Replacing a Head Gasket on an Outboard Motor
In this video I'll remove the cylinder head, talk about head gaskets, get the head and block surfaces cleaned up and checked for warp. Then I'll install the new head gasket and get everything torqued back up properly.
Replacing a Head Gasket on an Outboard Motor – Video Transcript
Hey there Dangar Stu here. Today's video proudly sponsored by MarineEngine com is going to be about out this Yamaha 20 horsepower because it's got a few issues.
This outboard runs but it runs fast, backfires all this kind of stuff so it's not in great shape. I also noticed there was a bit of water leaking from the cooling jacket cover so what I thought I'd do is just take it all apart so to show you the sections of a two-stroke head, what happens when a head gasket goes, show you where the water goes and where it leaks from in this case from that final cover. Maybe this will give you a better idea of what's been going on in that part of an outboard. What I'll do first though is I'm going to put a compression tester in and we'll just get some compression readings for those cylinders because I haven't actually done that yet. I've got a head gasket for this time, we'll swap it out I've got it I may as well but I'll be curious to sort of get some before and after readings. First step in doing this compression test is just pulling these plugs out as usual. These spark plugs haven't been in very long at all but they already look quite different to each other this is a bit whiter, this is blacker so there's definitely something going on.
I have been keeping aside spark plugs as they come out of boats if they've got something interesting about them, you know they're burning oil, burning rich, running lean whatever so I'll keep those aside and when I've got a decent collection we'll do a whole video on reading spark plugs. For now though we'll just do a compression test, really one of the most important things of doing a compression test is make sure that you reset the gauge, you don't want to sort of have the used it previously, still have a high reading, if this outboard has a lower reading you won't get a true reading. So I'm just going to throw this gauge into the cylinder number one and turn over a few times. They say you should have the throttle open for a compression test but I've got to say I was talking Aaron once and he said ah, I don't think it's true, he said we'll go on do one with the throttle wide open, do one with the throttle closed and the readings are the same, so he was right. That cylinder, top cylinder gave us 100 which is, you know, not catastrophic but not great.
I also am a firm believer in the fact they say you're looking for a difference less than ten percent difference between all your cylinders because these aren't highly calibrated instruments, you know there's nothing to say this is 100 percent accurate but we are using the same gauge to measure both cylinders and that we can trust. So this has given us 95 so they are within that 10 percent range, that's good, maybe it isn't a head gasket issue but got new ones I'm going to take it off anyway. Now I'll just give you a close-up of the engine itself and show you where it's been leaking from what's going on. On a two-stroke you don't have any valves, any camshafts, anything like that so all we've got is the block itself. the head which is this middle section and then a cooling jacket sort of cover here. If we unbolt this we're actually separating three sections, spark plugs at the top here. they go straight through this cover plate and then into the combustion chamber through the head here. Next thing we'll do is get these bolts out, I might have a look, let's have a look over here, it looks like this bracket for mounting the ignition actually bolts into the head here so we'll have to undo a couple on the side here.
Then on top here we've got the thermostat, I think even if this isn't a head gasket problem with this motor I think it's just going to be good exercise to look at how the cooling water works in an outboard like this,and how the paint doesn't work. Next step is to take all these head bolts out, given this outboard is quite corroded I'm gonna be quite careful with these because it's pretty easy to snap a head bolt if the outboard is a bit old. On this Yamaha all the head bolts have 12 millimeter heads on them, I definitely like using hand tools for head bolts because you, at least to break them because you get a really good sense of whether they feel like they're going to snap or whether they feel like they're going to come out, after that, yeah I may be more happy to use a power tool just to extract the rest of the way, but it's nice to get a sense of how it's going to start with. It's not as critical to remove them in the correct order, I don't think, as it is tightened in the correct order but ideally you're sort of starting from the outside and spiraling in, when you tighten them I think it's really important you start in the middle and spiral on the way out. Some of the bottom ones I can't really fit a socket in so I'm just going to spanner to get those bottom ones out. They're also feeling a little bit tighter than the higher ones which is not uncommon, often you'll find it's the bottom spark plug corroded, the bottom head bolts corroded, they just get a lot more saltwater on them is what it comes down to.
But fortunately so far none of them feel like they're going to break. To make it a little bit easier to get to these bolts I'm just going to take these two ignition coils off soon. I would never work around something you can take off easily if it's in your way just move it. It might seem like extra work but I always think it saves you time in the long run. Then I use a ratchet spanner to get these bottom ones out, ratchet spanners and I sort of have a bit of a love-hate relationship with, in that I find more often than not because the head looms large or because of the ratchet mechanism you can't get them in where you need to. You've also got to be really careful with these because if you put them on a particular fastener and that doesn't have a lot of clearance against something here, if you then sort of wind its fastener out and then it gets to an end and you can't get the spanner out and you can't get fastener out, there's no way to then move it back in because it's a one-way ratchet, the only way you can move it back in is to swap it over.
