Pair New GM Chevrolet 5.0L 305 Mercruiser Cylinder Heads – Casting Numbers 059 and 520 | 12552520 12558059 | No Core Required

New GM Chevrolet 5.0L 305 Mercruiser Cylinder Heads (Pair) – Casting #059 & #520

Professional Grade Marine Replacement – OEM Quality Performance

Restore your engine’s peak power and thermal efficiency with this matched pair of brand-new aftermarket cylinder heads, specifically engineered for GM Chevrolet 5.0L (305) Small Block V8 engine series. Featuring the high-performance #059 and #520 casting specifications, these heads are designed to exceed original equipment (OE) standards for long-term durability in high-demand marine and truck applications. Unlike remanufactured units that may have already undergone multiple thermal cycles, these “zero-hour” castings provide a fresh structural foundation, eliminating the risk of previous fatigue or micro-cracking common in used iron heads.

Each unit is manufactured from high-density cast iron for superior heat dissipation and structural rigidity, ensuring a superior head gasket seal under heavy loads. This matched pair arrives fully assembled and “drop-in” ready, featuring precision-seated valves and calibrated springs to ensure immediate performance restoration on the water or the road. Every casting undergoes a rigorous factory inspection protocol—including precision deck surfacing and high-pressure leak testing—to guarantee absolute dimensional accuracy and reliable operation in the most demanding environments.

🛠️ In-House Machining Process

• Debris-Free Preparation: Every new casting is chemically cleaned and blown out to remove all residual machining oils, metal shavings, and manufacturing debris. This ensures your oiling system remains uncontaminated from the first start-up.

• In-House Precision Machining: Unlike “shelf-stock” heads, we perform all final machine work in-house. This includes final guide sizing and precision valve seat cutting to ensure perfect geometry and clearances before the head reaches the assembly bench.

• Pressure & Integrity Testing: To go beyond the factory standard, we pressure-test every head to 60 PSI. This verifies that the casting is free of porosity leaks or internal defects that could compromise the cooling or oiling systems.

• Dimensional Inspection: Our technicians perform a multi-point inspection of the gasket-sealing surface and bore alignment. This ensures the head is perfectly flat and meets tight OEM tolerances for a flawless fit.

• Professional Assembly & Setup: Valvetrain components are installed and verified in a clean-room environment. We check spring heights and seal seating to ensure the assembly is balanced and ready for high-performance use.

• Vacuum-Seal Verification: Every intake and exhaust port is Vacuum Tested. This guarantees a 100% airtight seal between the valves and seats, ensuring maximum compression and peak performance right out of the box.

📊 Technical Specifications

• Engine Size: 5.0L / 305 CID
• Casting Numbers: 12558059 (#059), 12552520 (#520)
• Material: Cast Iron
• Condition: New
• Each Cylinder Head Includes: Valves and Springs Installed

🚛 Fitment Data

Common Applications:

• Chevrolet 1500 Pickup: 1996–1999 Chevrolet 1500 Pickup (5.0L)
• Chevrolet 2500 Pickup: 1996–1998 Chevrolet 2500 Pickup (5.0L)
• Chevrolet Express 1500 Van: 1996–2002 Chevrolet Express 1500 Van (5.0L)
• Chevrolet Express 2500 Van: 1996–2002 Chevrolet Express 2500 Van (5.0L)
• GMC 1500 Pickup: 1996–1999 GMC 1500 Pickup (5.0L)
• GMC 2500 Pickup: 1996–1998 GMC 2500 Pickup (5.0L)
• GMC Savana 1500 Van: 1996–2002 GMC Savana 1500 Van (5.0L)
• GMC Savana 2500 Van: 1996–2002 GMC Savana 2500 Van (5.0L)