• Home
• Videos
• Dangar Marine
• Outboard Motor Timing Belt Replacement

Outboard Motor Timing Belt Replacement

In this video I change the timing belt and tensioner pulley on my Honda BF40 outboard.

Outboard Motor Timing Belt Replacement – Video Transcript

Hey there, Dangar Stu here, today's video is about changing the timing belt on a four stroke outboard and it's proudly sponsored by MarineEngine.com.

Another viewer t-shirt photo before we get started, This one here is Steven Wilks from Brisbane. Thanks Steven!

The outboard we're going to be doing this on is the Honda BF 40 on the green machine. It's a pretty simple job in that it's a single cam. It's a little bit more complicated if it's a V motor but not much. To do this job I bought of timing belt from Honda, so that's the part number there. I've also bought an idler pulley that I'd like to change at the same time, and that's the part number for that. Now because we've got quite a broad range in people that view, some know a lot about motors some know next nothing. I thought because of that we might start with just a few of the basics about what a timing belt does. The job of a timing belt is to keep the crankshaft in time with the camshaft.

That's why it's called timing belt. It can get a little bit confusing if you're new to motors in that you'll often hear timing either referring to ignition timing or valve timing. So normally, when they say it's out of timing it is the ignition timing. With valve timing however, particularly a simple motor like this. You're simply locking the cam and the crank together with this timing belt and they'll always stay in sync, stay in time with each other. Some motors have variable valve timing which allows the camshaft to move in relation to the crankshaft, but we don't have that here. This is kind of the simplest case really. Single cam, fixed timing. when we put this timing belt on will be having the crank at top dead center for cylinder one.

So cylinder one's piston will be at the very top of us travel and we'll have the camshaft here at the top of the compression stroke for cylinder one. The cam shaft rotates once for every time the crank shaft rotates twice which kind of brings me maybe to a quick explanation of the four-stroke cycle. I'll do this really quickly because I know a lot of you will have seen this many times before. The basic idea with a four-stroke cycle is intake valve opens, piston comes down, draws in air and fuel. All the valves closed, piston comes up, compresses the air and fuel, spark fires, pushes the piston down on its power stroke, and then as it comes back up the exhaust valve opens and exhaust gases go out. So what you can see is the cam shaft rotates once for this whole series of states from induction, compression, power stroke, exhaust stroke, etc.

But the crankshaft just goes through one up, one down, one up, one down, what that means is the crankshaft actually doesn't know whether it's on a compression stroke or an exhaust stroke. It just keeps doing these up and down cycles. I find that knowing that stops people getting confused about the fact that they're putting the crank shaft onto top dead center like am i putting on to top dead center? do I have to rotate it one more time to get at top dead center for the compression stroke instead of top dead center for the exhaust stroke? And the answer is no, just you need the piston at the top done once we have a crankshaft at top dead center for cylinder one, we're going to turn our camshaft to top dead center on the compression stroke for cylinder one. Then we put our belt on, they're locked off, we check it, etcetera.

So now let's go through physically how you do it. There's one more bit of theory, actually it's worth knowing before we push on. And that is that there are two fundamental types of engines when it comes to valve clearance. And that's an interference engine, or a non-interference engine. With an interference engine, in order for a valve to open the piston has to be on its way down, and that does happen when all the timing is linked in normally. But if a timing belt should break, then the Pistons will keep coming up and down but the valve to stop. So a valve that's open will actually get hit by a piston and break it or bend it. With a non-interference engine, the valve can be open a piston can come up and they won't actually touch.

So if you break a timing belt on a non-interference engine you won't actually damage anything. you can just put a new one on. If you break a timing belt on an interference engine there's a really good chance you are going to do some damage. Now even if your belt hasn't broken, it's worth knowing this because what it means is that you can't have the belt off and start rotating the crank and not expect things to hit internally. You probably won't do as much damage because it's not moving at speed, but be aware it can happen. Most motors have a cover over the flywheel like a plastic cover so you're going to have to take that off. If it's a pull start outboard then it's also a recoil mechanism on here. in this case we don't have any of that. no pull start, no plastic cover.