So this really is the situation I was talking about. If I'm using a ratchet spanner to pull this fastener out and I get to the point where there's not enough room to take the spanner in this gap here, then I can get to the point where this, the fasteners out, the spanner won't come off, I can't wind it back in because the only way I can wind it back in is to take it out, turn it around and do this, so it's just a thing to be really careful of, because you can get yourself very stuck otherwise. The very bottom two, I can't pull all the way out, they do hit against the casing here but they're obviously all the way out of the threaded section of block so we're good to remove the head now. There are these two ten mil fasteners on this sort of ignition mounting bracket and I'll take those out because they do go into the head. There's a couple of extra fasteners that hold the water jacket cover plate onto the head but I'm going to leave those on for now because we can get those on once the heads off. Even though all the head bolts are out now, the heads quite often just quite stuck from the bit of corrosion, the head gasket, all that sort of thing so I'm just going to use a little pry bar just to push it away from the block. I usually try just to get it in a few different locations, see if you can find a part that sort of starts to give, so you're not putting too much pressure on on any one point, so that's coming away now. Somebody asked me to comment a while ago about a similar thing if the actual power head stuck on, I find prying it just gently work your way around, eventually you'll see one section will lift and then you can still work from there.
If it doesn't come easily at all or even with a fair bit of effort just double-check you have a Mr. Fastener somewhere because it's easy to do. There we go. Let's see, now we pull this off. Interesting, you can just see how different these cylinders are, this is quite black, quite oily, this top line is quite clean. Before we look at their head itself this is what the piston tops look like. I'll just go over this, so at this moment we got cylinder one close to top dead center, I'll just go and see and reach from here. Okay, we got cylinder two up, looks like there's a big chunk of something stuck up here in the water jacket. Hmm, nothing solid just salt and aluminum corrosion. Alright, so we can just take these bottom two bolts out now, I couldn't get them out while I was in situ, what I'll also do now is take out these last two ten millimeter fasteners so we can take this cover plate off from the head where the cooling jacket are. I'm not 100% sure that the actual head gasket had a leak in it but I do know for sure this cover plate was leaking.
Once again it's not going to fall off because of all this corrosion, I'm going to need to pry it off. These two ten mil fasteners are the cover plate for the thermostat so I'll take that as well. Once again there's a little gasket for the cover plate, and you can see where this outboard has been running without a thermostat in it. Running an outboard without a thermostat isn't great,, it's better than running an engine with a thermostat that stuck closed, you know, no question there, because I'd rather have it running too cool than having an overheating, but if it's running too cool because of the absence of a thermostat then you're not going to get sort of complete combustion it'll carbon up a whole other stuff like that so definitely the best option is to have a good working thermostat in the engine all the time. When a head gasket truly blows, it doesn't necessarily, you know have any sort of catastrophic failure but you will just see sections where combustion gases have been crossing oil cooling water, whatever and this doesn't seem to have that, it seems okay in that sense.
But you can also see it's got a lot of corrosion, a lot of salt all over it so I think it's really good time to replace it anyway. The compression test we did sort of verified the fact there wasn't an obvious failure of one, one cylinder so I'm almost thinking this is probably a bit of a ignition or a carburetor problem that was causing this. Having said that, now I've got this off I'll be replacing the head gasket here and then the gasket that covers the water jackets here. So I'm just going to once again try and find an edge that I can pry up here, here we go, just so we can get under it, and then struggle, start trying to get this head gasket off. There we go. Changing a head gasket on a two-stroke outboard isn't anything to really be that freaked out about, changing the head gasket on a car can be much harder, you've got you know sort of all your timing gear, timing chains, inlet manifold, exhaust manifold all sorts of things that you need to take up before you can even get to it. But on an outboard like this it's pretty simple.
Once you pull the bulk of the gasket off you can see here if I just sort of start scraping there's lots of material left behind so I'm really looking to get this back to a clean surface before I put the new one on. So now I'm just going to go and scrape all the rest of this off and make a huge mess. Once I scrape the bulk of the material off with a scraper like this I'm just going to use a wire wheel like a bench grinder just to clean the surface up a little more. As you can see it's getting pretty cleaned up now, the wheel on there is a sort of brass wheel so it's quite a soft metal so it's not really damaging the surface here, but it gets all that last little bit of gasket material off. This is the outboard I was going to look at quite a while ago, when I did that remote starter video where the water pump had pretty much just melted, I had an impeller kit but even the housing had melted so I'm a little bit worried that this is overheated to such an extent that this head is now walked so I'm going to use a straight edge, just put it across various angles to see if this does have any sort of tap or bowl or straight edge so I guess we'll just have to use this one. The idea is that you just lay it across the surface of the head and you're looking for an area where it's touching at the edges but not touching in the middle. If you look up in the service manual for a motor and you look at how flat the head of a motor needs to be to be serviceable, it'll give you a particular maximum deflection and that's a measurement you can get the right feeling gauge for.
