Outboard Motor Spark Plugs

Everything you wanted to know about spark plugs plus more, except if there is something you wanted to know that isn't in here. In that case it is almost everything you wanted to know about spark plugs.

Dangar Marine

About Dangar Marine

MarineEngine.com proudly sponsors the Dangar Marine YouTube Channel. Our friend Dangar Stu draws on his experience as a commercial boat skipper and mechanic to make some great how-to videos. Dangar Marine videos cover a range of relevant topics in outboard repair, boat repair/upgrades and seamanship. Subscribe on YouTube to be notified when new videos are released.

Outboard Motor Spark Plugs – Video Transcript

Hey there, Dangar Stu here, Today's video is about the humble spark plug and is proudly sponsored by MarineEngine.com.

Before we get started, I've got another viewer t-shirt photo. This his one here is Paul Thomas from Marlow in the UK and this is taken at the Southampton Boat Show, so thanks for flying the flag there.

Alright, so I'll go no to the bench and I'll show you some of plugs I've got there. As you all no doubt know, spark plugs are there just to ignite the fuel air mixture that's in the cylinder. Spark plugs are a pretty simple construction, but we'll go through it sort of top to toe. It's worth understanding them because it can help you a lot with spark plug selection, also understanding what's wrong with a plug itself, and then also help you diagnose how a motor is running. As I said a few weeks ago. I've got a handful of old spark plugs that have failed in interesting ways but to start with we'll just go through a new spark plug and sort of do the basic anatomy of it. At the top of the spark plug you've got what's called the terminal which is where the HTE lead connects from the ignition coil, and you've got two main styles. This sort of push on style and then a threaded one here. Often this threaded style will also be a push on connector so it just slips on other times on things like say lawnmowers and that, you'll have a ring connector that goes over here and a nut that goes down the thread. Often spark plugs that have this thread will also convert, so there's just a screw cap to change to the push on style as well.

So if ever you got a spark plug like this and you need the threaded type just be aware that you may be able to screw this connector off and get the style you need. This terminal at the end here goes all the way through the plug and then ends here, at the very tip here, and it's insulated from this outer casing, this threaded section, which is ground by the porcelain insulator, this section here. You can see also the porcelain has got these ridges on it here, and those ridges are designed to increase the effective length of the spark plug. So if you imagine the surface area of the porcelain, if it was straight it would be shorter than if you followed the path of all those ridges. So the reason they have those ridges is to increase the resistance between the terminal here, and the casing here, so that even as a spark plug starts to get dirty, you don't have electricity jumping from this center one straight to the outer casing. So those ridges are there to stop that Happening. Because spark plugs are a point on the block where combustion gases could escape, they've got some sort of seal. In this case, this is an NGK spark plug and it comes with a little crush washer there, you can see it just there, and some spark plugs actually have a taper on them and the taper is designed to go into a matching taper on the block.

So there's a couple of ways that they seal against the block to make sure those combustion gases can't escape. You'll have to excuse my dodgy diagram but it's probably a bit easier to show you the next few points using a diagram rather than a real spark plug. So these are all the basic components we've just talked about, and it's this gap here that has the spark plug firing. Generally a spark will jump from this central electrode to this outer electrode here. The main reason for that is because this Center electrode here is sort of pointy and is also the hottest pilot plug, and it takes about forty-five percent less voltage for the spark to jump from a hot pointy section to this ground electrode here, than it does to go the other way. The one exception from that, that I know of anyway, is where you have what's called a wasted spark system. So you have your coil up here, and then this coil has two HT leads to come out and go onto the two plugs.

And the two plugs fire at the same time, but on one plug this spark will jump that way, and on the other plug it'll jump that way. What that means is you'll start to see the ground electrode wearing away on one, and here the central electrode wearing away on the other. So you'll see different patterns of spark wear on a wasted spark system. The gap here, between the ground electrode and the central electrode, has to be a very specific measurement so we'll talk about that next. The bigger that gap is, the stronger the spark is, but the more energy the high-voltage takes to jump the gap. So, if it gets too large for the amount of energy you're putting in, the number of volts you're getting, you might get a spark that's intermittent or not happening at all.

