# Outboard Spark Plugs: Reading the Old Ones and Choosing the Right NGK **By Sawley's Auto and Marine** · 2026-09-09 A spark plug is the only part of an outboard that comes out of the combustion chamber and lets you look at it. Pull one after a run and it tells you, in colour, what has been going on inside that cylinder for the last few hours. Most owners change plugs without reading them, which is a waste of the one free diagnostic the engine offers. ## How do you read an outboard spark plug? Hold it electrode up in good light and look at the insulator nose, the white ceramic around the centre electrode. - **Light tan to grey.** Correct. The mixture is right, the heat range is right, the plug is doing its job. Put it back or replace it on schedule, nothing to chase. - **Black and dry, sooty.** Rich mixture or a plug that is too cold for the engine. On a carburettor outboard this often means a choke that is not fully releasing, or a lot of idling and trolling and not enough time at speed to burn the deposits off. - **Black and wet, oily.** On a two-stroke, too much oil in the mix or an oil injection system over-delivering. On a four-stroke, oil getting past rings or valve guides. Either way, note which cylinder it came from. - **White, chalky, or blistered.** The plug is running too hot: lean mixture, over-advanced timing, a cooling problem, or a plug one heat range too hot. Blistering or tiny metal specks on the insulator is detonation and needs attention before it costs a piston. - **Wet with fuel.** The plug is not firing. Fuel is arriving and not lighting, so the fault is spark, not fuel. One plug reading differently from its neighbours is the useful case. Four black plugs is a mixture problem; one black plug is that cylinder's problem, and the plug has just told you where to look. ### Reading the code before you buy The letters and numbers on an NGK plug describe its construction, and two parts of the code matter more than the rest. **The number in the middle is the heat range** — how quickly the plug sheds heat into the cylinder head. NGK's scale runs the opposite way to most other brands: a **higher number is a colder plug**. A BR8HS is colder than a BR6HS. Outboards generally want a colder plug than a car engine of similar size, because they spend long periods at high sustained load, which is why the marine listings are full of 7s, 8s and 9s. Fit what the engine maker specifies rather than what looks similar; a plug one step too hot is the quickest route to the blistered insulator above. **An R in the code means a resistor plug.** The resistor suppresses ignition interference that would otherwise upset the engine's own electronics, a fish finder, a VHF radio or a tacho. If the engine was specified with a resistor plug, fit a resistor plug. Substituting a non-resistor version because it was the one on the shelf causes faults that are miserable to trace. ### Standard, iridium or platinum? The standard nickel plug is what most outboards were designed around and it does the job well. Where iridium earns its place on a boat is fouling resistance: the fine-wire centre electrode needs less voltage to fire, so it keeps firing through the oily, low-speed conditions that foul a standard plug on a two-stroke that spends its life trolling. It also lasts a good deal longer. On an older magneto-ignition engine with modest spark energy, the standard plug is often the safer choice; on anything with electronic ignition, an iridium plug in the correct heat range is a genuine improvement rather than a marketing one. ### Gap, torque and the things people skip Check the gap on every new plug before it goes in, even when the box says it is pre-gapped. The gap for your engine is listed alongside the plug in the tables below. Thread the plug by hand until it seats, then tighten it properly — a plug that is too loose does not transfer heat into the head and runs hot; a plug that is overtightened stretches the shell and never seals again. Do not use anti-seize on a plated NGK plug; the plating is there to do that job and anti-seize on top of it leads to overtightening. Replace plugs as a set, and keep the old ones in a row on the bench in cylinder order for a minute. That row is the diagnostic. Throw them out after you have read them, not before. ### Finding the right plug for your engine We have put NGK's full marine application listing onto the site, one page per outboard make, with the standard plug, the premium upgrade where one exists and the gap for every model: [Mercury](/pages/ngk-spark-plugs-mercury-outboards), [Yamaha](/pages/ngk-spark-plugs-yamaha-outboards), Evinrude and Johnson, Suzuki, Tohatsu and the rest are all listed from the [reference charts page](/pages/resources). Every plug code in those tables links straight to the plug in our [NGK spark plug range](/collections/ngk-spark-plugs-marine-automotive-small-engine-ignition). If the plugs are telling you something you would rather not hear — one cylinder reading differently, a set that keeps fouling, a plug that has been running hot — that is worth a conversation before it is a rebuild. Ring [0412 532 984](tel:+61412532984) and bring the old plugs with you. **Tags:** Guides, Outboards, Spark Plugs --- > Source: [Sawley's Auto and Marine](https://www.sawleysautoandmarine.com.au/blogs/guides/outboard-spark-plugs-reading-and-choosing-ngk)