Our Stage III ThunderSport heads are the perfect choice for 3.875 – 3.937” bore Twin Cam® engines with all the highs – high compression, high flow throttle body, high flow exhaust and high output at all RPM’s! Our original design has been fully refined to fulfill the airflow needs of the highly modified Twin Cam® engine with displacement ranging from 95 to 107 cubic inches.

• Advanced port and valve designs works great with domed pistons
• Special ductile-iron step-lock valve guides
• 5/16” stem 1.900” / 1.630” stainless steel valves with severe duty coatings
• Performance valve springs for .580” lift standard* (*Optional springs available for higher lift applications)
• Fully machined 91cc** combustion chamber un-shrouds valves for increased flow (** 84cc on Twin Cooled® heads)
• Custom decked for your specific application
• Best quality workmanship and materials used
• Optional manual compression release machining available for non-ACR heads

After 5-axis CNC-porting is complete, we install and size Zipper’s super-concentric ductile-iron Step-Lock valve guides. Valve seats are machined with specially developed proprietary seat angles to accept oversized stainless steel valves treated with severe-duty coatings and flow-enhancing shapes. When full-radius seat and guide machining is complete, the head gasket surface is decked for combustion chamber CC requirements and trued for best possible gasket seal. Lastly, viton valve stem seals are installed and dual-coil performance valve springs are set up for your camshaft selection.

With the right combination of compression, camshaft, intake and exhaust choices, ThunderSport heads shine throughout the full RPM range, delivering the widest of powerbands. If your engine build includes domed pistons, larger throttle body, a “headwork required” camshaft and other related high performance components, this is the head for you! Zipper’s 1.900” Stage III ThunderSport CNC heads can easily position a 107” engine for triple-digit torque at 2000 RPM, with horsepower climbing past the 120 mark – using reasonable compression and today’s high-test pump gasoline.

Supply us with your piston dome volume and camshaft selection and we’ll calculate the final decking required for your desired cc volume/compression ratio. Contact us if you need assistance selecting the proper camshaft, piston compression volume and/or exhaust system for your application.

Work is performed on your supplied head cores; shop time is approximately 5 business days (allow for additional time during peak season). Exchange heads may be available if the condition of your cores qualifies. You can also purchase heads outright (without supplying a core set). We will supply clean, reconditioned factory castings for your application – add part #517-015 to your order.

International Customers – Import duties are not required when shipping your castings for modification. Special low shipping rates are available for your cores; contact your sales representative for details.

Supply us with your piston volume and camshaft selection for final cc volume through decking.

Remove heads from engine, wash off oil and ship to Zipper’s as-is. Improper cleaning methods can actually do more harm than good; we suggest you allow us to properly prepare your heads for modification. Extreme care is taken during our preparation process to preserve component finishes. A small cleaning charge applies to heads sent in for modification.



Zipper’s Job Order Sheet


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

Weight 1 lbs

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