And if you have the gap really small, you might get a very consistent spark, but that spark may be too weak to ignite the fuel mixture. All spark plugs regardless the manufacturer have some sort of part number. And this part number actually gives you quite a lot of information. On these NGK plugs, the last number is the distance the gap comes when you buy that spark plug off the shelf. So you can see here with these two spark plugs, one has a ten, which is a one millimeter gap and the other is an 11 so it comes with a 1.1 millimeter gap. As a spark plug gets used that gap tends to open up over time so you might find that you need to re-gap it every now and then. Whether you re-gap a spark plug or replace it, depends a bit on the cost to the plug, to be honest with you I think, and the availability. Some spark plugs are really cheap, five dollars no dramas, very common, others are made more from some exotic metal and you'll start seeing these platinum plugs and they can be thirty dollars each. So you know, it really depends a lot, to my mind, if it's a cheap spark plug, it's been in for a while and you're doing a service, I'd probably just replace it. There's a couple of ways that you can check and adjust the gap on a spark plug.

So one common way is with a little tool like this, and you can see here hopefully, it starts quite thin at this end then as it rotates round it gets thicker and thicker and thicker. This particular tool has got thousandths of an inch on one side and then millimeters on the other. Another way to do it is just with the more traditional feeler gauges, just swapping between them until you find the one that gives you that sort of snug, but not too tight fit. NGK spark plugs come with this little sort of cardboard tube on the end and that stops the spark plug from bouncing in the box and slowly closing that gap up, so it should be pretty accurate when you get it, but it never hurts to check it before you install it anyway. When you do go to install them, it's important to get them to the right torque setting, and rather than actually having, like a torque wrench setting, it's done on just rotating a certain amount, and that depends whether it's a tapered one or has a crush washer. If it's a tapered fitting it just goes one sixteenth of a turn past snug, and if it has a crush washer, then it goes to about two-thirds of a turn.

So, the idea there is to thread the spark plug down until it's seated, and then just turn it either another half to two thirds if it's got a crush (washer),or 1/16 of a turn if it's tapered. Now, another important thing you can see from the part number of these spark plugs is the number in the middle. In this case you can see that this one is an 8, and this one here is a 5, so it's a B8 for this one and a BKR5 for the other one. That number indicates how hot the spark plug is. The lower the number the hotter the plug, the higher number the cooler the plug. The way a spark plug's temperature is set is by how readily the heat from the central electrode can dissipate out of the engine. So how quickly can it get from the tip to the engine block and then radiate out to the atmosphere. And what effects that is how much sensor insulation there is. So, if we have a look at the eight, which is the cooler plug, and then the five, which is the hotter plug, you can see a distinct difference in the insulation and how much that tip protrudes. So there you can see, this one here is the hot plug and this one here is the cold plug. So there's a much greater path for the heat from the tip of this plug to get to the outer casing than there is for this plug to get from the tip to the outer casing, and that's what makes them hot or cold. Having a hot plug doesn't make the combustion chamber hot, it would a tiny, tiny amount you know, but what it does is, it makes the tip of the electrode hot and you need the tip of the electrode to be hot because that's what keeps it clean.

It's a really important part of the self-cleaning process. If the tip of the electrode doesn't get hot it's going to carbon up, if it gets too hot it's gonna kind of melt, and carbon deposits will actually melt and become glazed as well. So it's really important to get it right. It isn't however, going to affect the temperature of the actual explosion inside the cylinder. Because modern engines, EFI engines, you've got thermostats keeping the call for the right temperature, you've got ECUs that are monitoring O2 sensors, giving the right fuel mixtures, they're pretty much to the point where you can get a spark plug design for a motor and it'll run nicely. There is some argument however, if you've got an older outboard so carbureted outboard, and you're doing a lot of idling, moving around just doing some fishing, you might want to go to a plug that's one or two hotter to make sure that doesn't carbon up.

But if you're doing a lot of high speed running you might want to go to a cooler plug because you're already generating lots and lots of heat. So there is a little bit of scope to move one or the other. If you're having trouble when you read the plug, which we'll talk about in a second, then you might want to consider experimenting one direction or the other. There are a few problems you can have if your spark plug is too hot or too cold though, so if it's too cold you end up with this sort of carbon, but it doesn't self clean, and that carbon is conductive so you'll end up with the current shorting from the electrode to the casing without actually forming a spark and just travel along that carbon. If it's too hot, the actual combustion doesn't get too hot in the cylinder, but the tip gets really hot and if the tip gets really hot you can actually have pre-ignition, a detonation happening, just because the tip's hot enough to actually ignite the fuel even without a spark, and then you can cause a bit of engine damage. So too hot can be a real problem. Of course the manufacturers have chosen what they consider the optimal plug for any particular motor, both from the point of view of thread, so for example if we go back to this diagram, this thread here should the thickness of the cylinder head. If the cylinder head's thinner, you're going to have the spark plug protruding into the combustion chamber and maybe actually hitting the piston.

