Mercury MerCruiser OEM Pipe Plug 38407 | 10-38407 | 1/8-27 NPT Sealing Plug

Mercury MerCruiser OEM Pipe Plug — Part 38407

• Part Number: 38407
• Full Catalog Number: 10-38407
• Brand: Mercury Marine / MerCruiser (Genuine OEM)
• Official Product Type: Hardware & Fasteners — Pipe Plug
• Thread: 1/8-27 NPT (National Pipe Thread)
• System: Engine block, cooling system, and related sealing applications on MerCruiser marine engines
• Typical Positions: Engine block drain ports, cooling jacket passages, thermostat housing ports, heat exchanger sealing ports, and other 1/8-27 NPT plugged openings — confirm exact position from the Mercury parts diagram for the specific engine model
• Weight: approx. 0.05 kg

Confirm the exact port location from the Mercury parts diagram for the engine model: 38407 / 10-38407 is a 1/8-27 NPT pipe plug used in multiple positions across MerCruiser engine block and cooling system applications. The same plug can appear in several locations on the same engine — block drain ports, cooling system passages, thermostat housing ports, and related sealing positions. Verify the correct position from the Mercury parts diagram for the specific engine model and serial number before ordering to confirm 38407 is the right plug for the application being serviced.

What It Is

The 38407 / 10-38407 is the genuine Mercury OEM 1/8-27 NPT pipe plug used to seal engine block drain ports, cooling system passages, and related port openings on applicable MerCruiser marine engines and drives. It is a standard NPT tapered pipe thread plug, which seals by thread interference as it is tightened into the port. Mercury specifies OEM plugs for these positions to ensure correct material specification and thread fit for the marine environment. This plug appears across a wide range of MerCruiser sterndrive and inboard applications wherever a 1/8-27 NPT sealing plug is called for in the Mercury parts diagram.

Compatible Applications

Confirm fitment by engine model, serial number, and the Mercury parts diagram for the specific position before ordering.

• MerCruiser sterndrive and inboard engines — engine block drain and sealing port positions — where the Mercury parts diagram specifies 38407 / 10-38407
• MerCruiser cooling system components — thermostat housing, heat exchanger, and related cooling passage ports — where specified by diagram
• MerCruiser accessory and ancillary system positions using 1/8-27 NPT plugged ports — where specified

Not sure it fits? Send us your MerCruiser engine model and serial number before ordering.

Part Numbers

• 38407 — Mercury / MerCruiser OEM part number
• 10-38407 — Full Mercury catalog number

When to Replace

Replace the pipe plug when:

• Cooling water or engine fluids are seeping at the plug threads
• The plug head is damaged, rounded, or corroded to the point where it cannot be removed cleanly
• The plug threads are stripped or cross-threaded in the port
• The plug has been removed for engine flushing, winterizing, or cooling system service and shows any corrosion or thread damage
• Engine block or cooling system service procedures call for plug replacement
• The plug is missing from a port that should be sealed

Given the low cost of 38407 relative to the labor involved in engine or cooling system service, replacing the plug rather than reusing the original is a sound practice any time the port is accessed.

Service Interval

No fixed interval — inspect on condition. 38407 plugs are typically accessed during engine block flushing, winterizing, cooling system service, or engine overhaul. Inspect for corrosion and thread condition whenever the plug is removed. Replace any plug that shows thread damage, corrosion at the sealing threads, or a damaged head. In saltwater environments, corrosion at the plug-to-block interface is the most common failure mode and warrants replacement at each service where the plug is removed.

Installation Notes

• Confirm 38407 is the correct plug for the specific port from the Mercury parts diagram for the engine model and serial number
• Clean the port threads in the engine block or housing before installing the replacement plug — inspect for corrosion or damage in the female thread
• Apply the appropriate sealant to the plug threads as specified by the Mercury service manual for the application — NPT pipe plugs typically require thread sealant to seal correctly
• Do not overtighten — follow the Mercury service manual torque specification for the specific port; overtightening a tapered pipe plug can crack the housing or strip the threads
• After reassembly, run the engine and inspect the plug for seepage before returning to service

Technical FAQ

Q: What thread does 38407 use?
38407 is a 1/8-27 NPT (National Pipe Thread Taper) plug. NPT is a tapered thread standard — the plug seals by thread interference as it is tightened into a matching tapered female port. Do not install an NPT plug in a straight-thread (UNC/UNF) port, or vice versa — the thread forms are not compatible.

Q: Do I need thread sealant on a pipe plug?
Yes, for most applications. NPT tapered pipe threads seal primarily by thread interference, but a thread sealant (such as PTFE tape or a liquid pipe thread compound rated for the fluid in the system) is standard practice and is typically called for in Mercury service procedures. Confirm the specific sealant recommendation in the Mercury service manual for the engine and port being serviced.

Q: Can I reuse the original 38407 plug after removing it?
If the plug is undamaged and the threads are in good condition, it can often be reused with fresh thread sealant. However, given the low cost of a replacement plug and the labor involved in accessing these ports during engine service, replacing the plug at each service is the more reliable approach, particularly in saltwater environments where thread corrosion can develop between services.

Parts Desk Insights

38407 is one of those small parts that gets overlooked until there is a cooling water leak or a missing plug discovered during a service. The most common scenario is winterizing or engine flush service where the drain plug is removed and either lost or reused with corroded threads. Recommending a replacement plug alongside any cooling system or block service order is an easy add-on. The second most common situation is engine overhaul or gasket service where all block plugs should be replaced as a matter of course — they are inexpensive insurance against a seep that requires the engine to come back apart.