Okay we'll start taking the old timing belt off now. First thing I need to do that is take the flywheel off. These flywheels are a little bit different to many others, in that it has four 14 mil bolts. Others will just have a single larger bolt. I'm gonna use an impact gun to get these off. shouldn't be too bad, if you need to get a larger top bolt off and you don't have any impact tools chances are you're going to need to lock the flywheel off somehow. So there we go, flywheel off, fixed magnets inside. If you have a single larger nut on top of the flywheel it's going to be on a tapered shaft, probably with a Woodruff key. Which means you will probably need some sort of a puller to lift it off, it won't just pull off the way that one did. Puller kits aren't particularly expensive to buy.

You Can get them from an auto parts store. And the idea is, you have a large threaded section that goes through what I guess is the puller, then it'll have some sort of tip that goes on that gets centered in indent in the top of the crank shaft. And then you'll put whatever bolts match holes in your flywheel, Put three around then as you tighten the bolt in the center here, you'll pull the flywheel off. If we have a look now, we can see the timing belt comes under this coil here, so I'm going to take this one off, and then we're also going to take this lift eye off as well. there's a little bracket here that retains the wire for this coil. I've taken this up as well. it was stopping me from lifting this coil out of the way.

With this system here, we can't just put the nut back on the shaft and rotate the crankshaft using a standard tool. so what I'm going to do is actually cut a small bit of metal, drill this bolt pattern, so we can make a bit of a custom tool. to make this tool I'm just going to pull the dowel out from the top here. And then I'm just going to take a rubbing of the top of it to get the bolt pattern. you can actually see the bolts on this side, but the grease on the other side actually gives you a clearer view. So I'll use this, and we'll go and drill it out on the plate. I've cut the paper dead level with the edge of the holes to make it easier to Center on the middle. Taped it on, so it can't move.

Now I'll just punch the centers. One last thing before I drill, is just to go and find the diameter I need to drill. So these bolts are 9.8 millimeters in diameter, 10 mil bolts. I'm gonna drill a 10 mil hole I don't want too much slop in it. I can always drill it out to ten and a half I need to. Just a quick test fit to make sure it's all good, which it seems to be. And now I'm just going to weld your bolt head on to here. Now we've got a larger hole on the other end that I can fit our bolt through. So my plan is to cut this bolt just off to the thickness of the metal, and then weld it on from the back.

Okay, bolt shank cut to the thickness of the metal. So I've loaded it up with heaps of filler metal, but kept it reasonably flush still. So obviously very hot, we'll let it cool down, then we'll give it a whirl. Here's the finished product four holes, and 19 mil bolt head on the other end so we can attach this on and start using that to rotate the crankshaft. Before we start rotating the crankshaft though I'm going to take a spark plugs out. Having them out gives no compression, so it's easier to turn and on top of that when you have compression, the piston will come up it's compressing the air and them against top dead center that air or sort of want to push it down again so it stops you're having that thing we're just getting close the next thing it falls off top dead center.

So a couple of good reasons to take them out first. These Honda spark plugs are an 18 Mill socket. This particular socket I got which is quite thin walled because there's not a lot of room in there. And it was from, Forgot my glasses, MP it's got a part number on there. Hopefully you can read it. It took me quite a while to find a socket that worked well with these out boards. All right, we'll install our tool, just with the bolts that held the flywheel on. This bit of metal is obviously thinner than the flywheel, so we'll see how far these bolts go in I don't want to be levering against the thread, because these are the bolts I need to use later on. If they don't go in, we might need to find some other bolts. Okay, there was plenty of depth to those threads, so we're all good to go there.

When you're putting a puller in on the standard flywheel, be careful with the depth of the threads though, because the bolts can actually go right through and damage the coils beneath. Just something to be aware of in that situation. now we've got the plugs out, our old belt still on, tension is still on. We can start rotating the crank shaft using this tool. what I'm going to do, is rotate the crank until the timing mark for the cam gear is lined up, and I'll show you that. On the Honda's this is the mark for the pulley itself. And there's the mark on the block we're lining up. So once those two were lined up. The cam is in good position. It seems that on the crank side, it is the dowel that I removed that's underneath this tool now. So what I'm going to is and take it off, confirm it's at the front edge here, so it should be in line with the bolts along here. If so, I'm actually gonna mark it on the tool and then we'll put the tool back on.

