​how to repair cracked cylinder head？

Before attempting any repair, confirming the presence, location, and severity of the crack is non-negotiable. According to “Automotive Engine Repair: Principles and Practices”, misdiagnosis is the primary cause of failed cylinder head repairs—so getting this step right lays the foundation for success.

Start with a visual inspection: Remove the cylinder head (refer to your engine manual for torque specifications—such as 105 N·m for Doosan DE12 models) and check for obvious signs: coolant residue, oil leaks, or visible cracks around valve seats, spark plug holes, or coolant passages. A flashlight and magnifying glass will help spot hairline cracks that might otherwise go unnoticed.

Next, pressure testing is the most reliable method for heavy-duty cylinder heads. Secure the head to a test bench, seal coolant and oil passages, and apply 10–15 psi of compressed air. Submerge the head in water; bubbles indicate a crack. For aluminum cylinder heads (common in modern machinery), use a dye penetrant test instead to avoid water damage to sensitive components.

Don’t forget to check for related damage: A cracked cylinder head often stems from overheating or improper torque. Inspect the cylinder block deck for warpage, head gasket damage, and piston tops for coolant contamination—addressing these issues is vital to prevent the crack from reoccurring.

Step 2: Evaluate—Should You Repair or Replace?

Not all cracks are repairable. “Cylinder Head Refinishing Guide” emphasizes that the decision hinges on three key factors: crack location, size, and the cylinder head’s material (cast iron vs. aluminum). Understanding these will help you avoid wasting time on futile repairs or unnecessary replacements.

Repair is a viable option if you’re dealing with hairline cracks (less than 0.004 inches wide) that don’t extend to combustion chambers, valve seats, or coolant passages. It’s also suitable for cracks in non-critical areas—such as the outer edges of cast iron cylinder blocks for Cummins ISF3.8—and when repairs are cost-effective.

On the other hand, replacement is the safer choice for cracks that reach combustion chambers, valve guides, or coolant/oil passages—these pose risks of leaks or combustion gas escape. Severe cracks (more than 0.008 inches wide), multiple cracks (common in aging Kubota D1402 or Doosan DE12 cylinder heads), and aluminum cylinder heads with warpage exceeding 0.003 inches (aluminum is less forgiving than cast iron for repairs) also call for replacement. Additionally, if the cylinder head has already been repaired multiple times, its metal integrity is compromised, making replacement the more reliable long-term solution.

At CustomsVehicle, we stock OEM-quality replacement cylinder heads for all major heavy machinery brands—including the Cummins ISF3.8 (part number 5258274), Doosan DE12 (150113-00040A), and Isuzu 6BD1 (11110-80002)—each undergoing 100% pre-shipment pressure testing and backed by a 1-year warranty. For many customers, replacement is faster and more dependable than complex repairs, especially for critical equipment that can’t afford extended downtime.

Step 3: Professional Repair Process for Repairable Cracks

1.Thorough disassembly and cleaning: Use a heavy-duty engine-specific valve spring compressor to remove the valves, valve springs and guides from the cylinder head. Remove engine oil, carbon deposits and coolant residues using a parts cleaning machine with a degreaser — use a wire brush for cast iron cylinder heads and a plastic scraper for aluminium to avoid scratches. Completely dry the cylinder head to prevent moisture in cracks from affecting the repair.

2.Crack repair preparation: Use an angle grinder fitted with a carbide drill to grind a 'V-shaped groove' along the full length of the crack, matching the wall thickness of the cylinder head to provide a stable surface for the repair material. Remove burrs from the groove with a grinding wheel and wipe clean with acetone to remove metal fragments and impurities.

Choose the right repair method for your cylinder head:

Weld repair (for cast iron cylinder heads): Suitable for thick-walled cast iron engine blocks and cylinder heads. Preheat the cylinder head to 260–315°C to prevent thermal shock, use nickel alloy welding rods compatible with cast iron, and perform short-distance alternating welding to avoid overheating. After welding, allow slow cooling for 24–48 hours to reduce stress and prevent new cracks.

Epoxy repair (suitable for aluminium cylinder heads or small cracks): Suitable for aluminium cylinder heads (such as Kubota D1402) or cracks in non-critical areas of cast iron cylinder heads. Select engine-specific epoxy resin capable of withstanding temperatures up to 260°C, mix according to the instructions and fill into the V-groove to ensure full penetration, then clamp and cure for the recommended time (usually 24 hours).

3.Refurbishment and assembly: Resurface the cylinder head top using a surface grinder or milling machine to ensure flatness. Repair valves and guides — check valve wear, grind valve seats to restore sealing, and replace worn guides. Install new valve seals, springs, and circlips, and tighten all fasteners to OEM standard torque.

After completing the assembly, be sure to perform a pressure test again to confirm there are no leaks — this step is crucial to ensure the effectiveness of the repair and must not be omitted.

Step 4: Post-Repair Best Practices to Prevent Re-Cracks

Address the root cause first: overheating is the primary factor leading to cylinder head cracks. For construction machinery, mix the coolant in the proper ratio according to the climate (coolant to water 5:5).

Use high-quality replacement parts: when replacing the cylinder head gasket and related components, prioritise OEM or equivalent aftermarket parts.

Strictly follow torque specifications: when reinstalling the cylinder head, use a torque wrench and tighten in the OEM-specified sequence (usually from the centre outwards). After 50 hours of engine operation, retighten the bolts — this is a crucial step for diesel engines to prevent bolt loosening and uneven pressure.

Monitor closely after repair: during the first 100 hours, regularly check the coolant level and engine temperature. If signs of overheating or leakage are detected, stop the engine immediately.