Cracked rings - what's the cause?

[Bob95065]

I picked up a Stihl MS290 from a local shop yesterday. The plastic is in excellent condition but the saw had no compression. I tore it completely down to find that the rings were in pieces.

When I used to work on small 4 cycle engines I was told that excessive use of starter fluid can crack the rings this way. Could starter fluid be the cause of the same problem in a 2 cycle engine?

The saws in great shape otherwise including the inside of the cylinder. I'm going to put a new piston and rings in it along with cank seals and a carb kit. She ought to be good as new soon.

Bob

 

[jughead500]

Probably a bad batch of rings.If that much Ether was used it would have probably have some major wear elsewhere in the motor from the Parts being "Washed" Down.the Cylinder walls being the First Having Major Scratches and Wear though the Plating.Hate to say it but Probably a Bad Batch of Rings or from What I've seen Sharp places at the Port edges.The Edges should be tapered a little bit.I have an 025 that wore Flat Spots on the rings at the exhaust port from not being Tapered.
Can you get us some pictures of the rings and port edges?
The 025 I just talked about was rebuilt last week due to the rings.I lost all compression and it wouldn't Idle.I done a Complete Rebuild on it.Fired it up and it ran maybe 10 Minutes and Lost Compression again.the saw looked Brand new even though it was 15 years old but only cut camp fire wood a few times a summer.I tapered the ports during the rebuild but I believe the rings being worn funny May have wore the Bore a Little crooked above the Exhaust Port.Time for another Tear down.This time I'll install a different Motor to use while I inspect this one.

 

[fearofpavement]

Most of the broken rings I've encountered have been from excessive wear. Never heard of starting fluid breaking rings but hey, could be?

Check the skirt of the piston and see if it shows wear. ie, one side thinner than the other side?

 

[jughead500]

Yeah if it were Ether we would have a Lot of Broken down Tractors around here without any Compression.

 

[woodchuck357]

Them 2 stroke tractors can be a bit difficult to start

:msp_sleep:

Yeah if it were Ether we would have a Lot of Broken down Tractors around here without any Compression.

 

[Itsme7]

Ether damage is more common in glow plug equipped diesel applications, as it causes severe pre detonation, which can break rings, ring lands, fracture pistons, melt craters in pistons, and can even mess up starters by stalling the starter while cranking because of pre detonation. Ether explodes where as diesel burns slowly, which is where the damage comes from. Gas engines dont have as many issues, but using way too much ether can do damage thats for sure in either application.

 

[Bob95065]

This saw is very clean and has very little use. The rings came out in pieces - probably 4 or more per ring. I didn't see any wear at the piston skirt. The only wear was in the area around and above the rings. It is hard to say whether that was a cause or result of ring failure.

The cylinder looks pristine. When I got the saw I was expecting straight gas in the tank. I was surprised to see mixed gas when I dumped it out. I pulled the muffler expecting to see deep scores and aluminum transfer. The piston had s little wear and the rings looked good. I tore the saw down and when I removed the piston from the cylinder the rings fell off in several pieces.

I am planning on replacing the piston and rings with an aftermarket set from Northwood Saw. Sal has a Meteor set listed for $34.95. I plan on replacing the crank seals and rebuilding the carb too. I wanted to eliminate the cause of the cracked rings and it sounds like I will if I replace the piston. I bought the saw to repair and resell but was thinking if I was to keep it I probably would have considered reringing it which may have been a mistake.

Thanks for the information..


Bob

 

[Termite]

Off topic but. My 2003 Dodge diesel had starting problems. I would prime it with 2 cycle mix from a pressurized spray container. It would start right up. The problem was one injector bypassing fuel back to the tank. Anyway, while talking to a Cummins mechanic about it and priming with 2 cycle mix, he said they use the hell out of starting fluid on Cummins diesels with no damage!! I was surprised by the comment.
I remember having reversed the two ignition wires on a 1960 Pontiac 389, years later when we tore the motor down, one cylinder had a broken ring.

 

[Chris-PA]

This saw is very clean and has very little use. The rings came out in pieces - probably 4 or more per ring. I didn't see any wear at the piston skirt. The only wear was in the area around and above the rings. It is hard to say whether that was a cause or result of ring failure.

The cylinder looks pristine. When I got the saw I was expecting straight gas in the tank. I was surprised to see mixed gas when I dumped it out. I pulled the muffler expecting to see deep scores and aluminum transfer. The piston had s little wear and the rings looked good. I tore the saw down and when I removed the piston from the cylinder the rings fell off in several pieces.

I am planning on replacing the piston and rings with an aftermarket set from Northwood Saw. Sal has a Meteor set listed for $34.95. I plan on replacing the crank seals and rebuilding the carb too. I wanted to eliminate the cause of the cracked rings and it sounds like I will if I replace the piston. I bought the saw to repair and resell but was thinking if I was to keep it I probably would have considered reringing it which may have been a mistake.

Thanks for the information..


Bob

That's a puzzler. Is the ring groove machined too wide or the ring too narrow? Was just thinking maybe the ring was loose and getting pounded around. Trying to figure out what kind of forces would make it crack.

 

[Wvhunter1]

maby it was ran way lean on air to fuel ratio? my buddys yz250 dirtbike had rings that disentigrated from a combination of that and racing on the hard road

 

[Plan-b]

Broken rings caused by ring flutter.

 

[stormpetrel]

Yeah if it were Ether we would have a Lot of Broken down Tractors around here without any Compression.

Starting fluid blew the head gasket in my 26 hp diesel tractor. Not gonna make THAT mistake again!!:bang:

 

[BigMoneyGrip]

My guess would be the end gap was too little. I've also seen it mess up the cylinder walls. The end gap gets over looked a lot of times, but it is just as important as piston to cylinder clearance.

 

[Bob95065]

What should the nd gap be on this saw? I m going to replace the piston and rings.

 

[BigMoneyGrip]

What should the nd gap be on this saw? I m going to replace the piston and rings.

I always use .0035~.004 per inch of bore. You might want to check a shop manual to be safe, though.

 

[Naked Arborist]

I'd put my money on spark scatter or breaking up of the ignition at high RPMs. The only other causes I have seen are broken spark plugs, rotten plug wires, loose flywheels, bad coil bridge, cracked coil case or CDI box and loose screws on the mounts of either one. To little ring end gap is very unlikely but possible.

Most broken rings are caused by faulty ignition events or TOO MUCH juice NO2