This is exactly what I was alluding to. It's pretty much either spark or fuel, and knowing that it isn't one means that it is the other.
MS 650 Now Has Zero Compression?
- Thread starter Wood Doctor
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This is exactly what I was alluding to. It's pretty much either spark or fuel, and knowing that it isn't one means that it is the other.
Gotcha.
Trx250r180
Saw polisher
any pics of the damaged parts yet ?
wigglesworth
Booned
Yep. Too much advance will cause the mixture to burn while it's still in the squish band, causing hot spots on the crown, and will cause eventual failure. I'd really like to see a pic of said hole. Was there any peppering or deformation on the rest of the crown?
From the few piston crowns I've seen killed from detonation, they are usually void of much carbon due to the excessive heat, and peppered all to crap, with a nice big melted hole in em, usually on the exhaust side. But ima newb, wut does I no.....
From the few piston crowns I've seen killed from detonation, they are usually void of much carbon due to the excessive heat, and peppered all to crap, with a nice big melted hole in em, usually on the exhaust side. But ima newb, wut does I no.....
My understanding is that pre-ignition, where the charge starts to burn before the spark arrives, produces a melted hole, because the crown is overheated while compression still is increasing. If you are "lucky", the problem will self-correct by making the piston concave and reducing the compression ratio.
Detonation, on the other hand, is the result of a second flame front being produced in addition to that generated by the spark, producing excessive pre-heating that leads to a burn rate closer to an explosion than it is to the desired steady pressure increase. Pitting of the crown definitely is a sign of this, but I'm told the hole looks more like it was physically punched through than it looks burned through.
Either way, it sounds like a bad thing...
None in Sight--Yet
I don't see any other deformation or peppering on the crown but I still haven't got the cylinder head removed. Cracked yet another screwdriver while trying to loosen the mounting screws. I ordered a special tool to help and may have to try some chemicals. This thing is a bear. Soon as I get the piston off, I'll post a pic. I may have to start another thread on the topic.
Yep. Too much advance will cause the mixture to burn while it's still in the squish band, causing hot spots on the crown, and will cause eventual failure. I'd really like to see a pic of said hole. Was there any peppering or deformation on the rest of the crown?
From the few piston crowns I've seen killed from detonation, they are usually void of much carbon due to the excessive heat, and peppered all to crap, with a nice big melted hole in em, usually on the exhaust side. But ima newb, wut does I no.....
I don't see any other deformation or peppering on the crown but I still haven't got the cylinder head removed. Cracked yet another screwdriver while trying to loosen the mounting screws. I ordered a special tool to help and may have to try some chemicals. This thing is a bear. Soon as I get the piston off, I'll post a pic. I may have to start another thread on the topic.
Good idea...this one is starting to smell fishy.
Interesting that you would say that. What's "fishy" about it? The event happened. Here are the facts:
(1) The crown of a Stihl chainsaw's piston has a hole in it the the size of a pencil's eraser. The saw, therefore, has no compression whatsoever and stopped suddenly.
(2) The engine, about 6 years old, has delivered 500+ hours of operation.
(3) The saw was operated by a professional logger.
(4) The model number is MS 650, pulling a 36", 0.063" gauge bar, and a 0.375" pitch chain.
(5) The engine stopped at full throttle while doing a normal cutting operation of a big log destined for a sawmill.
(6) The hole in the piston is directly below the decomp valve.
(7) The piston's crown is not discolored.
Now, please tell me what on earth is fishy about this thread?
Heavy carbon build up will cause detonation, pr-ignition, and burn holes in pistons. Btw, 500 hours on a saw designed for 2500 is only 20% of it's life.
That's also what the owner told me (somewhat in disgust) as you would expect. But, if the piston's aluminum alloy crown failed after 500 hours, then it would seem that Stihl's engineers never tested the engine for 2,500 hours of typical operation.
I assure you that there is no heavy carbon build up on this piston crown. I also assure you that a Pic of this piston crown will be posted here as soon as I can get to it. And, if I cannot remove the cylinder head, I will find a mechanic who can help.
How about the fact that a professional logger gave a saw to someone to fix who doesn't seem to know the first thing about them?
Ohhhh daddy is not gonna be happy when he sees that one....
sefh3
Addicted to ArboristSite
How are you breaking screwdrivers? You need a T27 torx to remove the jug from the saw. Not a screwdriver????
Why are you so convinced that this has something to do with what the piston is made of, or somehow a general design flaw not caught by their own testing? You would need advanced knowledge of said items for that to be a logical conclusion. You've demonstrated that is not the case at all. Now, if you want help diagnosing this, we'll do what we can, but drop the illogical, uneducated accusations.
Wood Doctor...you must be screwing up to get Brad and I on the same page!
It is a Torx T27
I am very well aware of that. Both broken drivers were T27 Torx, also known as spline drivers or star drivers. They are/were, in fact, screwdrivers because they drive machine screws.
I'm not sure where you are coming from on this issue. I am sure that everyone here has at one time or another snapped the tip of a screwdriver or screw driver bit. It happens.
And, Brad Snelling, these are not intended to be accusations of any sort. They are simply conclusions that I am drawing from an observation.
An MS 650 has no compression, has a hole blown right through the top of a piston crown that occurred under normal operation, and it is being repaired. As soon as I have Pics of this piston to post that may be of interest to you, I shall do so. Thank you, for your feedback. As far as I am concerned, this thread is resolved.
I am very well aware of that. Both broken drivers were T27 Torx, also known as spline drivers or star drivers. They are/were, in fact, screwdrivers because they drive machine screws.
I'm not sure where you are coming from on this issue. I am sure that everyone here has at one time or another snapped the tip of a screwdriver or screw driver bit. It happens.
And, Brad Snelling, these are not intended to be accusations of any sort. They are simply conclusions that I am drawing from an observation.
An MS 650 has no compression, has a hole blown right through the top of a piston crown that occurred under normal operation, and it is being repaired. As soon as I have Pics of this piston to post that may be of interest to you, I shall do so. Thank you, for your feedback. As far as I am concerned, this thread is resolved.
So what caused it?
Dan_IN_MN
Addicted to ArboristSite
I'd like to know too!
Is the jug off yet? How much of the piston was melted? What does the cylinder look like? Is is salvageable?
How many saws does the owner have?
imagineero
Addicted to ArboristSite
(in my best monty python voice)
"run away! run away!"....
"run away! run away!"....
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