The idea is that as you lay your straight edge across the head you shouldn't be able to slide a feeler gauge between the head and the straight edge, anywhere along, if you can and you've got the correct feeler gauge for the specification then the warpage of the head is exceeding the specification and it'll need to be machined. In this case it actually doesn't look too bad so that's good news given the potential they've had over heaters. So you're looking at running a few diagonals, a few different parallels, looking for any obvious problems but this one looks okay. Unfortunately cleaning this up is only a quarter of the picture when you clean the other side here the other side of the cover and then the block itself. I've got both sides of the head and the cover plate cleaned up now, obviously you can't get the block itself to the bench grinder so I'm just going to use a brass brush on a die grinder to clean them up. Now all the old gaskets cleaned off the water jacket cover and the cylinder head. I'm just going to spray both of the gaskets with a bit of polymer and let it set for about ten minutes then we'll put back together. While that polymer dries I'm going to put these head bolts onto a wire wheel, clean the threads up and then go and see if I can find some torque specs for them. The polymer is dry enough now and as far as I can see we're looking at tightening these bolts in two stages forty Newton meters the first time then twenty-seven the second time. Because they're not symmetrical these head gaskets can only go on one way, same thing goes with the water jacket cover.
The water jacket had a couple of small bolts that held it on, so I'm just going to put these in and do them up, just to keep it all aligned and then we'll put the main head bolts through. Before I put each head bolt in I'm just going to put a little bit of oil on the thread and onto the head that way when you torque them up you're actually getting the right amount of tension you need for that torque setting, if they're binding up the torque wrench will click early. I'm also going to put these bottom two head bolts in now, because you can't actually get those in once the heads in place. Now that's in, I'm just gonna run along and just loosely put the rest of the bolts in. Now they run down lightly I'm going to torque them up in the standard sort of start the middle and spiral to the outside pattern to 40 Newton meters, these are also numbered on the head so just tighten them up in the order the numbers on the head tell you to. Sorry about the rain, hope it's not too loud. I can't get to the bottom two bolts with the torque wrench, without moving the power head and I'm not going to do that tonight I'm running out of time. So what I'm going to do is tighten by hand but use the torque wrench on the other bolts, it's been a reference to roughly the ballpark, ideally review something that's roughly the same length as the top right so you get a feel for comparing apples to apples.
What I'm going to do in this case is use a little terminal spanner but I'll put a second spanner on the end so that I've got a handle roughly the length of the torque wrench, except you get a feel for how tight it really is. To make the spanners longer all you will need to do is use this end, is take the ring end of another spanner and pop it on there and you kind of lock them and I'll end up with something that's roughly the length of torque wrench. That was starting to feel very similar to the others when the torque wrench quit, so I think it's in the ballpark. Now I've gone up to 27 Newton meters, I'll do the same thing again. Alright, gonna call that good. Now I'm going to put the ignition coils, the thermostat, all that kind of stuff back in and we'll see how it runs. I realized on the weekend when I went to edit this video up that I didn't really wrap it up properly, so what I'll do is, just do some compression tests again so we can get to, the after readings and then I'll just start it quickly so you can have a listen what it sounds like, it still runs terribly so it hasn't solved the problem but next week we'll look into the timing and all sorts of other issues that might be causing this backfiring, all this sort of thing.
Another point I should make about these compression tests is the ignition coils don't really like to be, have power supplied to them when they're not connected to ground, so just make sure you have your lanyard out so you've sort of essentially got the engine with the ignition turned off, that way you're pulling it over, the pistons are turning but you're not getting spark so you're not going to damage your coils at all. So that's up to 110 now slightly higher than it was. That one's about 105, so it's not a huge increase but it's one thing we can rule out and it's no longer leaking water so that's another problem solved. What I'll do now is fire it up, let you listen to it, but next week we'll actually have a full video on diagnosing the problem. Shouldn't have said that should I, now if I can't figure out what's wrong with it you're going to know when I don't do a video. I put them up all workshop. I'm going to put a fan On, go outside for a while. We'll wrap this video up here, I'll definitely be revisiting this outboard next week to start with diagnosing what's going on, I think we'll start by looking at how much oils in the fuel to start with but we'll go on from there, definitely be looking at some timing stuff. That really fast idle you can see, seems to be because the timings too advanced, so I'll be looking at how that's adjusted, I think there might be an issue with some of the springs, the return springs on it so we'll have a look at that whole section.
Hopefully this video gives you an idea what's involved in changing the head gasket on a small two-stroke motor, as you can see it's not a huge job, it can be done with the power head in situ, taking it off would let me get my torque wrench to those bottom two bolts but I think if you use a lever it's the same length as your torque wrench and you've had a really recent reference feel of what the torque spec is you can use it get it pretty close, they're pretty low tension on these heads as well so I think that kind of makes a bit less critical than some of the really high tension heads you might get on larger motors. Alright, well thanks again and I'll catch you next week.