MS 650 Now Has Zero Compression?

[Wood Doctor]

Casting Flaw?

Could it be as simple as a casting flaw in the piston, resulting in an area that was slightly thinner than its surroundings? Might not ever show on a normal saw, but showed on this high-hour saw?

I doubt that because the same owner has another MS 650 running flawlessly with the same number of hours. He also told me today that he rarely uses the Decomp valve. He has very strong arms and can drop start the saw even when cold.

The point I guess I am trying to make is that the hole in the piston crown is right below the Decomp valve--dead center below. If anything, the fact that the owner and operator rarely used the decomp valve when starting makes this an even greater mystery.

BTW, do you or anyone else know what metallic alloy that Stihl is using to make these pistons?

 

[timmcat]

Here is another thing to consider, the folks at the big orange mothership are starting to use the smaller 12 and 10mm spark plugs in their strato saws as a way to reduce the ununiform shaping of the combustion chamber, seems they were having failures caused by small pockets of air hanging around in the cylinder and causing uneven burn and detonation. If your 650 was adjusted right to the edge of lean and was making some really long duration cuts maybe the flat spot caused by the plunger on the decomp could cause the same phenomenon. I know its a stretch but you gotta think outside the box once and a while.

 

[morgaj1]

BTW, do you or anyone else know what metallic alloy that Stihl is using to make these pistons?

Quick research says that most small motor pistons are eutectic Aluminum Silicon Alloy. I couldn't find the exact alloy.

 

[Wood Doctor]

Sorry, I wasn't trying to be 'smart', just reading it like it was, Now you corrected your post and this may help solve your problem.

The written word is only as good what is written. I'm not a mind reader, but if I wanted the blue box but said to get the red one, would you get the blue one?

I thought it would be a bit strange for a plug color to be checked before a problem was known. I do hope you get the source of the problem found.

See how much better your post reads with correct punctuaton? I apologize for using one incorrect word in mine. Notice how one incorrect word threw you for a loop?

Now, let's forget this crap. I am trying to fix a good chainsaw that burned a hole in the piston crown. Does anyone know what metallic alloy that Stihl uses to make the MS 650 and or MS 660 piston head? It's possible that this is a major design flaw.

 

[Wood Doctor]

Aluminum Alloy?

Quick research says that most small motor pistons are eutectic Aluminum Silicon Alloy. I couldn't find the exact alloy.

If true, that may be the problem. Good grief. Why not a high quality steel alloy instead of aluminum that melts down to nothing in my wood stove? Are you trying to tell me that today we are using aluminum alloy in high quality, expensive chain saws to make piston crowns? Gasp!

 

[Cerran]

If true, that may be the problem. Good grief. Why not a high quality steel alloy instead of aluminum that melts down to nothing in my wood stove? Are you trying to tell me that today we are using aluminum alloy in high quality, expensive chain saws to make piston crowns? Gasp!

Weight. The high RPM in a modern saw demands a lightweight alloy for use in the piston and steel won't work in this application. I would imagine there are some stronger more exotic alloys available but the cost would likely be 2-3x.

Remember with mass produced goods there is often a balancing act played between several different factors such as performance, longevity and price.

 

[edisto]

If true, that may be the problem. Good grief. Why not a high quality steel alloy instead of aluminum that melts down to nothing in my wood stove? Are you trying to tell me that today we are using aluminum alloy in high quality, expensive chain saws to make piston crowns? Gasp!

Not just today. For a long time after today. Wait...prior to. 40 years maybe?

They use them in racing pistons too. Gasp!

Silicon content is what will vary with cost and quality.

You seem to be trying very hard to turn what should be a diagnostic into a witch hunt.

 

[blsnelling]

Seriously? Steel pistons?:msp_scared: A decomp causing holes in pistons?:taped: And decomps being a major defect in Stihl saws? This discussion is all over the place, most of it making little sense. Until you get that cylinder off and post pics, this is nothing more than an uneducated guessing game.

 

[showrguy]

i'm glad i saw this thread, but, now i am worried sick about this..

heading out to the garage to tear down all my saws with decomps in them, just to make sure they don't all have holes beginning to form in the piston crowns...

and, if i find out that stihl put aluminum pistons in any of MY saws, the chitz gonna hit the fan !!!

 

[edisto]

Seriously? Steel pistons?:msp_scared: A decomp causing holes in pistons?:taped: And decomps being a major defect in Stihl saws? This discussion is all over the place, most of it making little sense. Until you get that cylinder off and post pics, this is nothing more than an uneducated guessing game.

Thank God you came along.

In lieu of pictures, however, it would be helpful to know if the hole looks burned through, or "punched" through.

i'm glad i saw this thread, but, now i am worried sick about this..

heading out to the garage to tear down all my saws with decomps in them, just to make sure they don't all have holes beginning to form in the piston crowns...

and, if i find out that stihl put aluminum pistons in any of MY saws, the chitz gonna hit the fan !!!

Just run a lag bolt through decomp...problem solved.

 

[Wood Doctor]

Rebuttal

Not just today. For a long time after today. Wait...prior to. 40 years maybe? They use them in racing pistons too. Gasp! Silicon content is what will vary with cost and quality. You seem to be trying very hard to turn what should be a diagnostic into a witch hunt.

