S&S Cycle 1250 Big Bore Kit For Harley Sportster 1986-2022
Extreme Bolt-On Power: S&S Cycle 1250 Big Bore Kit for Harley Sportster (1986–2022)
The S&S Cycle 1250 Big Bore Kit is one of the most popular and effective performance upgrades for the venerable Harley-Davidson Sportster platform, offering a significant displacement increase and a dramatic boost in torque and horsepower. This kit is a versatile power solution, designed to fit nearly three decades of Evolution Sportster models, covering the range from 1986 all the way through 2022 (excluding the newest Revolution Max-powered Sportster S/RH1250S and the XR1200 models).
This upgrade is specifically designed to convert stock 1200cc Sportster engines up to 1250cc, or to be used as a final stage in a complete 883cc to 1250cc conversion. It delivers the renowned quality of S&S Cycle engineering in a package that riders can install without complex engine disassembly.
Key Engineering and “Bolt-On” Advantage
The central appeal of the S&S 1250 kit is its ability to offer high-level performance while maintaining a relatively straightforward installation process.
True Bolt-On Installation
Crucially, the S&S 1250 kit is engineered to be a true bolt-on upgrade for 1200cc models. This means no machining or rebalancing of the stock crankcases or flywheels is required. The new 3-9/16″ bore cylinders are designed to perfectly fit the stock case openings, allowing the entire job to be performed with the engine still in the frame, saving considerable time and labor costs. This “weekend wrenching” accessibility is a major selling point for the Sportster community.
Forged Pistons and Cylinders
The kit comes complete with two new cylinders and matched forged pistons. Forged pistons are vastly superior to cast stock units, offering greater strength and durability to handle the higher cylinder pressures and heat generated by the increased compression ratios.
The kit is available in two main compression ratios to suit different performance goals and fuel requirements:
10.3:1 Compression Ratio: A reliable, high-performance option that is typically manageable with pump gas and an appropriate tune.
11.2:1 Compression Ratio: A higher-performance option that delivers maximum power but may require higher octane fuel or more aggressive tuning to prevent detonation.
The pistons also feature coated skirts for reduced friction and longer life, and larger than stock valve pockets to accommodate up to 2.00″ valves, should the rider decide on CNC ported heads as a future upgrade. The cylinders are supplied in a finish that matches the stock Harley aesthetic, such as Wrinkle Black or Silver powder coat.
Performance and Conversion Details
The results of the Sportster 1200 to 1250 conversion are immediately noticeable, providing a substantial increase in acceleration and usable street power.
Performance Gains
While the displacement increase from 1200cc to 1250cc may sound small, the combination of the larger bore, high-flow piston design, and increased compression ratio translates into a significant boost in performance, particularly when paired with the proper supporting modifications (high-flow air cleaner, performance exhaust, and a proper tune). Riders can expect dramatic improvements in mid-range and top-end horsepower and torque, often pushing power figures well beyond the stock factory limits.
883cc Conversion Requirement
For owners of 883cc Sportster models (which have smaller combustion chambers in their cylinder heads), this kit can be used to perform a radical 883 to 1250cc conversion. However, S&S specifies that 883 cylinder heads cannot be used with this 1250cc kit to achieve the correct compression ratios. XL1200 cylinder heads (or an aftermarket equivalent) are required to complete the conversion correctly, due to the different valve and combustion chamber design of the 883 heads.
Required Supporting Modifications
Achieving reliable peak performance from the S&S 1250 Big Bore Kit requires addressing the engine’s supporting systems:
Fuel and Ignition Management: This is non-negotiable. The increased air demands a corresponding increase in fuel. Carbureted models (pre-2007) will require new jets and possibly an ignition module. EFI models (2007-later) will require a flash tuner and a professional dyno tune to adjust the fuel maps and ignition timing.
Air and Exhaust: A high-flow air cleaner and a performance exhaust system are essential to allow the engine to breathe efficiently and extract maximum power.
Camshafts (Recommended): For ultimate power, especially when converting an 883, the addition of performance S&S cams (like the S&S 482 or 500 series) is highly recommended to take full advantage of the increased displacement and compression.
The S&S Cycle 1250 Big Bore Kit remains one of the most impactful and widely trusted ways to inject serious, track-ready power into the iconic Harley Sportster XL engine.
| Product Style | AmericanV-twin Item # | Catalog Number | Store Sku | MFR. Product # | Availability |
|---|---|---|---|---|---|
| Black / 10.3:1 Compression Ratio | 1181564 | 207-083 | 910-0691 | Only 5 left | |
| 10.3:1 Compression Ratio / Silver | 1181565 | 207-082 | 910-0689 | In Stock: Ships next business day | |
| Black / 11.2:1 Compression Ratio | 1181566 | 207-085 | 910-0692 | In Stock: Ships next business day | |
| Silver / 11.2:1 Compression Ratio | 1181567 | 207-084 | 910-0690 | Only 2 Left: Ships next business day |
This Product Fits
Harley Davidson
Sportster Custom XL1200C 1996-2019
Sportster Custom XL883C 1998-2009
Sportster Deluxe XLH883DLX 1989, 1990, 1991, 1992
Sportster Forty-Eight Special XL1200XS 2018, 2019
Sportster Forty-Eight XL1200X 2010-2022
Sportster Hugger XLH883HUG 1988-2003
Sportster Iron XL1200NS 2018, 2019, 2020, 2021
Sportster Iron XL883N 2009-2022
Sportster Low/SuperLow XL883L 2005-2019
Sportster Low XL1200L 2006-2011
Sportster Nightster XL1200N 2007-2012
Sportster Roadster XL1200CX 2016, 2017, 2018, 2019, 2020
Sportster Roadster XL1200R 2004, 2005, 2006, 2007, 2008
Sportster Roadster XL883R 2002-2012
Sportster Seventy-Two XL1200V 2012, 2013, 2014, 2015, 2016
Sportster Sport XL1200S 1996-2003
Sportster SuperLow XL1200T 2014, 2015, 2016, 2017
Sportster XL883 2004-2009
Sportster XLH1100 1986, 1987
Sportster XLH1200 1988-2003
Sportster XLH883 1986-2003
