Piston Failure on Stihl 046

[Wood Doctor]

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.

 

[blsnelling]

What brand of kit? Looks like one of the black painted pistons.

 

[watsonr]

Looks like it caught a ring, what brand was it?

 

[dieselfitter]

Post some pictures of quality please.

 

[stihl for life]

thats not a over heating scorring . i would think there was somthing in the that wasent suppose to (a piece of metal or somthing ) that would of scored that piston . is that a oem/ mentor or aftermarket

 

[Wood Doctor]

Looks like it caught a ring, what brand was it?

What is "It"? What did the ring catch on? The cylinder? If so where?

 

[watsonr]

Pictures are terrible, I'd say the roof of the port most likely, could have been the floor as well. Any damage to either of those areas, as in excessive damage?

What was the brand of that cylinder and piston... it?

 

[Wood Doctor]

Pictures are terrible, I'd say the roof of the port most likely, could have been the floor as well. Any damage to either of those areas, as in excessive damage?

What was the brand of that cylinder and piston... it?

Please try to get around the lousy Pics. I don't own a close-up lens. The point is that this piston failed in one hour or less. A large chip broke off the edge of the piston. Several other fragments disappeared as well. The rings broke in several places. Whatever is left of them is lodged in the piston grooves. The cylinder shows little damage.

All of this took place in lest than one hour of normal operation. I still have no idea what caused this.

 

[SEAM]

Please try to get around the lousy Pics. I don't own a close-up lens. The point is that this piston failed in one hour or less. A large chip broke off the edge of the piston. Several other fragments disappeared as well. The rings broke in several places. Whatever is left of them is lodged in the piston grooves. The cylinder shows little damage.

All of this took place in lest than one hour of normal operation. I still have no idea what caused this.

A bad piston/kit can cause this... defect in alloy, bad weight (distribution), poor shape...

 

[PA Dan]

What brand of cylinder and piston did you use?

 

[Wood Doctor]

What brand of cylinder and piston did you use?

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.

 

[watsonr]

If the port was miss shaped and had a flatter opening instead of rounded, it will catch the ring as it slides past causing the piston to take undue stress and directly in the ring groove. Couple thousand cycles and the rings fail early or blow the top off the piston.

 

[watsonr]

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.

Me too! I stood right there when he revved the saw and BAM!

 

[watsonr]

what brand and was there any warranty?

 

[Wood Doctor]

Agreed.

If the port was miss shaped and had a flatter opening instead of rounded, it will catch the ring as it slides past causing the piston to take undue stress and directly in the ring groove. Couple thousand cycles and the rings fail early or blow the top off the piston.

Now that makes a lot of sense. I imagine that either port could have caused this? The primary piston failure did occur on the exhaust side. Either the top or bottom lip of the port could have caught the ring and triggered the event, setting up stresses that eventually broke apart the piston cap.

Now we have to consider that breaking in the engine slowly might have saved it. Parts have to wear in, and I wonder if that would have helped a little--perhaps running the engine at 4000 RPM for awhile with little or no load. I recently read a thread here that advised running at WOT for a break in, and I shook my head for awhile in disbelief.

 

[watsonr]

Now that makes a lot of sense. I imagine that either port could have caused this? The primary piston failure did occur on the exhaust side. Either the top or bottom lip of the port could have caught the ring and triggered the event, setting up stresses that eventually broke apart the piston cap.

Now we have to consider that breaking in the engine slowly might have saved it. Parts have to wear in, and I wonder if that would have helped a little--perhaps running the engine at 4000 RPM for awhile with little or no load. I recently read a thread here that advised running at WOT for a break in, and I shook my head for awhile in disbelief.

5000 or 10,000 won't matter if the port is shaped wrong. Caber rings may have helped a little as they are a cast ring, flex a little and softer metal to absorb that kind of abuse.

 

[z71mike]

Can we get a pic of the ports?

 

[PA Dan]

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.

I never did care or listen to what Hillary had to say but would like to know what brand you used? Any reason your not sharing?

 

[04titanse]

I am curious what type of piston, cylinder and rings were used.

 

[promac850]

Well, I'll go out on a flimsy limb and say it doesn't look like a Tecomec piston.