All right, so it's in the right position, which makes sense given the motor was working. but the dowel is in this direct line with the bolts. And hopefully you can see down in here, there's a little triangle protrusion below the pulley. And that's what you're lining this dowel hole up with. All right little improvement to the tool, I just ground a little notch out of it, put some yellow paint in it. now when I put the tool on I'm going to put that little notch with the yellow paint where the dowel hole is on the crankshaft. Because this sits underneath a little bit, I'm also actually going to take both these coils off, just so it doesn't sort of snag on them as it goes around. All right, we're taking a little bit of a slow road to Rome. But I wanted to check a few things on this motor first. i.e. that all timing is correct to start with. And also to cover a few situations where you might not have that may timing marks.

The reality is if the motor is running nicely, you don't really need any timing marks. You just need to make your own and then make sure it goes back exactly the same as it was. You don't need to actually have everything sitting at top dead center. You just need to make sure that the relationship between the crankshaft and the camshaft doesn't change. If for whatever reason you don't have timing marks you can use like this outboard has. There's a few ways you can go. To find top dead center on the crankshaft, all you need to do is have a look down the spark plug hole for cylinder number one. Then watch the piston coming up and down when it's at the very top as travel, that's sort of top dead center.

But, I have to say, as the piston comes up, the amount of travel relative to the degrees the crank shaft rotates decreases. So it's actually not super accurate. But, if you're stuck, it's one way to go. With the camshaft, you can have a look at the way the cam lobes are pointing. I'll quickly take this rocker cover off and show you that. What you can hopefully see is this cam lobe here, the exhaust side for cylinder one is pointing down, with the flat section against the follower, and there's a little bit of a gap on this rocker. And same on the exhaust side, it's coming here and it's going down which means there's no pressure on this rocker so both these valves here are completely closed. Hopefully it's easier to see on the board here, but the situation that our outboard's in at the moment is top dead center on the compression stroke.

If you imagine looking straight down on the outboard from above, this outboard somewhat unusually rotates counterclockwise. And then we had this sort of cam follower sitting just here. Both the lobes were pointing away towards the crankshaft when we're looking at it, and it means that both valves are closed. And we know that's the compression stroke because of the way the lobes are pointing away. If both lobes are pointing towards the follower, like this, it's also top dead center. The cranks are top dead center but it's top dead center of the exhaust stroke, and in that case there's actually a slight overlap we've got the exhaust closing here the exhaust past because the motors turn this way, the exhausts comes past having the exhaust valve fully open. The intake valve is starting to open, but the exhaust valve hasn't fully closed. So there is a little bit of overlap on the exhaust stroke to aid scavenging and getting the gases to flow. When you rotate a motor it's good to rotate it in the direction the motor naturally goes, in this case counterclockwise. actually guys, rotating it clockwise briefly before, it's not gonna kill it.

But it's not great for it just because the cam lobes everything were in the direction it's used to going. So I'm gonna set this back up on top dead center using the timing marks rather than looking at the cams or anything because they're here. Next thing I take off is the tensioner pulley. The tensioner puller has got a spring here and it's slotted, so if we take this little cover off you can just see under here that the bolt goes through a slotted hole so you can have tension put on and locked off. So we're gonna undo this nut, looks like another 14mm, and get the spring Off here so we can get some slack in the belt. Taking this off is just a matter of unhooking it. You can see there it's just to hook at both ends, and the longer end was going towards the tensioner pulley.

This bolt here is holding the tensioner pulley in place. if I wanted to move the pulley I could just back it off. In this case we want to remove it so I'll take it out completely. So there we go, Just comparing the parts, making sure they're the same, and correct. So we'll pop the new one on. Now the tensions off the belt, it's really easy to take off. And there's our old one. Signs of the belt going bad is this just belt powered rubber, powder you can see, it's sort of been grinding away a little bit so if you see a lot of that powder around the place you know it's bit to your belt coming off. sometimes that can mean there's a bit of a misalignment of bearings going on, something like that. but it also happens with age. this belt is actually not too bad.

