1250cc Big Bore Conversion Kits / '86-'22 Sportster

The S&S 1250cc Big Bore Conversion Kit takes an '86-'22 Sportster to 1250cc using 3-9/16" bore cylinders and forged S&S pistons.

Sportsters respond to displacement more obviously than most Harley platforms, because there is less mass for the extra torque to move. Going to 1250cc with a 3-9/16" bore is the established route, and S&S sells it as a complete kit so the bore, the piston and the gaskets all come from the same place.

What Comes in the 1250cc Conversion Kit

S&S 1250cc pistons ship with rings and wristpins. The pistons are forged with enlarged valve pockets that accept valves up to 2.00", so the kit does not become the limiting factor if you go further with the heads later. Skirts are coated, and each piston carries a measurement window and the S&S logo. MLS head, base and exhaust gaskets are included along with assembly lube and instructions. Made in the USA.

1250cc Conversion Kit Specifications

• Fits '86-'22 Sportster models
• S&S 3-9/16" bore cylinders
• Forged pistons with coated skirts, rings and wristpins included
• Enlarged valve pockets accept valves up to 2.00"
• Compression options: 10.3:1 flat top pistons, or 11.2:1 with 5.5cc domed pistons
• MLS head, base and exhaust gaskets included
• Assembly lube and instructions included
• Finishes: wrinkle black or silver
• Made in the USA

Before You Order the 1250cc Kit

883 models require XL1200 cylinder heads to reach the correct compression ratios, so budget for heads on top of the kit if you are starting from an 883. This kit is not compatible with XR1200 models.

⚠️ VERIFY your model, year and head configuration before ordering. A displacement change of this size requires retuning.