Grenade!!!!!!!!!!!!!!!!! Warning, GRAPHIC content!!!
- Thread starter blsnelling
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Welcome to the world of hot rod chain saw failure. Having had all the various things happen to me or friends we got pretty good ideas what caused it. I will throw this out and you decide for yourself.
Everytime a circlip comes out it leaves up and down grooves. You will see the evidence in the piston or cylinder wall. Most times the clip is gone or embedded in the grooves. I don't see it and the clips look in too good of condition. Same if a screw of washer goes down the throat. Steel is hard and aluminum is not. Saw an actual imprint of a screw once on top of piston. It was the throttle shaft butterfly screw.
Ring snag is generally caused by lack of proper bevel. You don't need bevel at the side of the port only the horizontal part. A little more toward the middle tapering out toward the edge works well. Thing is a ring snags more at low RPM than high because it has more dwell time to jump into the port. If a ring is goint to snag you will feel it most when you are trying to pull the starter. If it is bad you will have the piston stop. Now if it is just slight the ring will beat a tell tale mark where it is catching. It doesn't just jump in there one time and explode (it was idling for 5 minutes). It will also leave a slight widened mark in the ring groove. I don't see that so have to discount that posibility.
OK then what? When you widen the intake port there is less support for the piston skirt. The side thrust in this area is tremendous. It increases with RPM and load. The aftermarket pistons seem more failure prone. I am no metalurgist but the alloys of aluminum are innumerable. Some are more brittle some tougher and some softer. When that piston is down near BDC the thrust is trying to force the piston into the intake port. When the skirt cracks, breaks and starts churning in the crankcase, pieces get ground up. Pieces get caught at the transfers. Pieces get between the top of the piston and the top of the cylinder. Castrophic failure is the result. It doesn't take much time, at say 8K, to do that kind of damage.
One last possibility is the material above the top ring breaks and the pieces destroy the engine. That always leaves lots of pinging, stippling, gouging on the top of the piston. Generally happens when we get to exeuberent in removing material for the popup and the ring is not supported above like it should be. Don't see that though. Brad? Mike
Everytime a circlip comes out it leaves up and down grooves. You will see the evidence in the piston or cylinder wall. Most times the clip is gone or embedded in the grooves. I don't see it and the clips look in too good of condition. Same if a screw of washer goes down the throat. Steel is hard and aluminum is not. Saw an actual imprint of a screw once on top of piston. It was the throttle shaft butterfly screw.
Ring snag is generally caused by lack of proper bevel. You don't need bevel at the side of the port only the horizontal part. A little more toward the middle tapering out toward the edge works well. Thing is a ring snags more at low RPM than high because it has more dwell time to jump into the port. If a ring is goint to snag you will feel it most when you are trying to pull the starter. If it is bad you will have the piston stop. Now if it is just slight the ring will beat a tell tale mark where it is catching. It doesn't just jump in there one time and explode (it was idling for 5 minutes). It will also leave a slight widened mark in the ring groove. I don't see that so have to discount that posibility.
OK then what? When you widen the intake port there is less support for the piston skirt. The side thrust in this area is tremendous. It increases with RPM and load. The aftermarket pistons seem more failure prone. I am no metalurgist but the alloys of aluminum are innumerable. Some are more brittle some tougher and some softer. When that piston is down near BDC the thrust is trying to force the piston into the intake port. When the skirt cracks, breaks and starts churning in the crankcase, pieces get ground up. Pieces get caught at the transfers. Pieces get between the top of the piston and the top of the cylinder. Castrophic failure is the result. It doesn't take much time, at say 8K, to do that kind of damage.
One last possibility is the material above the top ring breaks and the pieces destroy the engine. That always leaves lots of pinging, stippling, gouging on the top of the piston. Generally happens when we get to exeuberent in removing material for the popup and the ring is not supported above like it should be. Don't see that though. Brad? Mike
I will rise again
Please tell me that is a phoenix reference, and not a Jesus complex...