If the thread isn't as long as the cylinder head, carbon is going to be filling these exposed threads here. So that's one measurement that's pretty critical. Obviously then, you've got things like the diameter of the thread, all sorts of measurements that are really important to get right. So getting the recommended one for your motor is a great starting place. Most people are fine generally when they're doing a service, they simply take the spark plug that's in a motor, buy the same one and put it in and that's great if it is the right one. If you're a little bit worried and don't think it's the right one, then MarineEngine.com has a really good page on their website that shows you the recommend spark plug for your motor, so I'll show you that quickly. So if you come up to the top here where it says Boat Engine Parts and then I come down here to where it says NGK spark plugs, then I select it and then for example here, Yamaha motor, and then you can see here you're simply looking at a model, so we're seeing how many horsepower it is, and model numbers, so for example, if I come down here and I've got 40x m HL, over here, and I can see it's a BR7HS10. So I know from these numbers as well now, that it's a 7 in the temperature range and a 10 for the gap, one-mil gap. So resources like that can be a really good way just to quickly find and go yep that's the plug I'm supposed to have, or oh hang on, somebody's put the wrong plug in this so instead of replacing it with the same one I'll actually get the right one.

Each spark plug manufacturer as well will generally have a little bit of a guide to reading their part numbers. In this case, this kind of grubby crinkled greasy piece of paper is one I printed off a little while ago for Bosch plugs. You can see here though it's got sort of the breakdown of the part number here and what each section means. So they can be really good if you're looking to cross- match a plug to. You're saying, oh look I've got an NGK, I want a Bosch or vice versa, then you can also have a look and say right what's the thread pitch, the thread diameter, the heat range, etc. etc. and make sure you do get one that's compatible. There is also a really good website for cross-referencing spark plugs so if you've got one and you can't get that brand, you can type in the number on the website to find the one you need. A friend of mine, Justin, pointed out during the week to where I need to speak English not Australian. When I said I was a bit crook, I'm not actually a thief I'm just not well.

Right, so where were we? We've put the plug in, we've got the right plug for the motor, we've put it in, we've got the right terminal for the type of HTV we've got, we've got it torqued up right, everything's good. Now it's been running for a while, you might want to take a plug out and just see what it looks like. Now spark plugs change pretty quickly. So if you're looking to see how a spark plug's going for your motor, the best thing is to run it at the normal operating speed you use it at, and then do what they call a plug shot. You pretty much just cut the ignition and shut it down, get the plug out. If you test at speed and then idle back to a boat ramp for ten minutes it's going to change what the plug looks like and you don't want hat. I'll bring you over now and have a look at a few of the plugs here and we'll have a look at some of these that have come out of motors. This one is pretty garden-variety, rusty so there's obviously been in a bit of water in the engine. This one here as you can see, the porcelain's actually cracked on it. So that's not good either. This one here's very gummed up with deposits which can be burning oil if it's a two-stroke or, running really sort of a lot of oil from the 4-stroke and too rich with oil.

This one here is very black, which can be running really rich, but it's also got a bit of a glazed look which can be, again, oil or maybe even running so hot that it's melting the deposits and causing them to glaze up. This one's interesting, hopefully you can see if you look at the insulator in the center between the central electrode and the outer, hopefully you can see there; See it's got a bit of a black stripe down it so it's all white all the way around except for that. Get it in the light there, you can see there's got a bit of a black stripe down it, and that can be where it's actually just arcing down the insulation. Every now and then as well you'll see spark plugs leaking around the tops here, you'll see there's carbon here and the combustion gases are blowing past here, where it's sort of delaminated from its casing, and that's not good either. This one here though is running pretty Normally, you're looking for a light brown gray sort of coating, just because it's used, no buildup, no obvious wear, there's still quite a lot of metal left on the actual electrodes, and no sort of big oily or carbon deposits building up so that's kind of what you're looking for. The NGK website also has a really good set of photos showing a particular spark plug and what the problem is, so if you're interested in reading plugs like that, I really recommend you jump on those websites.

