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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesHarley-Davidson’s Milwaukee-Eight 121 is a 1,977 cc, 45-degree V-twin with an 11.4:1 compression ratio, a long 4.625-inch stroke and liquid-cooled cylinder heads. What sets it apart in the 2025 CVO lineup is that “121” covers distinct factory configurations: the VVT 121 in the CVO Street Glide and the more powerful 121 HO in the CVO Road Glide ST.
What is the Milwaukee-Eight 121?
Harley-Davidson’s 2025 owner’s manual describes the Milwaukee-Eight 121 as a two-cylinder, four-cycle, 45-degree V-type engine with one camshaft and one balance shaft. Its displacement is 121 cubic inches, or 1,977 cubic centimeters.
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The engine’s key dimensions are a 4.075-inch (103.5 mm) bore and a 4.625-inch (117.5 mm) stroke. The relatively long stroke and 11.4:1 compression ratio are defining specifications, but they do not by themselves establish how the 121 performs against another engine; that requires comparable motorcycle configurations and test methods. Harley-Davidson’s 2025 owner’s manual is the source for these specifications.
Is the Milwaukee-Eight 121 liquid-cooled?
Partly. The manual specifies liquid-cooled cylinder heads, a frame-mounted radiator and electric pump, and an oil-cooled crankcase. Calling the entire engine simply “liquid-cooled” loses that distinction: the cylinder heads use coolant, while the crankcase is oil-cooled.
#1 Best Overall
- Compatible with Harley-Davidson, Drag Specialties, All Balls, Denso. Please verify your OEM part number from your original unit to help ensure fitment.
- Replaces: Oem Part Numbers: / Harley-Davidson: 31400-53, 31400053 / Drag Specialties: 2110-1102 / All Balls: 80-1017 / Cross Reference: 17044 / Specifications: / Voltage: 12V / Power: 1.6Kw / Starter Type: Plgr (Planetary Gear Reduction) / Rotation: Cw / Tooth Count: 10 / Gear Od: 28Mm / Overall Length: 168.5Mm / Oe Manufacturer: Denso / Condition: New
- Applications: Harley-Davidson Electra Glide (2017-2023): / Flh Electra Glide Revival 1868Cc 114Ci (2021) / Flhfb Electra Glide Highway King 1868Cc 114Ci (2023) / Flht Electra Glide Standard 1690Cc/1746Cc (2019-2022) / Flhtcu Electra Glide Ultra Classic 1746Cc 107Ci (2017-2019) / Harley-Davidson Road King (2017-2024): / Flhr Road King 1745Cc/1746Cc 107Ci (2017-2022) / Flhrc Road King Classic 1690Cc (2017-2019) / Flhrxs Road King Special 1746Cc (2017-2019), 1868Cc (2019-2024) /
- Applications: (2019-2024) / / Harley-Davidson Street Glide (2017-2024): / Flhx Street Glide 1746Cc 107Ci (2017-2023) / Flhxs Street Glide Special 1746Cc (2017-2018), 1868Cc (2018-2023) / Flhxse Cvo Street Glide 1868Cc (2017-2019), 1923Cc (2018-2022), 1982Cc (2023-2024) / / Harley-Davidson Road Glide (2017-2024): / Fltrx Road Glide 1746Cc 107Ci (2017-2023) / Fltrxs Road Glide Special 1746Cc (2017-2019), 1868Cc (2019-2023) / Fltru Road Glide Ultra 1746Cc (2018-2019),...
- Starter Motor Source: Aftermarket
How much horsepower and torque does it make?
Harley-Davidson publishes different outputs for the two 2025 CVO configurations. These are manufacturer specifications for the named motorcycles, not results from an independent or same-bike comparison.
| 2025 motorcycle | Engine designation | Published output | Named distinctions |
|---|---|---|---|
| CVO Street Glide | VVT 121 | 115 hp; 139 lb-ft of torque | Harley-Davidson’s model page identifies the engine as VVT 121; it does not name the HO hardware listed for the Road Glide ST. |
| CVO Road Glide ST | 121 HO | 127 hp; 145 ft-lb of torque | High-performance camshaft, Heavy Breather intake and rev limit increased to 5,900 rpm, as described by Harley-Davidson. |
The Street Glide figures come from Harley-Davidson’s 2025 CVO Street Glide page; the Road Glide ST figures and named hardware come from Harley-Davidson’s 2025 CVO Road Glide ST page. The pages describe different motorcycles, so the ratings do not isolate the effect of a single component or establish a controlled, same-bike comparison.
What makes the 121 HO different?
For the CVO Road Glide ST, Harley-Davidson identifies the 121 HO as the higher-output configuration. Its page names a high-performance camshaft and Heavy Breather intake and says the rev limit is increased to 5,900 rpm. The page presents those features alongside the 127-hp and 145-ft-lb ratings; it does not provide a controlled test attributing a precise share of the output difference to each feature.
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Rank #2
- Compatible with: This air cleaner fits Harley motorbike 2018-2024 Cruiser Softail, 2017-2025 Touring (Except Center Cooled Models) and Trike Milwaukee-Eight engines models (Note: The above models must be equipped with a Screaming Eagle Heavy Breather Elite Air Cleaner.)
- Premium Filtration for Engine Protection: Multi-layer filter media traps dust, dirt, and debris effectively, delivering clean air to the engine and maintaining performance
- Washable & Reusable for Long-Term Value: Durable, reusable construction—rinse, dry, re-oil (filter-specific), and reuse. Cuts down on replacement costs over time
- Pre-Oiled Design for Quick Installation: Factory pre-oiled—no messy prep needed. Just remove the old filter and snap this one in for instant, hassle-free setup
- Warm Tips: Regularly checking and replacing the air filter is key to maintaining the vehicle's performance. Follow the moto parts manufacturer’s recommendations for replacement intervals to ensure optimal engine efficiency
Harley-Davidson calls this the strongest factory-installed engine used in a production Harley-Davidson on that model page. That is the manufacturer’s promotional claim, not an independent industry ranking.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.What should owners know about fuel and maintenance?
The 2025 owner’s manual specifies premium unleaded fuel. Its listed capacities are reference figures, not instructions to fill a system by capacity during a service; follow the procedure for the exact motorcycle and market.
- Total engine oil capacity: 5.0 qt (4.7 L).
- Service oil change amount: 4.75 qt (4.5 L).
- Coolant capacity: 0.82 qt (0.78 L).
For service intervals, checking levels, fluid specifications and procedures, use the manual for the specific motorcycle rather than assuming every Milwaukee-Eight 121 installation has identical requirements.
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