Everything you said makes sense to me. So your theory is that this was a case of the intake skirt failing due to lack of support, and possibly due to a brittle material? It was definately MUCH harder than normal. Your comments about lots of pieces churning around and getting ground up fit the bill. I was amazed how much small debris there was for an immediate and catastrophic failure.
AUSSIE1
Al.
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Yeah that's a real bummer. 
It doesn't look like you have overly relieved the piston.
What brand clips did you use?
I did notice in the "A tale of three saws" (I think it was) you like very wide transfers.
I suppose you get so wide it probably requires a bit of curvature like the Ex and In.
It doesn't look like you have overly relieved the piston.
What brand clips did you use?
I did notice in the "A tale of three saws" (I think it was) you like very wide transfers.
I suppose you get so wide it probably requires a bit of curvature like the Ex and In.
AUSSIE1
Al.
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You don't need a lot of bevel. Coating thickness is usually all that's needed.
Too much bevel is only compensating for a lack of curvature at the top or bottom of the port.
But as Rupedoggy states more in the centre and lessening toward the sides would give a slight curvature.
Too much bevel is only compensating for a lack of curvature at the top or bottom of the port.
But as Rupedoggy states more in the centre and lessening toward the sides would give a slight curvature.
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Yeah that's a real bummer.
It doesn't look like you have overly relieved the piston.
What brand clips did you use?
I did notice in the "A tale of three saws" (I think it was) you like very wide transfers.
I suppose you get so wide it probably requires a bit of curvature like the Ex and In.
I used the clips from the kit. I really don't think it was a failure of the clip. Right now I'm thinking skirt failure or ring caught on the transfer. I'm still surprised to think a ring would catch in one of these transfers. They're not near as wide as an intake or exhaust. Plus I've ported too many cylinders like this and never had a problem. I guess there's always a first though. Fortunately, this was my saw, reguardless of the cause.
The transfers were beveled, both top and bottom. I bevel them with a stone, and then finish by hand with 600 grit paper. I still have a hard time imagining them hanging.
File a claim with UPS .....
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File a claim with UPS .....
I filed this one with Snellerized Saws. They've already got it back up and running. You can't beat their service:greenchainsaw:
Miniburbs36
ArboristSite Operative
Seems like snellerized saws has got something going for it, maybe I should send them a 346xp for porting once I get done clearing.
btw did you see my muffler mod? you might only have to do porting as I am pretty proud of it.
btw did you see my muffler mod? you might only have to do porting as I am pretty proud of it.
Seems like snellerized saws has got something going for it, maybe I should send them a 346xp for porting once I get done clearing.
btw did you see my muffler mod? you might only have to do porting as I am pretty proud of it.
Both of your mods look good
outdoorlivin247
Addicted to ArboristSite
File a claim with UPS .....
.
I filed this one with Snellerized Saws. They've already got it back up and running. You can't beat their service:greenchainsaw:
No offense Steve, but :hmm3grin2orange:...LOL
I have to go with the brittle piston and a bad batch of casting on the pistons for this. As you said the piston was extremely hard which makes me think it was a bad casting or a mis-match alloy for the casting which ended up being extremely brittle. Where a normal piston probably would have survived what happened, yours exploded because it was extremely brittle. Different alloys can also have slightly different expansion rates which could also cause problems. I would let whoever sold you the kit know that they may have a problem with that run of pistons to prevent others from having the same problem.
Miniburbs36
ArboristSite Operative
Both of your mods look good
Haha, I sortof had to do it, will that be okay for porting? its like a 3/8 to 7/16, it melted a tiny bit while welding.
Sorry bout off topic :/
spacemule
The Peanut Gallery
That's what happens when you try to "fix" a perfectly good saw. Shame.
Maybe a cooling fin found it's way in there?
Maybe a cooling fin found it's way in there?
Miniburbs36
ArboristSite Operative
One could say "improve" but nobody will ever learn anything if we don't fail a few times. Many of the greatest improvements have been made with trial and error.
spacemule
The Peanut Gallery
I've seen enough of Brad's "improvements."
bent connecting rods, broken fins, slower than stock runs, etc. etc.
Miniburbs36
ArboristSite Operative
Haha, this is probably one of the best yet.
