Piston Failure on Stihl 046

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Since it seems to have started on the intake side...Where the ring gaps are...Can you see any indication of the rings catching the intake port? You would think there would be a dent,chip, or some damage on the cylinder to indicate where the rings snagged?
 
As Hillary Clinton said, "What difference at this point does it make?"

I watched a Stihl OEM piston/rings/cylinder blow out the same way after a rebuild by a dealer's mechanic.

"What difference at this point does it make?"................. :buttkick: will be delivered by the masses, that is the difference it will make.........:hmm3grin2orange:
 
My guess is the kit is a aftermarket knock off. I have heard stories of piston detonation from those types of kits from the ports not being the at the right specs. I also knew a guy that used a kit and had the same problem only it seemed to be the alloy in the piston is what caused his problem.

Sent from me to you using my fingers
 
My guess is the kit is a aftermarket knock off. I have heard stories of piston detonation from those types of kits from the ports not being the at the right specs. I also knew a guy that used a kit and had the same problem only it seemed to be the alloy in the piston is what caused his problem.

Sent from me to you using my fingers

Looks like one of those "Sum Ting Wong" kits to me too
 
Actually, I hated to post this. I rebuilt a 1997 Stihl 046 with a new cylinder kit. I imagine the saw had over 1000 hours on it before it gave up. Prior to my rebuilding, the saw would not even start and the compression was minimal. After rebuilding it, the engine started and ran flawlessly for me. I set the carb according to service manual specs. I checked it several times but not in the field under load with bar and chain. Fabulous compression and power. The owner, my good friend, then ran it for less than an hour while bucking large logs.

Then it stopped and he could not restart it. He gave it back to me the next day and I let it sit for a day. I started it and it ran for a few seconds, but it died suddenly. So, I took the muffler off and metal parts fell out--a collection of ring remnants and piston chips. One chip was huge in my book, bigger than any I had seen before. Here's a Pic of the exhaust side:



And here is the intake side:


Please pardon the focus. My camera is not the best. Rings were cracked in several locations and came apart. So, this piston and ring assembly died in less than an hour. Does anybody have any idea what happened here? Please advise.

Don't think anyone has mentioned ring end butt?
 
Rings "butting" together does that type of damage. Everyone thinks it snagged a ring in the port, not always the case.

This type of damage happens to four stroke pistons also. Aren't any ports to snag a ring on four strokes, but when the rings butt it usually looks very similar.

The lands break (usually the top).
 
Ring butt failure two and four stroke;

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Ring butt failure two and four stroke;

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That's it. Ring butt failure. It was an aftermarket kit, imported from China. I used Caber rings, however, and not the ones that came with the kit. I snapped one of the kit rings by accident and ordered a pair of Cabers to replace them.

I had the rings mounted correctly. The ring seats were on the intake side. Perhaps the Caber rings were incompatible with the piston and the cylinder and that set up the ring butt. The Cabers were the same thickness (1.2 mm) but a tad wider. So the inner diameter was less, and the piston grooves apparently were not deep enough to handle the expansion when they got hot. That could have done it. Who knows? It's just a theory, but it does make some sense..

Thanks for the far better picture. The jug seems OK, so I'll try another piston kit and see what happens. You live and learn.
 
That's it. Ring butt failure. It was an aftermarket kit, imported from China. I used Caber rings, however, and not the ones that came with the kit. I snapped one of the kit rings by accident and ordered a pair of Cabers to replace them.

I had the rings mounted correctly. The ring seats were on the intake side. Perhaps the Caber rings were incompatible with the piston and the cylinder and that set up the ring butt. The Cabers were the same thickness (1.2 mm) but a tad wider. So the inner diameter was less, and the piston grooves apparently were not deep enough to handle the expansion when they got hot. That could have done it. Who knows? It's just a theory, but it does make some sense..

Thanks for the far better picture. The jug seems OK, so I'll try another piston kit and see what happens. You live and learn.

I've ran into that before, between the Stihl OEM rings & Caber aftermarket rings for the 031 & 041 series saws. Both are for 44mm bores, though the pistons will differ in ring groove depth. The beam sections also vary in thickness per ring manufacturer.

You need at least 0.005" below the surface when the ring is seated all the way in, AND you need to check & set the ring gaps to the cylinder every time. I use 0.004" per inch bore for setting gaps, so if the bore is 54mm I use 0.008" minimum 'cause the top ring gets very hot. Just spin the ring around the piston to check that you are below the surface, of course you can't check right near the keeper pins as it pushes the ring out there.
 
