Yamaha 8HP 4 stroke OB. Change timing belt?

[ProDave]

We have an Yam 4 stroke 8HP OB that came with our new to us boat this season. It runs fine and has been reliable.

But I notice it uses a toothed timing belt. I don't know the age of the engine or it's service history, so like you do on a car, should I be changing this timing belt as part of the service this year?

The engine details, anyone got any idea of the age from that?





And the timing belt with the covers off




It looks pretty crude, no sign of a belt tensioner, and it looks like the flywheel would need to be pulled to change the belt.

Any advice please?

 

[jamie N]

For me, I'd change it. It's possible that penetrating oil might be needed to loosen some stuff, and you've got time on your side to reduce the stress of any unforeseen stuff.

 

[ProDave]

That's my gut feeling if it is not too difficult. Even if a snapped belt did not destroy the engine, it could spoil your day on the water.

But I wonder what the official service interval is? Though of course I have no clue when, if ever, it was last changed.

I am also keen to hear from anyone that has done it. If I pull the flywheel, am I safe to assume the crankshaft pulley for the belt will remain on the crankshaft, i.e. it won't come off with the flywheel?

 

[Davy_S]

Would it be possible/easier to remove the starter motor and simply put the belt over the flywheel?

 

[ProDave]

There look to be other things too close to the underside of the flywheel to do that. Again interested to find out what the official procedure is.

 

[CliveF]

as far as I am aware you need the info off the data plate on the bracket to establish age.

serial no and letter code

 

[ProDave]

Everyone I have spoken to locally says it is a good plan to change the timing belt so I want to go ahead with it.

Does anyone have the service manual? That would give me a bit of confidence. the main thing I want assurance on is pulling the flywheel is not going to pull the crankshaft belt pulley as well.

Someone also suggested to me there is a thermostat under that flywheel that you might as well change as well while you have the flywheel off. Is that true?

And where to get the belt? Buy a proper Yamaha one, or just take the numbers visible on the old belt and buy one the same?

I am surprised at the lack of replies to this thread so I assume everyone else just carries on without considering the potential of belt failure and that it might be a good idea to replace it from time to time?

 

[VicS]

Everyone I have spoken to locally says it is a good plan to change the timing belt so I want to go ahead with it.

Does anyone have the service manual? That would give me a bit of confidence. the main thing I want assurance on is pulling the flywheel is not going to pull the crankshaft belt pulley as well.

Someone also suggested to me there is a thermostat under that flywheel that you might as well change as well while you have the flywheel off. Is that true?

And where to get the belt? Buy a proper Yamaha one, or just take the numbers visible on the old belt and buy one the same?

I am surprised at the lack of replies to this thread so I assume everyone else just carries on without considering the potential of belt failure and that it might be a good idea to replace it from time to time?

It looks reasonably straightforward except that the hose on the thermostat housing will have to be removed ( that'll probably mean removing the housing but at least you'll then have access to the thermostat.)

Use the correct type of puller to remove the flywheel. When you reassemble be sure to refit it to a clean taper and torque the retaining nut correctly.
The belt drive gear is separately retained, and keyed, on the crankshaft under the flywheel so no problem there.

Put some timing marks on the camshaft gear and the flywheel before you start and on the belt drive gear after removing the flywheel.

The blue mark on the camshaft gear suggests someone has changed the belt previously. Maybe you'll be able to use the existing marks

In the absence of a 'shop manual these parts diagrams are very useful

https://www.boats.net/catalog/yamaha/outboard/2003/8hp/f8mshb-2003/generator

https://www.boats.net/catalog/yamaha/outboard/2003/8hp/f8mshb-2003/oil-pump

 

[ProDave]

Thanks that is very useful. There are two timing marks on the camshaft pulley and I would guess there are also timing marks on crankshaft pulley when it is revealed.

What do you mean by the proper puller? I have a unuversal puller set that basically has a collection of different legs of different lengths. If sets up as 3 legs that go under the pulley and then you tighten the centre bold down on the shaft. Is that what you mean?

That hose that i have already determined would be in the way. so that is the thermostat under that housing? As I had been advised to replace the thermostat, I presume I need a new thermostat and a new gasket for the thermostat cover then? And if removing the thermostat housing no need to disconnect the hose?

 

[VicS]

The correct puller is known as a "harmonic balancer puller". It bolts on to the flywheel with three high tensile bolts. A 3 legged gear puller is likely to damage the flywheel.
It is often difficult to pull the flywheel off.

Yes the thermostat is under that housing.
See https://www.boats.net/catalog/yamaha/outboard/2003/8hp/f8mshb-2003/cylinder-crankcase-1

You may need a new thermostat. You'll probably find it stuck open but it might clean up and be possible get free again. That's how I found mine last time looked at it but it cleaned up and after a bit of gentle persuasion freed up again.

Maybe you will be able to avoid disconnecting the hose ... ???

 

[ProDave]

I have taken the flywheel off this morning, it came off quite easily with my normal 3 leg puller without damage.


Changing the belt looks easy. The timing marks on both pulleys are clear. One magneto coil will have to come off and the thermostat housing, I want to replace the thermostat anyway.

so before i proceed I need to find a UK supplier of the timing belt, the thermostat, and a gasket for the thermostat housing.

 

[VicS]

Mark the position of the coil so that it can be refitted with the correct clearance between the polepieces and the flywheel.

Clean the mating surfaces of the taper and dont forget to refit the key in the crankshaft before you refit the flywheel.

 

[DownWest]

I doubt that you would run an OB enough to change for wear, but the age limit matters. On a Renault Clio, it had done about 40k km, but over several years and the belt was badly crazed.

Interesting thread.

I use an online suppler of belts, Good names and decent prices + quick delivery.

Here/ French version, but pretty sure they cover UK: Courroie | 123Courroies, spécialiste en courroies et poulies

 

[ProDave]

Disappointing that neither of the 2 local chandlers have bothered to reply for my request for a price for a new belt, thermostat and gasket.

I can get all the parts from other on line chandlers or even ebay for under £50. Is there anything I need to be aware of e.g is a genuine Yamaha OEM belt really any different to an after market equivalent belt?

 

[jamie N]

Sad about our 2 locals. They've both declined in quantity of gear over the past few years, particularly the one in Merkinch, where they're SO focused on 'online', that the shop's bare!
Equivalent would be my choice; same factory as OEM possibly etc.etc...

 

[lexi]

A cam belt on a car and one on an 8 horse engine are apples and oranges. These wee engines cam belts are under little load compared to Ohc diesels etc. Driving a light camshaft usually with a few valves. I kept same belt on Honda 8 20 years and even the dealer said forget about changing it. I mean change it by all means and you may as well get an OEM belt I think. Dayco or Gates might have belt size but best with Yammy item now you are in there.

 

[ProDave]

Just to confirm that I changed the belt and thermostat on Sunday and this morning put the OB in the tank and ran it. All good.

It was really quite a simple job. I searched for the belt on the belt part number rather than engine type. Total cost just under £50 for belt, thermostat and gasket.