Gearbox backlash in Mariner 3.3 two stroke

[chamac]

I have a 2006 Mariner 3.3 two stroke that likes to break shear pins.
I asked about this at the Mariner stand at SIBS. They suggested that the idle speed may be too fast. I turned it down but I still get the same breakages. I noticed that the backlash at the propeller is about 45 degrees which seems excessive. Thats with the forward gear engaged. I wondered if this was the problem. Does anyone have one in the garage that they could check?
BTW. They are stainless steel shear pins.

 

[m1taylor]

This does sound like a gearbox problem - something is too stiff. You should be able to put the engine in gear and turn the prop by hand without difficulty on this small engine (when it's not running of course!). If it is unreasonably stiff you could take the gear box apart and rebuild it - not difficult, but you will need a gasket, and take care not to damage the seals.

 

[William_H]

If it is anything like an Evinrude the gearbox has a drive gear on the end of the vertical shaft. This is in constant mesh with 2X 45 degree gears which are in constant meshbut each on a different bush on the prop shaft. Into the side of these gears are big square holes which are part of what is called a dog clutch. A dog is slld along the shaft from neutral between the two gears to forward where it's own square lugs fit into the square holes of the gear. The dog itself is splined onto the prop shaft. Or something like that if you get my drift.

The point is that backlash of the gearbox is where the dogs mesh with normally huge backlash. I wouldn't be surprised at 45 degees.
However it is possible that something is worn or broken. Eveen so it should not break shear pins.

If you can turn the motor over from the prop then I don't think you have any binding or excessive friction. presumably the motor is ok if the shear pin doesn't break. ie you can get full power/speed.

When you put the motor in gear the action of meshing the dogs is quite horrible. Just a sudden catch and crash. The motor is running and suddenly the prop is made to turn at motor speed. Hence your dealer is right the usual cause of broken sheer pin is if the motor is running too fast when you bang it into gear. However if you had fitted a stainless steel prop in place of aluminium or plastic the additional mass (inertia) might make pins break more readily.
olewill

 

[chamac]

Thanks lads for your replies.
The engine has only a couple of hours use. There is no stiffness in the gearbox. It has about 45 degrees of backlash at the propeller when in forward gear. I wondered if this was normal as it would add to the sudden jolt of being put into gear and the shear pin taking the load. The shear pins I was using were made from 4mm threaded bar. I reasoned that that way they would not be too strong if the prop hit something. I will try some solid ones and see how they work out.
I can use the engine by warming it in neutral, stopping it and starting it again in gear. The shear pin lasts a bit longer this way but still gets a bend in it. I am not using aggressive throttle.

 

[VicS]

45 degrees of backlash does not sound excessive. My Evinrude has slightly more than 90 degrees and my old Volvo Penta has almost the same.

I think idle speed is probably the key to the problem. Reduce it to the lowest at which it will idle smoothly and not stall when the gears are engaged.

Using sturdier shear pins than intended will put your engine at risk of damage if the prop grounds or hits an obstruction

Stiffness in the gear box would have no relevance as the shear pin is between the box and the prop.

BTW when engaging gear it is correct to do so with one snap action as doing it gently will cause damage to the dog clutch and may lead to a situation in which it won't stay in gear.

 

[William_H]

I suggest you try to get the correct shear pins. Threadeed bar has lost a lot of its strength in the thread cutting process. Hence breaks easier.
Or try a piece of nail of suitable diameter.(if you don't want to but the correct shear pin). olewill

 

[ex-Gladys]

There are optins around the Tohatsu/Mariner/mercury 3.3 shear pin; there are brass ones and also steel I think.