Not true at all. The hole in this piston crown is there, believe me, right below the decomp valve. I will post a Pic if at all possible. There is no witch hunt here--just a professional loggers chainsaw engine that has 500+ hours on it and a 1/4" dia. hole burned right through the piston crown.

Look, are racing pistons designed to last 500+ hours of operation? I doubt it. Are competition saws designed to last 500+ hours? Give me a break.

Guys, it's a machine, and even if it's a Stihl, it's going to eventually fail, even when used by a trained professional. This MS 650 is now being rebuilt and the holy piston is being replaced.

 

[Trx250r180]

was the top of the saw covered in black goo like tar if the decomp valve was leaking ?

 

[ZeroJunk]

There are a multitude of Stihl saws with decomp that have a hell of a lot more than 500 hours on them still running strong.

Normally if you keep them tuned right with good mix the rest of the saw will fall apart before the piston does.

 

[edisto]

Not true at all. The hole in this piston crown is there, believe me, right below the decomp valve. I will post a Pic if at all possible. There is no witch hunt here--just a professional loggers chainsaw engine that has 500+ hours on it and a 1/4" dia. hole burned right through the piston crown.

I didn't say it wasn't there. I did, however, ask about what the hole looked like.

Look, are racing pistons designed to last 500+ hours of operation? I doubt it. Are competition saws designed to last 500+ hours? Give me a break.

Yup. No sense wasting money on a good piston if you are going to race.

Guys, it's a machine, and even if it's a Stihl, it's going to eventually fail, even when used by a trained professional.

That's what I've been trying to tell you. No point running around in circles looking for design flaws.

This MS 650 is now being rebuilt and the holy piston is being replaced.

I presume it is getting one of those sweet steel pistons?

 

[Jacob J.]

There are a multitude of Stihl saws with decomp that have a hell of a lot more than 500 hours on them still running strong.

Normally if you keep them tuned right with good mix the rest of the saw will fall apart before the piston does.

I worked for a dealership (Stihl) belonging to a large logging company and they had an 064 that went over 2,000 hours on a landing without a major rebuild. That's 2,000 hours in the worst working conditions that a saw will endure.

 

[madhatte]

Fwiw, a hole burnt in the piston top is usually caused by bad fuel, or too much ignition timing...

Which leads us back to the question of the flywheel key. Is it or is it not intact?

Also: where are the pics? These will tell the rest of the story.

 

[blsnelling]

How could a flywheel key have anything to do with a saw loosing compression? If this piston wasn't moving, then it might give that impression, but that doesn't sound to be the case here.

 

[CTYank]

Not true at all. The hole in this piston crown is there, believe me, right below the decomp valve. I will post a Pic if at all possible. There is no witch hunt here--just a professional loggers chainsaw engine that has 500+ hours on it and a 1/4" dia. hole burned right through the piston crown.

Look, are racing pistons designed to last 500+ hours of operation? I doubt it. Are competition saws designed to last 500+ hours? Give me a break.

Guys, it's a machine, and even if it's a Stihl, it's going to eventually fail, even when used by a trained professional. This MS 650 is now being rebuilt and the holy piston is being replaced.

Give up on the decomp valve. No chance in hell- if there were, you'd hear that thing tooting like a ship's whistle. Blowing OUT.

Re piston materials, check the early history of internal combustion engines. Pistons were made of cast iron. Some early diesels were limited to less than 600 rpm because of the forces involved in accelerating those slugs up and down (among other things). Note that the forces are proportional to the square of the rotational speed; double the speed, quadruple the forces.

At the rotational speed of modern saw engines, any sort of steel/iron pistons would be doing a taffy-pull with the crank. Not to mention providing great glow-plugs on their crown.

We need to see the edge of the hole to formulate a guess as to whether the cause was manufacturing failure, thermally problem, or ingested debris. Why? To help you and us avoid recurrence.

 

[Jacob J.]

How could a flywheel key have anything to do with a saw loosing compression? If this piston wasn't moving, then it might give that impression, but that doesn't sound to be the case here.

I think Wiggs was referring to the ignition timing being off which can burn a hole in the piston...

Another thing on the alloy piston- expansion under heat and pressure. A steel piston would expand too much.
That's why they haven't made a reliable connecting rod out of titanium. There's alloy ones...

The Silicon content helps with hardness and resistance to detonation pitting. Jonsereds used to
make their pistons out of a Silicon/Tin/Aluminum alloy.

From what I've read, aluminum and other alloys have been used in two-stroke pistons since
the very early days of two-stroke engines. My IEL model F has an alloy piston in it and it
was made in 1945.

 

[edisto]

How could a flywheel key have anything to do with a saw loosing compression? If this piston wasn't moving, then it might give that impression, but that doesn't sound to be the case here.

Every time I've seen a damaged key, the engine wouldn't run, but pre-ignition can hole the piston, which makes it hard to get good compression numbers.

Of course, detonation can hole a piston too...and detonation can heat things up enough to cause pre-ignition...and pre-ignition can lead to detonation...and...ummm...

Edit: how it it the sleepy guy can type faster than Beaker?