What causes a piston to do this

[Boosted3g]

I was losing some compression and had a suspected oil tank leak so today I rebuilt my ms361. I installed a new oem piston and rings and upon inspection of the original piston it is cracked on exhaust side of the pin on both sides. The crack is light and I can't open it up with pressure. The pin was also very tight and took some work to push out. I'm glad I didn't cheap out and replaced everything but if I was just reringing I might have missed this. This was the first this saw had been apart so could hours have caused this? The saw was smoking a bunch before the tear down but the case gasket was in rough shape and it was sucking bar oil. It was run for about a tank in this condition. Saw has only seen motomix for years and only has a muff mod. Has always run, started and idled perfectly. Is there a such thing as spark knock on a 2 stroke that may have beat it up?

 

[Full Chisel]

Probably just a casting defect...it happens occasionally.

 

[Boosted3g]

I'm surprised there is not more forged or performance replacements. I saw brads post on a domed Weisco with coated skirts and really liked the design. Heck even a higher compression cast piston would be nice. This was my first time splitting a chainsaw case and it was a good time. Plenty of older threads and some knowledgable guys here to get me through.

 

[blsnelling]

Wow. You're fortunate to have caught that!

 

[Full Chisel]

Wow. You're fortunate to have caught that!

Yeah, that would NOT have been pretty when it let loose!

 

[lefturnfreek]

Go buy a lotto ticket cause your sooo lucky there....

 

[Boosted3g]

I used the saw today and it ran great. Burned less than a tank of fuel and grabbed the 260. As fun as the 361 is I just love the 260. I'm just a homeowner and the big oak came down today so the 361 will probably make its way to eBay. I'm glad I fixed that 361 up so the next guy don't have any issues.

 

[diezelsmoke]

Probably was that way from day one. Remember a 2 stroke is always a power stroke, no intake stroke so pressure is always pushing down.

 

[Cliff R]

When you die and go to heaven pray that you don't get reincarnated as a chainsaw piston or a valve spring!

That piston spent a good part of it's life coming to the top of the bore around 200 times per second, and it has to completely stop at BDC and TDC for each cycle, relatively heavy for the size, dragging a couple of rings with it, and getting yanked around by the connecting rod with pretty good authority, and each time it comes up to the top of the bore there is a mild explosion on top of it.

Considering the workout a chainsaw piston gets, in a dirty environment, with nothing more to lubricate it than a little oil coming in with the fuel mix, it's a miracle they last more than a few minutes anyhow.

Glad you caught it before the big "boom". Although I'm not surprised by the cracking, especially for the "windowed" design. It is however, far more common with chainsaw pistons to see damage to the sided of them and smearing material across the rings and onto the bore than catastrophic failure.

I can't say as I've ever seen a forged piston in a chainsaw engine. Cast pistons are used not only due to cost of ease of manufacture, they are also very stable dimensionally with minimal growth when they heat up. Casting are also very strong, but unlike a forging they will crack when they reach their elastic limit. I've pulled down race car engines using forged pistons that had valves dropped on them and other than the valve head being imbedding deep in the piston they were in pretty decent shape. I've pulled down race engines with hypereutectic cast pistons in them that were hit with a shot of nitrous and you couldn't find a piece of piston bigger than a half dollar anyplace! What would you want in your engine?......Cliff

 

[Boosted3g]

Funny you speak of race car engines since that is my background before I retired from the sport in 2006. The 2 stroke process just interests me since I haven't had much experience with it. I've spent many hours on here just reading about porting and tuning. It's different terms but some it relative to 4 stroke engines. I fully understand when we talk about degreeing an engine and adjusting the squish but I thought there would be more of a performance aftermarket. The only aftermarket I find is the Chinese aftermarket. In the past I've made piston and combustion chamber molds and had Pistons custom made. Before Dart and Edelbrock started producing race blocks and heads for the Honda 4 cylinders we were already machining plates and sleeving the cylinders taller to use 4" Chevy rods. We were making 8+hp per cubic inch and turning rpm that will rival a chainsaw and not one forged piston failure ever. Basically what I was saying is that I've seen a ton of broken cast pistons and thought with the amount of guys modding saws that we would have some options. Last company I used was CP and they were very reasonable. Maybe the rod or cranks are not up to the task? I don't know I'm just thinking out loud and when I'm looking for power I look at internal parts.

 

[Smokepole]

+1 Boosted3g. I'm surprised that I don't here more about coating pistons for race and woods ported saws. Back when my brother and I dragged sleds, it was cheap insurance to get the domes coated and the skirts polymoly'd . Used to drop them to Swain tech and get them done. I guess they are probably still in business. Considering what a new pro saw cost, it seems a reasonable cost on a performance rebuild.

 

[Cliff R]

"Funny you speak of race car engines since that is my background before I retired from the sport in 2006."

We have 5 cars that we drive/race out of my shop, and I'm still full time in the industry rebuilding engines, transmissions, carburetors and distributors. I also work on a good bit of small power equipment, and cut wood to heat my house and shop. We've been downsizing over the last 10 years or so, and now parts sales, book sales, and carburetors work are the "bread and butter" here.

As for forged pistons for these engines, the market for them would be very small. Forged piston technology has really improved in the past couple of decades. The new alloys don't expand much more than cast pistons, so we can fit them really tight in the bores, which improve ring seal, reduce ring wear, skirt wear, and make more power as well.

For small 2 stroke engines I stick with OEM P/C's, and have had a 100 percent success ratio with them. Some folks go aftermarket, most of that stuff is offshore production, and quality ranges for pathetic to OK from what I've seen come in here....FWIW.....Cliff