S&S 107" to 129" Big Bore Kit for M8 Models

• S&S 107" to 129" Big Bore cylinders
• Forged pistons
• Piston rings
• Wrist pins
• Circlips
• Required gaskets
• Required installation hardware

Converts Gen 1 107" M8 Engines to 129"
Transforms compatible Gen 1 Harley-Davidson Milwaukee-Eight 107" engines with stock 4.375" stroke crankshafts into a 129 cubic inch high-performance setup.

Built for Serious M8 Performance
Designed for riders looking to increase displacement, horsepower, torque, and overall engine performance on a Gen 1 107" Milwaukee-Eight platform.

Designed for Stock 4.375" Stroke Crankshafts
Engineered specifically for use with factory 4.375" stroke crankshafts found in Gen 1 107" Milwaukee-Eight engines.

Forged Pistons Included
Includes forged pistons designed for strength, durability, and high-performance engine use.

11:1 Compression Ratio
Built with an 11:1 compression ratio to support increased horsepower, torque, and throttle response.

Works with Stock Crankcases

Works with Compatible M8 Crankcases
Designed for compatible Gen 1 Milwaukee-Eight 107" engine applications.

Gaskets and Hardware Included
Comes with the required gaskets and installation hardware needed for assembly.

Available in Highlighted or Non-Highlighted Finish
Choose wrinkle black with highlighted fins for a contrast-cut look or wrinkle black without highlighted fins for a more understated engine finish.

Built for High-Output M8 Builds
A strong upgrade for riders building a serious Milwaukee-Eight engine with increased displacement, stronger torque, and higher horsepower potential.

Fits:

• Gen 1 Harley-Davidson 107" Milwaukee-Eight engines with stock 4.375" stroke crankshafts

Important fitment notes:

• Fits Gen 1 107" M8 engines only
• Designed for engines with stock 4.375" stroke crankshafts
• Does not fit 114" or 117" M8 engines
• Does not fit Gen 2 M8 applications
• NOT compatible with VVT models

Always confirm your exact year, model, engine size, crankshaft configuration, tuning setup, camshaft selection, exhaust, intake, and supporting components before ordering.

Important Notes:
For 107" M8 Engines with Stock Stroke Only
This kit is designed to convert compatible 107" Milwaukee-Eight engines equipped with stock 4.375" stroke crankshafts to 129". Confirm your engine size and crankshaft before ordering.

Professional Installation Required
Big bore kit installation requires advanced mechanical knowledge, proper tools, engine assembly procedures, and tuning. Professional installation is strongly recommended.

Tuning Required
Proper tuning is required after installation to ensure correct fueling, ignition timing, drivability, reliability, and maximum performance.

Supporting Components Recommended
For best results, supporting upgrades such as a performance camshaft, intake, exhaust, clutch, and proper tuning may be recommended based on your performance goals.

What does the S&S 107" to 129" Big Bore Kit do?
This kit converts compatible Gen 1 Harley-Davidson Milwaukee-Eight 107" engines with stock 4.375" stroke crankshafts to 129 cubic inches for increased displacement, horsepower, and torque.

What engines does this kit fit?
This kit is designed for Gen 1 Harley-Davidson 107" Milwaukee-Eight engines using stock 4.375" stroke crankshafts.

Does this fit 114" or 117" M8 engines?
No. This kit is intended for Gen 1 107" M8 engines only.

Does this fit Gen 2 M8 models?
No. This kit is intended for Gen 1 107" M8 applications only unless otherwise confirmed.

What crankshaft is required?
This kit is designed for use with the stock 4.375" stroke crankshaft found in Gen 1 107" Milwaukee-Eight engines.

Are pistons included?
Yes. Forged pistons are included.

Does this kit include gaskets and hardware?
Yes. The kit includes the required gaskets and installation hardware.

What finish options are available?
This kit is available in wrinkle black with highlighted fins or wrinkle black without highlighted fins.

Will this increase horsepower and torque?
Yes. Increasing displacement from 107" to 129" can significantly improve horsepower and torque when paired with proper supporting components and tuning.

Is tuning required?
Yes. Proper tuning is required after installation.

Is professional installation recommended?
Yes. Professional installation is strongly recommended for proper engine assembly, clearances, torque specs, tuning, and long-term reliability.