S&S Cycle 128″ Big Bore Kit For Harley 114″ / 117″ Milwaukee Eight 2017-2026
Dominating Displacement: S&S Cycle 128″ Big Bore Kit for Harley 114″ / 117″ Milwaukee-Eight (2017–2026)
The Milwaukee-Eight (M8) engine, introduced in 2017, marked a new era for Harley-Davidson performance. While the factory 114-inch and 117-inch engines deliver respectable power, S&S Cycle, the industry leader in V-Twin performance, has pushed the limits further with the 128″ Big Bore Kit. This kit is the ultimate bolt-on displacement upgrade for the 4.5″ stroke M8 platform, maximizing cubic inches without requiring the costly and complicated machining of the stock crankcases.
This precision-engineered solution converts your factory 114-inch or 117-inch M8 motor into a colossal 128 cubic inch (2097cc) powerhouse. It’s the most straightforward path to unlocking incredible rear-wheel horsepower and torque that can easily exceed the 140+ HP mark when properly supported, making it a favorite for riders looking to dominate the streets and the dyno sheet.
The Bolt-On Advantage: Precision Engineering
The key feature of the S&S 128″ Big Bore Kit is its innovative design which makes it a true bolt-on replacement for the existing cylinders on 114″ and 117″ motors.
Crankcase Compatibility
S&S specifically engineered the cylinder spigot outer diameter to fit perfectly into the stock crankcase bore of the 4.5″ stroke M8 engines. This eliminates the need for expensive and labor-intensive crankcase machining, drastically reducing installation time and complexity. This feature is the pinnacle of Harley M8 performance parts design, offering maximum power with minimal modification hassle.
High-Performance Components
The kit is centered around superior components built to handle the extreme demands of the increased displacement and power:
Big Bore Cylinders: New S&S cylinders with a large 4.250-inch bore are included. They maintain the necessary wall thickness and strength to ensure long-term durability, often finished in factory-matching styles such as Wrinkle Black, Black Granite (with or without highlighted fins) to maintain a stock aesthetic.
Forged Pistons: The kit includes high-strength forged pistons designed for the M8’s 4.500-inch stroke. Forged construction is essential for reliability at high cylinder pressures. The pistons are engineered to deliver a powerful 11:1 compression ratio when used with stock M8 cylinder heads, maximizing combustion efficiency and power output. They come complete with rings, pins, and clips, often with the ring gaps pre-set for immediate installation.
Complete Gasket Set: All necessary high-quality gaskets, including head and base gaskets, are included, ensuring a perfect seal and simplified assembly.
Unleashing the Milwaukee-Eight’s True Potential
The transformation from a 114″ or 117″ to a 128-inch M8 engine is dramatic. The increase in displacement delivers massive low-end grunt and mid-range torque, allowing for effortless acceleration and power when passing at highway speeds.
Required Supporting Modifications
While the cylinder and piston installation is bolt-on, maximizing the kit’s potential requires attention to the engine’s supporting systems. The 128″ engine demands significantly more air and fuel than the stock configuration:
Performance Camshaft: To achieve the highly advertised power figures, a performance M8 camshaft is essential. S&S often recommends their own cams (like the S&S 550 cam) designed for big bore applications, which utilize higher lift and duration to fill the larger cylinders.
High-Flow Air and Exhaust: A high-flow air cleaner and a performance 2-into-1 exhaust system are non-negotiable for enabling the engine to breathe effectively and scavenge exhaust gases efficiently.
Fuel Management and Tuning: A proper EFI tuner (such as a Dynojet Power Vision or ThunderMax) and a professional dyno tune are absolutely critical. Running the engine without correctly adjusting the fuel maps and ignition timing will lead to lean conditions, overheating, and potential engine failure.
Clutch Upgrade: The significant boost in torque produced by the S&S 128″ Big Bore Kit will likely overpower the stock clutch, necessitating a performance clutch upgrade with heavier springs or an aftermarket unit to reliably transmit power to the rear wheel.
The S&S Cycle 128″ Big Bore Kit is the gold standard for riders of 2017-later Harley Touring and Softail models looking for an aggressive, reliable, and install-friendly increase in power, converting a high-performance cruiser into a true street monster.
| Product Style | AmericanV-twin Item # | Catalog Number | Store Sku | MFR. Product # | Availability |
|---|---|---|---|---|---|
| Black Granite w/ Highlighted Fins | 10555181 | 10555181 | 910-0733A | In Stock: Ships next business day | |
| Black w/ Highlighted Fins | 10555183 | 10555183 | 910-0684A | In Stock: Ships next business day | |
| Black w/ Non Highlighted Fins | 10555182 | 10555182 | 910-0685A | In Stock: Ships next business day |
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