M50139 Upper Deck Counterbore Tool – Cat 3306 / 3406 / 3176 / 3408 / C15 Shim Cutter

The M50139 Upper Deck Counterbore Tool is a professional, hand-operated counterbore shim cutter engineered specifically for restoring the upper counterbore (shim ledge) on Caterpillar 3306, 3406, 3408, C15 and 3176 engine blocks. Designed for heavy-duty engine rebuilders, this low-profile tool accurately machines the cylinder liner support surface, restoring proper liner projection and head gasket sealing without requiring a large stationary machine.

Unlike universal counterbore cutters, the M50139 was developed specifically for Caterpillar engines and features a compact three-leg design that provides excellent stability while requiring less clearance around the engine block. This allows many repairs to be performed in chassis, reducing teardown time compared to larger counterbore systems.

The M50139 is supplied as the cutter Main Tool only. It accepts interchangeable cutter plates, allowing the same base tool to service multiple Caterpillar engine families by simply changing the plate. This modular design reduces tooling costs while providing OEM-quality accuracy for different bore diameters. Compatible cutter plates include:

• M10228 / PT-2400-13 / 5P-1659 – Caterpillar 3306
• ATC10225A / PT-2400-15A – Caterpillar 3406, 3408, C15
• Additional cutter plates available for other engine platforms using the same cutter housing.

During an overhaul, the upper counterbore supports the cylinder liner flange and establishes liner projection, one of the most critical measurements in any Caterpillar rebuild. Over time, combustion loading, liner movement, cavitation, overheating, and corrosion can damage the shim ledge, resulting in:

• Loss of liner protrusion
• Counterbore fretting
• Head gasket failures
• Coolant leakage
• Reduced engine reliability

The M50139 restores the upper deck to a true, concentric surface so the liner shim seats correctly, providing the proper foundation for accurate liner height and long-term sealing performance.

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Key Features & Benefits

• Designed specifically for Caterpillar 3306, 3406, 3408, C15 and 3176 engines
• Hand-operated upper counterbore (shim ledge) cutting tool
• Compact three-leg, low-profile design for improved access
• Often allows in-chassis repairs, reducing engine removal time
• Modular design accepts interchangeable cutter plates
• Restores accurate cylinder liner support surfaces
• Helps maintain correct liner projection and head gasket sealing
• Heavy-duty construction built for professional machine shops and engine rebuilders
• Compatible with cutter plates used on Kent-Moore Porta-Tool style systems

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Engine Coverage (With additional Cutter Plate)

Designed for Caterpillar engines including:

• Caterpillar 3306
• Caterpillar 3406A
• Caterpillar 3406B
• Caterpillar 3406C
• Caterpillar 3406E
• Caterpillar 3176
• Caterpillar 3408
• Caterpillar C15

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Typical Applications

• Upper counterbore restoration
• Shim ledge resurfacing
• Cylinder liner projection correction
• In-frame engine repairs
• Complete engine overhaul
• Fleet and machine shop engine rebuilding
• Heavy-duty Caterpillar engine service

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Why It Matters

Correct liner projection begins with a perfectly machined counterbore. If the shim ledge is uneven or fretted, no amount of shimming will produce consistent liner height, often resulting in premature head gasket failure and repeat repairs.

The M50139 gives rebuilders the ability to restore the counterbore accurately while minimizing engine disassembly. Its compact footprint is one of its biggest advantages over larger counterbore systems, making it a favorite among engine rebuilders who regularly perform in-frame Caterpillar repairs.

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