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Darkslider

Has my Stihl BG86 Seized? (Please watch attached video :))

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Though I've done a bit of car mechanics I haven't much experience with 2T engines, so had a few questions about our Stihl BG86 blower that seized (?) while running at work today.

 

First things first, fuel mix was spot on, no issues there. Multiple other machines (Stihl chainsaw, second blower and strimmers) were all running from the same stuff with no issues so this isn't a lubrication related failure I don't think.

 

As you can hopefully see in the low quality phone video the crankshaft is only completing half a rotation, the piston is locking up against something solid at the top of the stroke and at the bottom as well.

 

There doesn't appear to be much in the way of scoring to the piston that I can see and the rings look intact, however the skirt has turned a lovely brown colour. Is this normal or a symptom of overheating?

 

I'd always thought a motor seized due to lack of lubrication the piston would expand with the heat and weld itself to the cylinder, however that doesn't seem to have happened here as the range in which it can move it feels completely free. Can anyone else suggest what might have gone wrong?

 

 

Something else which might not be related, but we've had two of these blowers from new and they've both failed around the same time. The first was a short while before this one and it shat itself catastrophically, piston rings broke up and the crankshaft main bearing shattered.

 

Any thoughts would be appreciated, don't want to waste any more time on it if it turns out to be irrepairable. But if it does look like a straightforward pot and piston job to get it 100% again then it could be worth fixing.

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Are you sure its locking internally?

 

A very common fault on these machines is that one of the 3 torx screws which hold the magneto mounting plate to the crankcase comes loose and unscrews until it locks the flywheel. Just look under the flywheel and you will see it clearly.

Remove the flywheel to tighten them.

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Just watched your vid and it seems like there could be something in the crank case as it stops top of stroke and bottom of stroke . Or most likely what Garden has suggested ( only just seen his post ) Piston looks ok

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That was my first thought Stubby, but a bit confused as to what would be going on in the crankcase to restrict the crank rotating? I'll check all the externals for loose screws as per Gardenkits suggestion next though I think.

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The screws come out and contact the raised areas on the back of the flywheel, preventing full rotation.

 

This is common and very easy to fix.

 

If this is not your problem then it will be much more expensive.

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The screws come out and contact the raised areas on the back of the flywheel, preventing full rotation.

 

This is common and very easy to fix.

 

If this is not your problem then it will be much more expensive.

 

It does feel like it's hitting something definitely solid in both directions, your theory would make sense. I would have thought if there was cylinder damage then a) the piston wouldn't move up and down so freely in the first place and b) when it did reach the damaged part of the cylinder it would feel rough or sticky, or I'd get a grating feeling or something. Not just the knock against something solid.

 

I will give the rest of the crankcase externals a good once over in the morning and let you know what I find :) thanks again for your help hopefully that's all it is and it'll be a five minute fix :thumbup1:

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Gardenkit is spot on, this isa classic symptom of one of the bolts come loose. You will need to pop off the flywheel to tighten the bolt but it will cure the issue I am sure!

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I've had a couple like this. They can over speed if the blades are worn or the air is blocked, and the result is the con rod gets bent.

 

But a bent con rod will make the engine hard to turn over/tight at different parts of the stroke - in the vid, there is a very solid mechanical knock at two very distinct places when the flywheel is rotated one way and the other.

 

I would bet £100 on this one being one of the flywheel alloy shroud bolts coming out and clouting the higher parts of the backside of the flywheel!

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