We’ve added some stroke to our successful 4-1/8” Big Bore kit for Twin Cams©!

Get Big Power all the way through the RPM range! Everything you need is supplied. This kit includes a balanced, fully assembled Darkhorse prepared 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 Carb bikes include a mapped ignition module and your choice of a ThunderJet equipped S&S ‘G2’ or Mikuni 48mm carburetor. All kits are available with gear drive cams if desired. 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

Country of Origin

Engine Type


Fuel / Air


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