Fuel Pump Diaphragm Kit – Fits Mercury/Mariner 35-70HP (Replaces 55278A4)

$6.29 $10.00

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Product Details

This Fuel Pump Diaphragm Kit is a direct replacement part for Mercury and Mariner outboard engines, specifically 2-stroke models in the 35-70 HP range manufactured between 1977 and 1990. It is engineered to restore original fuel system performance and reliability.

It serves as a direct interchange for several part numbers, including Mercury 55278A4, Mercury 55278A3, GLM 40340, Sierra 18-7809, and OMC/Johnson/Evinrude 0508428.

Engineered for direct replacement of the original equipment.
Designed for use with standard gasoline fuel systems.
Includes one complete diaphragm kit for a full repair.

Part Number Matrix

Use this matrix to confirm equivalent references across OEM and aftermarket brands.

Brand	Part Number
Mercury	55278A4
Mercury	55278A3
GLM	40340
Sierra	18-7809
OMC/Johnson/Evinrude	0508428

Engine & Model Compatibility

Confirm your engine model and year in the table below. While this kit is listed for some OMC models, specific compatibility data is limited.

For Mercury/Mariner engines, verify fitment using the model, year, and serial number ranges provided.

Brand	Model	Rated HP	Stroke	Years	OEM Part No.	Notes
Mercury/Mariner	KC-4A, 8A, 9A, 9B, 9C, 13A	35	2-Stroke	1984–1989	55278A4
Mercury/Mariner	WMK-38	35	2-Stroke	1984–1989	55278A4
Mercury/Mariner	WM-15	50	2-Stroke	1986–1990	55278A4	3 cylinder models
Mercury/Mariner	WM-13, 13A	60	2-Stroke	1984–1990	55278A4	Except models with square fuel pump
Mercury/Mariner	WM-6, 8, 9, 11	70	2-Stroke	1977–1983	55278A4
Mercury/Mariner	-	35	2-Stroke	-	55278A4	Serial No. 6445653-B110788
Mercury/Mariner	-	50	2-Stroke	-	55278A4	3 Cylinder, Serial No. 6428681-B122929
Mercury/Mariner	-	60	2-Stroke	-	55278A4	Serial No. 6428681-B122929
Mercury/Mariner	-	70	2-Stroke	-	55278A4	Serial No. 4571652-6428680

Technical Specs

Product Type: Fuel Pump Diaphragm Kit
Installation: Direct replacement for original equipment
Fuel Type: Gasoline
Dimensions: 6.38" × 4.0" × 1.13" (162.1 × 101.6 × 28.7 mm)
Weight: 0.03 lbs (13.6 g)

What’s in the Box

1 x Diaphragm Kit (Replaces 55278A4)

Installation Notes

Disconnect the battery and ensure the engine is cool.
Relieve any pressure from the fuel system. Place a rag under the fuel pump to catch spills.
Carefully disconnect the fuel lines from the pump.
Remove the mounting bolts securing the old fuel pump assembly.
Disassemble the fuel pump to access the old diaphragm, noting the orientation of all components.
Remove the old diaphragm and clean the housing surfaces of any residue.
Install the new diaphragm from the kit, ensuring it is correctly seated.
Reassemble the fuel pump and torque the screws to the manufacturer's specification.
Mount the pump back onto the engine and reconnect fuel lines.
Prime the fuel system and check for any leaks before starting the engine.

Troubleshooting & FAQ

How do I confirm this kit fits my specific engine?

The best way to ensure fitment is to check your engine's model, year, and serial number against the compatibility table. Also, visually compare your existing part to the product images. If your model is in a serial number break, it is critical to confirm which part your engine requires.

What are the signs of a failing fuel pump diaphragm?

Common symptoms include the engine hesitating or stalling, difficulty starting, rough idling, loss of power at high speeds, and visible fuel leaks from the fuel pump weep hole. A torn diaphragm can also allow fuel to leak into the crankcase.

How often should a fuel pump diaphragm be inspected or replaced?

There is no fixed replacement interval, but it's good practice to inspect the fuel pump and diaphragm during annual service. Due to modern fuel additives, diaphragms can degrade over time and should be replaced as a preventative measure every 5-7 years or at the first sign of failure.

Is it difficult to replace just the diaphragm?

For someone with basic mechanical skills, replacing the diaphragm is a straightforward task. It involves disassembling the fuel pump housing, swapping the part, and reassembling. However, careful attention to the orientation of gaskets and check valves is crucial.

OEM vs. Aftermarket?

This listing is an aftermarket replacement (not OEM). Built to OEM-level specifications with comparable materials. Manufactured in an ISO 9002 facility, batch-inspected for consistency.

Shipping speed & cost?

We ship worldwide quickly—typical transit 5–15 days. Standard shipping is free; expedited options appear at checkout where available.

Warranty & returns?

Covered by a 1-year warranty against manufacturing defects under normal use. If issues arise, contact support with your order details. Returns are accepted within our standard window (unused & in original condition). See our policy page or reach out for instructions.

Safety & Important Info

Verify images/specifications vs your original part. If anything differs—even if your model appears compatible—do not purchase.
This is an aftermarket replacement, not an OEM component.
Professional installation is recommended for safety and performance.

About Sea Sierra

Pro-Grade Standards — We supply marine parts engineered to OEM-level tolerances and tested for real-world conditions.
Fast Global Fulfillment — Free standard delivery to most regions in 5–15 days.
Manufacturer-Direct Value — Fair pricing without compromising on materials or QC.

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