The roar of a truck engine symbolizes power and productivity, but when cylinder head gaskets fail, it can spell serious trouble. Imagine being on a long haul when suddenly you notice engine overheating, mysterious coolant loss, and white smoke from the exhaust—these are telltale signs of a failing head gasket.

Replacement becomes essential, but with numerous Cummins gasket options available in varying thicknesses, selecting the correct one proves challenging. This guide examines cylinder head gaskets for the popular 5.9L and 6.7L Cummins engines to help prevent catastrophic engine failure.

Thicker head gaskets serve a specific purpose beyond standard versions. These reinforced components compensate for engine block or cylinder head imperfections caused by machining or thermal distortion.

When engine components warp from excessive heat or require resurfacing during repairs, the mating surfaces may no longer seal properly. Thicker gaskets restore proper compression ratios and sealing integrity by filling these microscopic gaps.

The durable 5.9L Cummins engine offers three MLS (Multi-Layer Steel) gasket options:

• 3958644: Standard 1.18mm thickness (Grade A) for unmodified engines with no machining history
• 3958645: 1.25mm thickness (Grade B) for engines with minor resurfacing or slight thermal distortion
• 3978517: 1.688mm marine-grade thickness for harsh operating conditions

The 6.7L powerplant provides two MLS gasket choices:

• 4932210: Factory-standard 1.2mm thickness
• 5271251: +0.020" oversize (1.7mm) for engines with significant machining or thermal damage

Visual differentiation proves difficult since thickness variations are minimal. Two reliable identification methods exist:

• Part numbers: Manufacturers designate unique identifiers for each thickness variant
• Hole patterns: Subtle differences in coolant and bolt passage configurations indicate specific gasket types

Buyers must weigh several factors when selecting between original equipment and aftermarket gaskets:

• Proven reliability from manufacturer testing
• Precise engineering tolerances
• Warranty coverage availability

• Cost savings over factory parts
• Specialized material formulations
• Custom thickness options

Certain manufacturers offer customized gasket kits that combine cylinder head gaskets with complementary sealing components. These comprehensive sets simplify procurement for complete engine overhauls.

For 6.7L applications, these kits may include:

• Exhaust manifold gaskets
• Intake manifold seals
• Turbocharger gaskets
• EGR system seals

Proper gasket selection remains critical for maintaining engine performance and longevity. By understanding thickness requirements and material specifications, operators can make informed decisions to keep their Cummins engines running optimally.