Get Big Power all the way through the RPM range! Everything you need is supplied. This kit includes a balanced, fully assembled Darkhorse prepared S&S 4-5/8” (124”) stroke crankshaft that is designed to drop right into your engine cases. Our 4-1/8” bore cylinders are precision-fitted with forged pistons; cylinder heads are upgraded to 2” intake and 1.625” exhaust valves, then treated to full port and combustion chamber re-shaping using the latest CNC machining technology. Compression releases are installed to ease starting, Red Shift cams, chrome-moly pushrods, and a full gasket kit are included.

Kits for EFI-equipped bikes include a ThunderMax EFI module with AutoTune, 54mm throttle body/manifold system with high flow air filter. All kits are available with gear drive cams if desired. Our EFI system includes a new ECM loaded with a dedicated base map developed by Zipper’s specifically for this kit. We’ve done the hard work for you – every part is ready to install, and the end result is Big Power!

Send your carefully packed heads directly to Zipper’s. Machining required for installation. Cases require boring to accommodate the Muscle 120 cylinders, and additional clearance in the cam chest will be required for high lift cams. ‘03-later cases should be converted to a Timken sprocket shaft bearing. Rocker boxes must be clearanced for larger valve springs. Heads are supplied on an exchange basis for quick turn-around, or may be purchased. These services, or a complete engine conversion, are available through Zippers. Heads must be in nearly perfect cosmetic condition. Previously modified heads are not eligible for exchange.

All kits can be purchased whole or in any combination of desired components, please contact our sales department at 410-579-2828 for more details and pricing. 



Zipper’s Job Order Sheet


This product is not for sale or use on pollution controlled vehicles.

Weight 1 lbs
Emissions Compliant

Engine Type


Fuel / Air


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