Price range: $5.00 through $103.00
The NGK BR9HS-10 Spark Plug, Stock No. 4551, is a resistor-type spark plug used in select Johnson/Evinrude 2-stroke outboard applications and is also commonly used in SST-45 / Formula Light racing engines.
Ideal for routine maintenance, ignition troubleshooting, powerhead service, or replacing fouled, worn, damaged, or questionable plugs.
The NGK BR9HS-10, NGK Stock No. 4551, is a resistor-type spark plug used in select Johnson/Evinrude 2-stroke outboards and performance marine applications.
For SST-45 / Formula Light racers, the BR9HS-10 is a popular plug choice because it combines the correct short-reach marine-style configuration with NGK’s colder heat range 9, making it well suited to high-RPM 2-stroke operation when appropriate for the engine setup.
NGK’s numbering system identifies the R as a resistor plug, 9 as the heat range, H as the 12.7 mm thread reach, and -10 as the 1.0 mm factory gap designation. NGK Spark Plugs
The spark plug ignites the compressed air/fuel mixture inside the cylinder.
On a 2-stroke outboard, plug condition can have a noticeable effect on:
Because the plug is exposed directly to combustion, it can also provide useful information about how the engine is running.
The BR9HS-10 is used as an NGK replacement plug in select Johnson/Evinrude marine applications.
NGK cross-reference information also identifies BR9HS-10 / 4551 as an equivalent for Champion QL78C.
NGK marine application guides list BR9HS-10 as the resistor option in several high-performance Evinrude applications using a 1.0 mm (.040″) gap.
Because Johnson/Evinrude used different plugs across different horsepower, ignition, and combustion-chamber configurations, always verify the recommended plug for the exact engine rather than selecting by horsepower alone.
For SST45 / Formula Light, spark plugs are more than a routine maintenance item.
At Liquid Revolutions, we regularly inspect plugs because they can provide a quick indication of how each cylinder is running and can help identify ignition, carburetion, or fuel-delivery problems before they become larger issues.
The BR9HS-10 is well suited to this type of use because its NGK heat range 9 is on the colder end of NGK’s scale. NGK’s numbering system works opposite of some other brands: higher NGK numbers represent colder plugs.
That colder heat range is useful in engines that operate for extended periods at high RPM, provided it matches the engine’s required specification and setup.
The NGK part number itself provides useful information:
B – 14 mm thread diameter
R – Resistor spark plug
9 – NGK heat range 9
H – 12.7 mm / 1/2-inch thread reach
S – Standard electrode design
-10 – Factory gap designation of 1.0 mm / .040 inch
This makes BR9HS-10 easy to distinguish from similar plugs that may have a different heat range, reach, resistor configuration, or gap.
The R in BR9HS-10 indicates that the plug contains an internal resistor.
Resistor plugs help reduce electrical interference generated by the ignition system.
This can be useful around:
The resistor does not eliminate the need for a properly functioning ignition system, but it helps reduce unwanted electrical noise.
The BR9HS-10 is manufactured with a 1.0 mm (.040″) gap designation.
However, NGK recommends checking the gap before installation because the factory gap may not be the correct specification for every engine that uses the plug.
For stock engines, follow the manufacturer’s specified gap.
For race engines, use the gap required by your ignition system and engine setup rather than automatically assuming the factory gap is ideal.
When adjusting the gap, move only the ground electrode and avoid placing pressure on the center electrode or insulator.
Replace or inspect the plug if you find:
Spark plugs are relatively inexpensive compared with the time spent troubleshooting an ignition problem caused by a questionable plug.
Spark plug appearance can provide useful diagnostic information, especially when comparing cylinders.
Look for differences in:
If one cylinder’s plug looks dramatically different from the other, that can help point toward a problem with:
Plug reading should be used as one diagnostic tool rather than the only indicator of mixture or engine health.
Before installing new spark plugs:
If the plug does not thread smoothly by hand, stop and inspect the threads before using a wrench.
For an SST-45 race engine, spark plug inspection is one of the fastest checks that can be performed between sessions.
At Liquid Revolutions, we use plug condition as part of the overall picture when evaluating how the engine is running. Differences between cylinders can help point us toward a fuel, ignition, compression, or cooling issue before we start replacing parts.
A fresh set of properly gapped plugs also removes one variable when troubleshooting a high-RPM miss or inconsistent engine performance.
The NGK BR9HS-10 4551 provides a proven resistor-style marine plug with the correct short-reach configuration for applicable Johnson/Evinrude and SST-45 applications.
For modified or racing engines, always use the plug specification and gap appropriate for the actual engine and ignition setup rather than relying only on a general stock application.
| Weight | N/A |
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| Dimensions | N/A |
| Package | 10 Pack, 25 Pack, Each |
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