But if you ever see any cracking, any real wear on the edges whatever, that's most definitely time to swap it out. Just going to quickly check this belt for length before I open it. Makes it easy to return if it's wrong, but looks spot-on. I'm gonna pop this belt on the crank pulley first. As a counterclockwise turning motor, this is the length of the belt that does all the work, this is actually the section that goes around the tensioner. So this is where we want to get it in the teeth here, and then really we'll align with the teeth here with quite a bit of tension across here. we don't want to put it in sort of one tooth of slack so it's loose here, we want all that slack to be on this side where the tension is going to go. All right, what I'll do now is drop the new tensioner in.

There's a little dowel here you can see. With this particular outboard it turns out to be easier to get the tensioner pulley onto that dowel there. Keep tension across this side, get as much slack as we can around this side, and then just slip the belt over. Now I'll put the bolt back in for the tensioner. I'm gonna leave that loose for now and put our spring back on. Next thing, I'm having a quick look for, is whether any of my timing marks have moved. Have these pulleys got bumped or anything. I didn't notice I'm getting bumped and they certainly seem dead on their mark still. So I think we're pretty good to go. The manual actually says then to turn this a few teeth in the opposite direction, and then torque this up to 45 Newton meters. Like all belts, chains, if they're really tight if they're too tight they can put a lot of strain on the bearings at the top of both these shafts. Now what I'm going to do, is put my spanner back on this little homemade tool, rotate the crankshaft few times, then double check the timing marks. looking straight down here, this timing mark looks alright, but if I look at the mark here and then the mark down here, the crank is actually further back than it should be.

So I'm going to take the tension off and move the belt one tooth. So I'm going to move this one tooth this way. Now it's going to rotate three or four times by hand so it will remove our custom tool. All right now I'll put my coils back on. Be really careful when you're putting things like those retainers back on because if a wire gets pinched somewhere and you get a little break or a short in it, it can be a nightmare to try and track that fault down later. The new tensioner pulley didn't come with a little rubber cap so I'll use the old one. Last thing to do now is put the flywheel back on.

So I'm just gonna put the little dowel that went at the top of the crankshaft in. you just wanna run these flywheel bolts down with the gun, then we're going to torque them up to 65 Newton meters. When it comes to torqueing these up to their Correct spec, Honda actually sells a tool that goes where this hanger goes and locks off the flywheel. We don't have one of those, so we'll think of something else. To torque these up here, I'm just going to put a pry bar in one of the teeth on the ring gear, and leaver it against the mount for this lifting point. Now the fly wheel is on.

There is actually another timing mark on these Honda's. I'll show you, the second timing marks on the flywheel, we can see that blue, and you can see it's offset from the original one used to put the belt on. And it's lined up with the mark on this mounting part for the lift point here. So we're lining this edge here, with this one here, and then our timing marks down here. Okay, we're pretty much ready to go now I've got the flywheel back on, everything's torqued up to spec, belts on, timing marks still line up after a few more rotations. Tension on the belt feels good. Not too Tight, not too loose. All we need to do now is put the spike plugs back in, and fire it up. Actually before I drag this outside and fire it up I've actually got to weld up a bit of a crack. I'll show you that. You can see here, there's a crack where can see the light shining through.

So I'll weld that up before we take it outside. A little bit hotter I think maybe but, hopefully won't leak anymore. All right, let's fire it up! All right I think we can call that good, let's head inside. That's probably more than you ever wanted to know about timing belts but hopefully it gives you some ideas if you need to do this job yourself. A few things in this Honda aren't very common, most outboards have a single nut on top of the flywheel, and a tapered shaft, so you'll need a puller for that. and more commonly outboards turn clockwise not counter clockwise. I did this particular video today because I'm waiting for parts on the Evinrude now, as well as plywood for the boat build. So both those projects are on hold. I'm going to head off now put this boat back in the water, so catch next week. See ya!