The Mercury / Quicksilver semi-surface gap spark plug is a proven OEM-style plug used across many legacy Mercury Marine and Mariner 2-stroke V6 outboards. You’ll find it referenced in parts catalogs under several Mercury/Quicksilver identifiers, including 13686Q, 13686, 13686 2, and the updated number 8M0176682.
This plug family is commonly associated with classic Mercury V6 two-strokes in the 2.0L, 2.4L, and 2.5L displacement groups and the horsepower ranges people search most: 135 HP, 150 HP, 175 HP, 200 HP, and 225 HP. It’s also frequently tied to performance model naming, such as XR2, Black Max, XR4, XR6, and 225 Pro Max platforms.
What “semi-surface gap” means and why it matters on a two-stroke
This Mercury/Quicksilver plug uses a semi-surface gap (surface discharge) design intended to reduce plug fouling in real marine conditions.
Two-stroke outboards often idle, troll, then immediately transition to load and RPM. Carbon buildup and oil residue are normal realities on many legacy two-strokes, and a surface-gap style plug is designed to help resist the deposits that can short a conventional J-gap plug.
In practice, this design supports more consistent ignition during low-speed operation and helps the engine clean up faster when you roll into the throttle—especially on older carbureted and EFI V6 platforms where plug condition can change quickly with idle time and fuel/oil mixture.
Common Mercury/Mariner platforms and search terms where it’s used
Owners most often encounter these plug numbers while servicing legacy Mercury/Mariner V6 engines in the 135 HP, 150 HP, 175 HP, 200 HP, and 225 HP groups, including popular performance and sport families such as XR2, XR4, XR6, and the Mercury 225 Pro Max. Many of these engines fall under the broader 2.0L, 2.4L, and 2.5L two-stroke categories.
Because “fits” can vary by year, ignition type, and ECU calibration, the most factual way to confirm is: if your Mercury parts lookup calls for 13686Q, 13686, 13686 2, or 8M0176682, this is the correct Mercury/Quicksilver spark plug family.
Technical specifications (shop-reference format)
These are the common service specs for this Mercury/Quicksilver semi-surface gap plug style used on many outboards:
Thread diameter: 14 mm (M14 x 1.25)
Seat type: gasket seat
Reach: 1/2" (12.7 mm)
Hex size: 13/16" (21 mm)
Center electrode: nickel
Design: semi-surface gap / surface discharge
Gap note (important)
Because this is a semi-surface gap design, there is no need to gap these plugs like a conventional gapped plug.
Installation best practices for Mercury V6 two-strokes
Correct installation supports heat transfer, reduces thread damage, and improves reliability:
Start the plug by hand to avoid cross-threading in the aluminum cylinder head. Seat the gasket properly, then tighten to the correct specification for a gasket-seat plug. Use a quality 13/16" spark plug socket and avoid over-tightening, which can distort the shell or damage threads.
Troubleshooting and plug-reading basics (useful in the real world)
On a Mercury V6 two-stroke, spark plug condition can quickly reveal cylinder-to-cylinder differences. If one plug repeatedly looks different—wet, unusually clean, peppered, or much whiter/darker—don’t just keep replacing plugs.
That pattern can point to fuel delivery imbalance, ignition strength issues, reed problems, or sealing/air leak concerns. Addressing the root cause early is cheaper than chasing symptoms after the engine has been stressed.
OEM Part Numbers (for ordering and cross-reference)
This Mercury/Quicksilver spark plug is commonly listed as: 13686Q, 13686, 13686 2, and 8M0176682.
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