler). And some kind of way of catching the oil under the filter. Hand tighten the wrench to contact + firmly hand-tightened.

These are the places oil needs to be changed on the Slim or other bikes with this motor:

  1. Engine oil
  2. Primary chaincase lubricant
  3. Transmission lubricant

While the standard oil change interval for the Softail Slim (and all M8-powered motorcycles) is 5000 miles / 8000 km, Harley-Davidson recommends you change the oil more often if you operate it under adverse conditions or in extreme cold without allowing it time to warm up. (See the above notes on temperature.)

Wheels and Tires for the Softail Slim

The Sotail Slim ships with Dunlop bias ply tires out of the box, custom-made for the various Harley-Davidson models.

Below are the standard tire sizes and recommended tire pressures. Of course, choose the tire pressures that best suit your riding conditions, riding style, load, etc.

WheelTyreRecommended pressure (cold)
FrontDunlop HD series, 130/90 B16 73H BW36 psi / 248 kPa / 2.48 bar
RearDunlop HD Series 150/80 B16 777H BW40 psi / 276 kPa / 2.76 bar
Harley-Davidson FLSL Wheels, Tires, and Pressures

About the Harley Davidson FLSL Softail Slim

2021 Harley-Davidson FLSL Softail Slim LHS action
2021 Harley-Davidson FLSL Softail Slim

In the Harley-Davidson “cruiser” line up, which is a merger of the former Softail and Dyna lines, the Softail Slim is an everyday urban cruise.

It’s most distinguished by being a middle-of-the-road easy-going motorcycle. The Softail Slim is a naked bike with a single headlight, floorboards, “slim” tires (aha! thus the name) and an easy riding position that’s comfortable for a wide range of rider sizes. In other words, this is the bike for everyone.

Harley-Davidson made the Softail Slim with the Milwaukee-Eight engine between 2018 and 2021. Prior, there was a Sotail Slim, with the older Softail (surprise!) chassis and the Twin Cam motor. But subsequent to 2021 Harley shelved it for the similar (but different) Softail Standard.

The heart of the Softail Slim is the Milwaukee-Eight in original 107 cubic inch size. It’s an air/oil-cooled engine with four valves per cylinder and a 10.0:1 compression ratio. Peak torque comes on low, at 3000 rpm. While the Softail Slim is not a race bike, it never feels slow.

The Softail Slim has high-profile tires on 16-inch wire-spoked rims. The stock handlebar is a swept back one that places it within easy reach of the rider.

The short seat height (it’s only 25.5 inches / 648 mm off the ground laden) and relatively short throw of the rear shock (only a 43mm / 1.7 inch stroke, giving it 86 mm / 3.4 inches travel) might make you think you’ll be in for an uncomfortable ride, but the high-profile tires soak up bumps and imperfections in the road with relative ease.

Harley-Davidson FLSL Controls and bars

The Softail Slim doesn’t just have slim tires; it has a slim profile, too. This is part of what makes it easy to ride, despite the over 300 kg / 670 lb weight in running order.

The ride gear on the FLSL is very simple. There are conventional forks up front, and a single brake disc, a 300 mm (11.8 inch) disc at the front with a 4-piston fixed caliper.

Unlike some of the FLSL’s stablemates, the Softail Slim only ever came in 107  — there was no 114 no 117.

Reference — Manual for the FLSL Softail Slim

The above maintenance guide for the FLSL Softail Slim came from the manual for the Milwaukee-Eight Softail line of motorcycles from 2018. It’s consistent between different years of the Softail Slim, which all were based on the 107 motor.

You can download service information for your Harley from here (login required).

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