What socket size fits the spark plugs on my Yamaha outboard?
I want to check the spark plugs on my Yamaha outboard, but my normal sockets won't reach properly under the plug caps. Which size spark plug socket do I need, and does it depend on the engine model?
Answer
Yamaha outboards commonly need a 16 mm (5/8 in), 18 mm or 21 mm (13/16 in) spark plug socket, depending on the engine and fitted plug. Check the plug identification or full engine model first, because Yamaha uses different plug hex sizes across its two-stroke and four-stroke ranges.
Check the plug number
Read the identification printed on the plug's ceramic insulator, or find the specified plug number in the engine's owner's manual. This identifies the required hex size more reliably than horsepower alone. Engines with the same advertised output can have different cylinder heads and plugs across generations.
These common NGK examples show the usual sizes:
- B7HS-10 and BR8HS-10: 20.8 mm hex, normally serviced with a 21 mm or 13/16 in spark plug socket. These plug families appear on various older Yamaha two-strokes.
- DPR6EA-9: 18 mm hex, requiring an 18 mm spark plug socket.
- LFR6A-11: 16 mm hex, serviced with a 16 mm or 5/8 in spark plug socket, as used on various Yamaha four-strokes.
These examples show socket sizes, not replacement-plug recommendations. Match the complete plug designation to the exact engine specification before ordering plugs.
If the lettering is hidden, record the complete model designation and serial number from the identification plate, usually on the mounting bracket. Use those details to find the correct manual. "Yamaha 50 hp" alone is not enough.
Choose a socket that fits
Use a deep, six-point spark plug socket with a rubber retaining insert or magnetic holder. Its depth clears the ceramic insulator, while the retainer helps lift the loose plug out of a recessed cylinder head.
A normal deep socket can sometimes turn the plug, but its internal shoulder may touch the insulator before reaching the hex. Four-stroke plug wells can also require a thin-wall socket. Even the correct nominal size will not work if the socket's outer wall jams in the well.
- 16 mm and 5/8 in: normal alternatives for this spark plug hex.
- 21 mm and 13/16 in: normal alternatives for the common 20.8 mm plug hex.
- 18 mm: buy the proper size; neither 16 mm nor 19 mm is a substitute.
A 3/8 in drive socket and short extension are a practical starting point. Check that there is room to withdraw the socket and plug before fitting a long extension. Keep the extension aligned with the plug to avoid pushing sideways against the ceramic.