Mercury Quicksilver Head Gasket 27-342731 | MerCruiser MCM 110 / 120 / 2.5L Inline 4-Cylinder

Cylinder Head Gasket for MerCruiser MCM 110, 120 & 2.5L Inline 4-Cylinder Engines

The Mercury Quicksilver 27-342731 is an OEM-specification cylinder head gasket for classic MerCruiser inline four-cylinder engines. It seals combustion pressure, coolant passages, and oil passages between the cylinder head and engine block — the three critical sealing functions that keep an engine running cleanly and efficiently.

This is the correct replacement for MCM 110, MCM 120, and 2.5L engines built between 1963 and 1989. Whether you are completing a top-end rebuild, addressing a coolant leak, repairing overheating damage, or returning a freshly resurfaced head to service, this gasket should be replaced rather than reused — a head gasket that has been torqued and heat-cycled will not seal reliably a second time.

Part Information

• Part Number: 27-342731
• Short Reference: 342731
• Brand: Mercury / Quicksilver
• Official Product Type: Cylinder Head Gasket
• Supersedes: 27-34273
• Approximate Weight: 300 g (0.3 kg)

Compatible Applications

• MerCruiser MCM 110 – 1963 to 1983
• MerCruiser MCM 120 – 1969 to 1989
• MerCruiser 2.5L – 1983 to 1989

Fitment Note: These early MerCruiser inline four-cylinder engines have decades of service history, and not every engine in this family is identical. Block changes, cylinder-head swaps, and prior repowers are common on boats from this era. Confirm fitment by engine serial number and the Mercury parts diagram before ordering — especially if the engine has been rebuilt or modified.

System & Diagram Position

This gasket is part of the engine top-end / cylinder head assembly. In the Mercury parts diagram it appears in the cylinder head section, at the interface between the cylinder head and the engine block deck. It is the primary sealing layer for all combustion chambers, coolant jacket passages, and oil return passages that cross the head-to-block joint. In the diagram it is listed as a single gasket covering the full cylinder bank.

Dimensions

Exact bore diameter and compressed thickness are not published in available catalog data for this part number. Confirm against the Mercury parts diagram for your specific engine serial number. The gasket covers the full inline 4-cylinder bore pattern for the MCM 110 / 120 / 2.5L block.

When to Replace

• Any time the cylinder head is removed — this is a single-use gasket
• Coolant loss with no external leak (possible combustion chamber intrusion)
• White smoke from the exhaust (coolant burning in the combustion chamber)
• Compression loss or cylinder pressure drop on a leak-down or compression test
• Overheating event that may have warped the head or damaged the gasket
• Oil in the coolant or coolant in the oil
• Head resurfacing or valve service requiring head removal

Service Interval

Head gaskets do not have a scheduled replacement interval. They should be replaced any time the cylinder head is removed, or when symptoms of gasket failure appear (coolant loss, white exhaust smoke, compression loss, or contaminated fluids).

Installation Notes

• Inspect the cylinder head and block deck for warpage using a straightedge before installing the new gasket — a warped surface will cause the new gasket to fail
• Clean all mating surfaces thoroughly, removing all traces of the old gasket material without scoring the sealing surfaces
• Check the head bolt threads in the block for corrosion or damage; replace any bolts that are stretched or corroded
• Follow the Mercury service manual torque sequence and specification for the cylinder head bolts — torque in stages, not all at once
• Allow the engine to reach full operating temperature and re-torque the head bolts if specified in the service manual for this engine
• Replace the coolant and inspect the cooling system for the root cause of any overheating before returning to service

Related Parts

• Mercury Quicksilver Fuel Pump Gasket 27-34213
• Mercury MerCruiser Seawater Pump Gasket 27-34897 – Pack of 2
• Mercury Sea Water Pump Gasket 27-881603001
• Mercury MerCruiser Water Pump to Gear Housing Gasket 27-85610
• Mercury Oil Filter Adaptor O-Ring 25-33756

Technical FAQ

Q: Can I reuse this head gasket after removing the cylinder head?
A: No. Head gaskets are single-use components. Once torqued and heat-cycled, the gasket material compresses and conforms to the sealing surfaces. Reinstalling an old gasket will not provide a reliable seal and is a common cause of repeat head gasket failures.

Q: Do I need to resurface the cylinder head before installing this gasket?
A: Not necessarily, but the head must be flat within the tolerance specified in the Mercury service manual. If the engine overheated or the head has been removed before, have it checked for warpage with a straightedge. A head that is out of spec should be resurfaced before a new gasket is installed.

Q: What causes head gasket failure on these engines?
A: Overheating is the most common cause on older MerCruiser inline fours. Cooling system neglect, a failed impeller, a blocked thermostat, or a stuck raw water pump can all lead to overheating events that damage the gasket. Before installing a new head gasket, identify and correct the root cause of the failure.

Q: Does this gasket fit both the MCM 110 and MCM 120?
A: Yes — 27-342731 is listed for MCM 110 (1963–1983), MCM 120 (1969–1989), and 2.5L (1983–1989) applications. Always confirm by engine serial number before ordering, particularly on engines that may have been rebuilt or modified.

Parts Desk Insights

Best use case: This gasket is the right call for any top-end service on a compatible MerCruiser inline four — whether you are chasing a coolant leak, repairing overheating damage, or doing a full rebuild. It is also the correct part to have on hand before pulling the head, since the gasket must be replaced regardless of why the head comes off.

Common issues: The most frequent issue on these older engines is installing a new head gasket without first checking the head and block deck for flatness. A surface that looks fine visually can be warped enough to cause the new gasket to fail within a season. Always check with a straightedge before installing. A second common issue is not identifying the root cause of the original failure — if the engine overheated and the cooling system problem is not fixed, the new gasket will fail for the same reason.