Most spark plug manufacturers probably have it, but it's a really good chance to see a textbook photo in a description of what the actual problem is. Another thing you can see is if there's little pitting on the plug, and that's actually detonation occurring, if it's mild you won't hear it but you will actually see it on the plug. When it's bad you'll hear this kind of pinging this sort of pre-detonation sort of happening, but you can also see it. They often describe it looking a little bit like sandblasting, like little flecks of sand that hit it and you just see these little pits, so that's something to be aware of too. In that case, could be the timings out, could be low octane fuel, could be compression ratio problems, all sorts of things. I won't go through all the different parts of a spark plug, model numbers, you can download those PDFs from the manufacturers and see what they all mean. But another really common one is whether it's a resistive plug or not. Resistive plugs are designed to try and reduce the EMF or electrical interference you'll get from the spark firing, that might affect radio reception and parts of the ECU and all that kind of thing. So I'll show you the effect that has. You can also get resistive HT leads will help with that, but I'm gonna do H2 leads in a separate video. So here I've got two pretty similar spark plugs.

The only difference with these, is that the heat rage changes slightly. One is an 8, one's a 7 and that one's got an R in it, so one's a resistive spark plug and one isn't. So I'll just pop these on the multimeter for a second and show you what that means. I'll just turn the light off to avoid the glare. But here we can see the one without it and this is the resistance from the terminal here, to the central electrode. So this is a piece of metal that goes straight through the spark plug. So once we hook that up, you can see you've pretty much got zero resistance, zero point one, point two, not much. If we do the same thing with the resistive plug you can see there we're getting about five kilo ohms, and that's what the R means on that plug. It's that five kilo ohms of resistance that reduces the interference the spark plug puts out and means that you don't get the other electronics in the boat getting affected.

So in case you're wondering, spark plugs need about twelve to twenty five thousand volts in order for the current to jump the gap on the spark plug. So they need very high voltage, which is what all the coils do. Obviously, if you've got trouble with the coil and are not getting the voltage you need, even if you got a brand new spark plug, you're not getting a spark, so diagnosing ignition and spark goes well past the spark plug. But I've got another video on CDI diagnosis that might help there. Before we wrap up, I'll just mention one other tricky thing that you might find with E-tec motors particularly, is what they call spark indexing. And what this is, is that the open end of the spark plug here, needs to face the fuel injector, so as you tighten it up you can't just tighten it and have it facing in any random direction. You actually do need them facing with the electrode open in a particular direction.

The way you do that, is to mark on the outer casing which way the spark plug is facing and that way you can see that mark you can see the injector coming in so you know it needs to face that way. The instructions for doing this basically say, bring it down finger tight till the crush washer touches the block and then turn it until the index mark faces the injector. Sometimes it won't be tight enough, you've only turned a quarter turn and you won't be able to go all the way around, and in that case it says to try a different spark plug. So it's a pretty fiddly process and you may actually need more spark plugs than the motor takes in order to keep trying until you get them facing the right way. It's a shame in some ways those E-tecs don't use a face gap plug like this where there isn't an electrode on the top. There's no, as you rotate them you can see, there's no sort of orientation at all. It would have eliminated the need for indexing, but obviously the reason they don't work in that context.

These plugs were actually invented originally for the Wankel rotary engines where the piston sort of is about triangle that circulates and goes round and it was designed so that there was nothing for that to catch on. They've tended to get used as well in some two-stroke motors. They don't give a great idle but they're pretty reliable, so if you do a lot of high speed running they're a good plug to have. The other good thing about these face gap plugs is there's no actual ground electrode to wear away so they do have a pretty long life as well. They're just another interesting design though that's different to the ones we've seen so far. You also get spark plugs where instead of just having this single ground electrode you can have 2 or even 4 coming over.

So, that's pretty much what I know about spark plugs. I don't know everything, I'm not an expert on them at all, but I think there's a few basic things that's really nice to know. Because you will see different types if you do a lot with motors, and it can be good just to know what the difference is between them, whether you can swap them or not, what risks you're taking, that kind of thing. All right what take care. I'll catch you next week.