For riders chasing serious horsepower without turning their engine into a full-blown custom build, S&S Cycle’s new 131ci kit for 2017-up Milwaukee-Eight models delivers a compelling middle ground. Designed as a true “sleeper” upgrade, this bolt-in package increases displacement, compression, and overall performance—while keeping a clean, near-stock appearance.

At its core, the kit bumps displacement to 131 cubic inches using a 4.250-inch bore and 4.625-inch stroke combination. That translates to a noticeable jump in torque and throttle response across the rev range, especially in heavier touring models where extra grunt makes all the difference.

One of the biggest advantages of the S&S 131 kit is its straightforward installation. Unlike many big-inch builds that require case machining or extensive teardown, this kit is designed as a true bolt-in solution. That means:

• No machining required
• Stock engine height retained
• Complete gasket kit included

For performance shops and DIY builders alike, that simplicity reduces downtime and overall install cost—making it one of the more accessible big-bore upgrades on the market.

With a compression ratio set at 11.0:1, the S&S 131 kit strikes a balance between aggressive performance and streetability. Riders can expect:

• Stronger low- and mid-range torque
• Improved throttle response
• Increased overall horsepower potential

Paired with the right cam, intake, and exhaust setup, this kit becomes a serious foundation for a high-performance M8 build without sacrificing rideability.

What sets this kit apart visually is its stealth approach. The cylinders feature a finish that closely matches the stock Harley-Davidson look, keeping the upgrade under the radar. Unless you opt to install the included “131” cam cover badge, most people won’t know what’s hiding inside.

With more than six decades in the V-twin performance game, S&S Cycle continues to refine what riders expect from aftermarket upgrades. Founded in 1958 and rooted in American manufacturing, the brand has built its reputation on durable, performance-driven components for Harley-Davidson platforms.

The 131ci kit is a natural extension of that legacy—combining modern engineering with practical installation and real-world gains.

For Milwaukee-Eight riders looking to step up in a big way without diving into a full custom engine build, the S&S Cycle 131 kit checks all the right boxes. It’s powerful, practical, and stealthy—exactly what a true sleeper performance upgrade should be.

Whether it’s for a torque-hungry touring bike or a street-focused performance build, this kit offers a serious bump in performance without unnecessary complexity.