I've ran into that before, between the Stihl OEM rings & Caber aftermarket rings for the 031 & 041 series saws. Both are for 44mm bores, though the pistons will differ in ring groove depth. The beam sections also vary in thickness per ring manufacturer.

You need at least 0.005" below the surface when the ring is seated all the way in, AND you need to check & set the ring gaps to the cylinder every time. I use 0.004" per inch bore for setting gaps, so if the bore is 54mm I use 0.008" minimum 'cause the top ring gets very hot. Just spin the ring around the piston to check that you are below the surface, of course you can't check right near the keeper pins as it pushes the ring out there.
I imagine these Cabers (54 mm bore) were really tight to the inside diameter of the piston groove because they were considerably wider than the kit rings. It's rather remarkable with such little play that I managed to get them assembled inside the cylinder. Once the ring got hot, it had to expand somewhere. In this case, it had no place to go, so it crashed through the roof.

I suppose another word to the wise here is that if you buy an aftermarket kit, be sure that any rings that you replace come from the same supplier of the kit. That's what I failed to do, primarily because the company that sold me the kit had no replacement rings available. So, I bought the Cabers because the bore and thickness matched. The entire assembly worked great--for about 10 minutes.
 
So are we willing to say that Caber's may not be all there cracked up to be? OR maybe you need to measure everything every time... EVEN with OEM parts? OR that rings supplied with the kit should stay with the kit?

I've seen this exact thing several times on rebuilds I've done, especially with Huskies like the 55 and 350. Even seen the kit come with rings that didn't fit correctly. Maybe they just make the piston and added another manufacturers rings from what I can tell. Maybe we're really talking about how the components match each other to make a good kit.....:msp_thumbup: and because of the great success of Caber rings in various applications, assumed they were "good to go".

I'm all ears... feel like were about to learn something here.. REALLY.
 
One other thought Ed. When the engine is running under load the exhaust side of the piston becomes the thrust side. The piston will move around a couple of thousandths in the bore, and the rings will also move in their lands. When the rings move out into the exhaust port, the purpose of the port bevels is to guide them back into their lands. Look closely at the locating pins, sometimes there is a top orientation required of the ring ends.
 
So are we willing to say that Caber's may not be all there cracked up to be? OR maybe you need to measure everything every time... EVEN with OEM parts? OR that rings supplied with the kit should stay with the kit?

I've seen this exact thing several times on rebuilds I've done, especially with Huskies like the 55 and 350. Even seen the same kit that came with rings didn't fit correctly. Maybe they just make the piston and added another manufacturer's rings from what I can tell. Maybe we're really talking about how the components match each other to make a good kit.....:msp_thumbup: and because of the great success of Caber rings in various applications, assumed they were "good to go".

I'm all ears... feel like were about to learn something here.. REALLY.
I think rings that are supplied with the kit must stay with that kit. Substituting is hazardous as I have already found out and reported here, regardless of who else makes the rings. I believe that replacement rings must be supplied by the same MFG who made the piston (and perhaps the cylinder) because the tolerances are so precise.

The Caber rings that I bought to replace just one kit ring that I broke, destroyed the piston in about 10 minutes of run time. That piston could have been made by anyone, including Stihl, and the engine may have still failed. However, if Caber makes the rings that Stihl buys, it may have worked fine. Until I replace rings from a vintage Stihl piston using Caber rings and evaluated the engine's performance, I would not be able to judge.

Let's keep this thread going.
 
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I think rings that are supplied with the kit must stay with that kit. Substituting is hazardous as I have already found out and reported here, regardless of who else makes the rings. I believe that replacement rings must be supplied by the same MFG who made the cylinder because the tolerances are so precise.

The Caber rings that I bought to replace just one kit ring that I broke, destroyed the piston in about 10 minutes of run time. That piston could have been made by anyone, including Stihl, and the engine may have still failed. However, if Caber makes the rings that Stihl buys, it may have worked fine. Until I replace rings from a vintage Stihl piston using Caber rings and evaluated the engine's performance, I would not be able to judge.

Let's keep this thread going.

But should be able to after a few measurements. I'm a believer in keeping the kits in tack'd... but I never miss a measurement.. ever! If you only broke one ring, why not go with just one ring... more RPM and wouldn't have made much of a difference.
 
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