Did finger ports kill this 394?
- Thread starter marti384
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Maybe, but I'm not seeing where the ring dug into anything. Maybe its my old eyes and there is, I don't see it. Of course if there is evidence then obviously the answer is yes, those finger ports let the ring ends float and eventually fail. Even if there wasn't a catastrophic "dig" and it was a cyclical failure I would expect to see some evidence in or around the finger port if the rings were moving out of their grooves & being forced back in against their will... Failure Might be cause be a "flutter" because of change in bearing surface, piston to cylinder wall clearance, or something else. Could have been a marginal piston....lots of things. Maybe it didn't like the new RPM's allowed by the greater transfer port area so while not directly caused by the shape, the resultant dynamics killed it. Best way to find out??? Stuff another piston in there and run it... 
Either way if it fails again you have to put another top end on it! Might as well test the theory and get some more use out of that one...
I have to confess to NOT liking anything that would let the open ends of the rings "float" unsupported. Common sense flags that as a way to wreck a saw. Are there any single ring piston options with a more favorable pin location? How wide is the exhaust port? What would happen if you ran the piston backwards should you see evidence of "trauma" around where the ends of those rings ride in the bore and still want to run that cylinder. Would those pins ride on the cylinder wall vs. across the exhaust port?
Either way if it fails again you have to put another top end on it! Might as well test the theory and get some more use out of that one...I have to confess to NOT liking anything that would let the open ends of the rings "float" unsupported. Common sense flags that as a way to wreck a saw. Are there any single ring piston options with a more favorable pin location? How wide is the exhaust port? What would happen if you ran the piston backwards should you see evidence of "trauma" around where the ends of those rings ride in the bore and still want to run that cylinder. Would those pins ride on the cylinder wall vs. across the exhaust port?
The piston would typically be installed backwards or the pins otherwise moved on a build like that. Was the piston replaced by someone other than the builder?
bikemike
loud pipes save lives
Would it have caught a ring if the piston was put in backwards?
cus_deluxe
Commie Satan turd
Hard to tell if its just the lighting or what, but almost looks like a little shiny spot on transfer port roof, that looks like a more likely spot for a ring end to hang but hard to tell wothout being roght there to be able to line it up. Im pretty novice when it comes port timing/mods but is that a normal sized transfer port? Seems very wide. Those fingers look very nice
cus_deluxe
Commie Satan turd
I mean normal sized transfer port for ported 394 of course
Trx250r180
Saw polisher
As long as the exhaust port was not widened into them ,one of my saw has fingers and the piston has to be in backwards because of what likely happened here .
bikemike
loud pipes save lives
Yeah it seams to be a common rule with fingers to reverse the piston if it cleans the exhaust port with the ring tips
not me, I wouldn't trust myself to do port work like that. I bought it with another one as parts saws.
I also couldn't really see anywhere that the ring dug into the cylinder, that is why I was asking. It seemed the most likely scenario to me that the ring end got caught. I like the idea of putting another piston in and seeing what happens. But maybe backwards if the ends clear the exhaust port.Maybe, but I'm not seeing where the ring dug into anything. Maybe its my old eyes and there is, I don't see it. Of course if there is evidence then obviously the answer is yes, those finger ports let the ring ends float and eventually fail. Even if there wasn't a catastrophic "dig" and it was a cyclical failure I would expect to see some evidence in or around the finger port if the rings were moving out of their grooves & being forced back in against their will... Failure Might be cause be a "flutter" because of change in bearing surface, piston to cylinder wall clearance, or something else. Could have been a marginal piston....lots of things. Maybe it didn't like the new RPM's allowed by the greater transfer port area so while not directly caused by the shape, the resultant dynamics killed it. Best way to find out??? Stuff another piston in there and run it...
I have to confess to NOT liking anything that would let the open ends of the rings "float" unsupported. Common sense flags that as a way to wreck a saw. Are there any single ring piston options with a more favorable pin location? How wide is the exhaust port? What would happen if you ran the piston backwards should you see evidence of "trauma" around where the ends of those rings ride in the bore and still want to run that cylinder. Would those pins ride on the cylinder wall vs. across the exhaust port?
Can't answer that, don't know who built it or if it was ever replaced.
I think it is a normal size transfer, it looks like the only porting done was the fingers and some weird groove by the exhaust. Maybe the uppers were raised, but not sure.
I don't see any sign of a ring ever hanging in that cylinder.
I do see a slight wipe on the intake side from the piston....betting that piston had a lot of slap. The thing that can confuse things is fractures like that usually start at stress risers. Sharp edges inside the ring grooves.... those pin holes etc. So just because it fractured there doesn't mean the actual trauma happened there, its just the point where the fracture actually formed that ended up in failure. Think about busting a piece of Hershey's chocolate off one of those candy bars...you grab a hold of the piece and it snaps off at the weak point.
The answer in a 394 with finger ports is to use the 395 piston, because it has its' rings pinned near the center of the intake side thus being well clear of those fingers. I agree with Walt though on probable piston slap- the 394s and 395s can take a lot of hours and when they get a lot of miles on them, the piston skirts get very thin.
tomdcoker
Addicted to ArboristSite
I a on a learning curve here, so I need to know if the teardrop opening is on the exhaust side or intake side. I believe it is on the exhaust side, but not sure. Tom
Wish I had the skirt to see how thin it was. Piston slap makes sense to me since I couldn't feel anywhere in the cylinder where a ring caught. So should a 395 piston work in the correct orientation then?
That is in the exhaust port, have no idea what the point of it is.
ncfarmboy
Addicted to ArboristSite
I remember reading builders used to cut that in but I can't remember what for.
Shep
Shep
