MS 650 big end

[bobh]

Big end went today on my MS650.

Was playing up a little yesterday, thought it was the plug so changed that last night - it was fairly black - and gave it another go today, same problem - cutting out when ticking over when hot, fine if cold.

Decided to lean up the H jet as the plug was fairly black, less than 1/4 turn and it cut out and could not pull the starter cord - seized! Time to head home.

Stripped cylinder off to find pot and piston in reasonable condition, just a few scores on piston.

 

 

Not such a good story on the big end bearing though

 

 

 

 

Signs of heating on the conrod

 

 

Anyone got any ideas as to the cause of this? Saw was bought secondhand back in the spring so I don't know the full history.

Also any recommendations for parts suppliers, don't really want to Stihl prices unless the aftermarket ones are rubbish.

Bob

[sawfinger]

Is that a chip missing from the piston skirt?

[spudulike]

Yes - are you sure that this piece of metal is from the big end bearing - has the big end lots of play in it and is all sloppy in the up and down direction - usually the cage cracks and disintegrates leaving the rollers loose and causes a lot of play.

 

It is very unusual for a big end to let loose when an engine seizes - usually the piston and possibly the cylinder goes and possibly the rod bends but is pretty uncommon as the saw usually just grinds to a halt and looses power.

 

This is a big saw and the bottom ends are pretty robust - only seen one saw with a big end gone and that is in my garage - Stihl 028:thumbdown:

[bobh]

Thanks for the replies folks, just been back out to the shed for further inspection.

Sawfinger, well spotted, that is a chip off the skirt, but it looks freshly done, don't think it's been running with it like that.

Spud, Just taken the piston off to try and get at the offending piece. Turns out it's just one of 3 pieces, tried prising them out and although they will move a little they won't lift out. What it looks like to me is parts of the cage as it is starting to collapse.

Loads of play side to side and a small amount up and down. If I look down the side of the conrod one side looks normal ie I can see the cage, on the other side I think I am looking down to the bearings, the 2 sides certainly look different anyway.

I've had a look for parts on the net and can't find a piston apart from a genuine Stihl one, anyone know if the 640 piston fits? or where to find a non oem piston?

[spudulike]

Thanks for the replies folks, just been back out to the shed for further inspection.

Sawfinger, well spotted, that is a chip off the skirt, but it looks freshly done, don't think it's been running with it like that.

Spud, Just taken the piston off to try and get at the offending piece. Turns out it's just one of 3 pieces, tried prising them out and although they will move a little they won't lift out. What it looks like to me is parts of the cage as it is starting to collapse.

Loads of play side to side and a small amount up and down. If I look down the side of the conrod one side looks normal ie I can see the cage, on the other side I think I am looking down to the bearings, the 2 sides certainly look different anyway.

I've had a look for parts on the net and can't find a piston apart from a genuine Stihl one, anyone know if the 640 piston fits? or where to find a non oem piston?

 

The sideways movement is normal, there should be no movement up and down, the bits in the case should not be aluminium if it is from the cage as it sounds like it is.

You will need a new crankshaft and that will be a reasonably big job. Will loook at the piston tonight unless Megatron etc know without looking!

[elfinwood saws]

well there is a crank for you. CRANKSHAFT FITS STIHL 066, MS650, MS660 CHAINSAW 10MM | eBay

[elfinwood saws]

The 064 piston should fit as the bore is 52 mm check that your bore is 52mm incase your saw has had a 660 top end fitted or even a 56mm bigbore kit, I,ve known at least half a dozen ms650,s that have knocked their bigends out, British Alcan used ms650,s for cutting up alli billets and scrap they were used 7 days a week 8 hr,s a day so have a fantastic number of hours on them, plus i know that someone bought lots of these saws with damaged cranks, bigends gone and clutch nose snapped off and repaired the cranks by splitting the flywheels with a hammer and wedges and re build using the same hammer to true them up no v blocks of dial gauges used. they tend to be 2007 and 2008 saws the give away for these saws is the black ali dust baked on to the saws in those hard to clean areas, most ms650 in this country have been through British Alcans hands, i have one and love it, but the crank went when it was eighteen months old!

[bobh]

Thanks for the update guys

Elfin, I think you have just described my saw exactly, I wondered what those black deposits were when I first got the saw. Mine is a 2010 model so would be about 18 months old now.

Been looking at that crank also one here eBay - The UK's Online Marketplace

Also looking at this gasket set eBay - The UK's Online Marketplace

Apart from piston is there anything else I would need to do this myself (we have the technical ability) thinking about grease, sealants, special tools etc.

Thanks

Bob

[elfinwood saws]

You need crankcase gasket and base gasket, crankseals and bearings would be a good idea. plus make sure you get the right crank depending on which flywheel you have, it should be the plastic flywheel.

[Megatron]

So it looks like the conrod seized instead of the piston. That black dust will most certainly have been a contributary factor. What air filter did the saw have, the wire mesh one?

 

To rebuild you will need a way of extracting the crank out of the case without causing damage to the cases. The Stihl tools are £££, if you can weld you can relatively easily make your own tool to push the crank out of the clutch side case. Then you can either use a press or big clamp to push the crank out of the flywheel side or again you can make your own tool. (I wouldn't recommend using a mallet to extract the crank out of either side).

 

If the bearings stay in the case you can push them out with sockets and a big vice and if they stay on the crank then use a bearing splitter or jaw puller etc You can reinstall the new crank and bearings into the case using heat...