I have been helping my neighbor by replacing the piston on his ms251 that he ran with the wrong fuel oil mix. I ordered a piston from Baileys which took 2 months to finally get here. I was just installing it into the cylinder and following the service manual to the T. Everything oiled and aligned. I applied maybe 5 lbs of pressure to seat it into the cylinder and ping. The top ring is now 2 pieces. I have already spent way too much time on this saw and really dont feel like waiting on a ring to come in the mail. I am hoping you guys will say "put her in as is, it will be fine". But I get the feeling the response it going to be "order another ring". Also can anyone offer advice as to how to avoid this in the future, the ring seemed super fragile.
Please Help Broke New Piston Ring
- Thread starter tim o
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Get another ring! You cannot use a broken ring. Did you have the ring installed on the locating pin correctly? was it upside down? Find out why the saw burned a hole in the piston. I doubt it was the mix ratio. Air leak, too lean, very poor fuel ect. Is the bore even useable? CJ
The rings came preinstalled on a factory stihl piston. I checked that the pins were in between the gaps in the rings as I was sliding it in. Everything has thorough coating of oil. The service manual said "the bore is tapered for ease of installation". I figured this meant that the bore was going to compress the rings for me as I slid the piston down. I dont know how it can do that without being able to apply a few pounds of pressure.
My neighbor said he mixed 2 gallons to one of the small oil containers which is meant for 1 gallon. The saw was in good shape otherwise and reasonably new. I doubt there was a air leak, maybe bad gas, but the wrong mix seems likely to me.
My neighbor said he mixed 2 gallons to one of the small oil containers which is meant for 1 gallon. The saw was in good shape otherwise and reasonably new. I doubt there was a air leak, maybe bad gas, but the wrong mix seems likely to me.
Pistons get holes in then from water in the fuel. It creates a hot spot and will blow a hole right through them. I've seen it many times in snowmobiles. IDK what the cylinder looks like but make sure if there is any transfer on the walls that it's removed properly. Even though the cylinders have a slight chamfer you typically need to compress the piston ring a great deal to get then to slide in. I use my first fingers and the flat edge of a small screw driver. It's a pain in the ass using that method but it works. It's easier if you buy a ring compression.
Big_Wood
westcoast dweller
the hole in the piston looks inline with a decomp that has dropped into the cylinder. i've seen it happen on a few combustion chamber decomp saws. enough where i refuse to run them without installing a plug.
Thanks marshy, thats what I needed to know. I wish the manual had been a little more specific.
westcoaster90, the decomp you are referring to, is this a decompression valve? I don't see one in my parts diagram, nor do I recall seeing one during disassembly. Are you suggesting this valve dropped into the cylinder and caused this? If so, I didn't find remnants of it.
westcoaster90, the decomp you are referring to, is this a decompression valve? I don't see one in my parts diagram, nor do I recall seeing one during disassembly. Are you suggesting this valve dropped into the cylinder and caused this? If so, I didn't find remnants of it.
I re-read the manual. It calls for a tool to install the piston/rod sleeve ( I found this totally unnecessary as mine slipped in with finger pressure). It does not call for a ring compressor anywhere that I see. It does however read "check that the gaps in the rings meet at the fixing pins". I thought this meant to make sure the pins were somewhere inside that gap. What is really means is compress the rings until they touch those pins.
Westcoaster90, I actually noticed the hole was in the thinnest part of metal, if you looked from the underside of the piston. It looked to me like they intentionally designed a weak spot in the piston to give way so you don't cook the cylinder too. I really didn't find much of any transfer to the cylinder.
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Why don't you just get a ring from your Stihl dealer?
cuinrearview
Red saw lover
In the few saws I've rebuilt a zip tie or two made fine ring compressors.
The dome of the piston is thin because if it was thick then you would have a lot of expansion. The dome is taking all of the heat from combustion so the thinner they are the easier heat can be transfered. when you get water in your fuel you end up with a lean combustion because of the vaporized water adds oxygen to the mix. You torch a hole right through the slug.
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Stihl sells the Piston assy. for $32.49, how much did the aftermarket one cost?
I just called them, and they can get it for a reasonable price. I was avoiding them because my neighbor has had bad experiences with them. I personally haven't dealt with them, but my neighbor isn't exactly easy to deal with, so maybe its just him.
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Here is the pick of the old one that he put up.
Yes the pin has to be between the end gaps of the ring. They usually have a good amount of spring to them so they need to be compressed some amount to get them into the bore. Make sure the arrow on the piston is pointed towards the exhaust port. Pitting new rings on your new piston can be a PITA the first time. Hope you don't break another ring. Maybe consider buying a ring spreading tool.
Im not saying there wasn't water in the fuel, with my neighbor its totally possible. I will remind him of the importance of quality fresh fuel in a machine that spins 13k rpm.
I already gave him the piston as a trophy otherwise I would post a pick of the underside. The thin part of the piston was just barely larger than the hole and did not extend to the center.
I already gave him the piston as a trophy otherwise I would post a pick of the underside. The thin part of the piston was just barely larger than the hole and did not extend to the center.
