066 Magnum Big Bore Project

Thanks for the quick response;

blsnelling said:

[*]Was there any breakin time before putting it on the mill?

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No milling, I bucked up a couple of logs, cut a bunch of cookies, and ripped a 12" diam x 2 ft log to see some curlies fly. All up less than one tank of mix.

Did you pressure test the saw? You may have a bad seal that caused both this and the original piston to seize. Yes, it can run well and still have a leak. I learned that the hard way.

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Piston was not seized, it was running really well.
I didn't pressure test, but how does pressure generate bits of metal?

[*]What kind of fuel and lube?

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Premium gas and Stihl standard lube 40:1.

Before starting I coated the inside of the cylinder, rings and piston with lube, greased and lubed the little end, and poured a capful of lube inside the crank and turned the saw over and over so it was well coated.

Now here's some new info, the piston scoring is on exactly the same place on the old cylinder as the new one.

Thanks again for your help.

I just thought on one other thing. I inserted the decomp valve after I put the thing together, I wonder if some pieces fell off the cylinder top when I did that?

Howdy,
Did you have any scoring 180 degrees from the scoring you've pictured?

blsnelling said:

I have a few thoughts and questions.

[*]Did you pressure test the saw? You may have a bad seal that caused both this and the original piston to seize. Yes, it can run well and still have a leak. I learned that the hard way.
[/LIST]

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BobL said:

Thanks for the quick response;

Piston was not seized, it was running really well.
I didn't pressure test, but how does pressure generate bits of metal?

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What Brad means is did you pressure/Vac test the crankcase for leaks.

If you have a leak in the case (impulse hose, intake boot, crank seal, etc.) it will allow air into the mixture causing a lean seize and the scoring you are seeing on both your old and new pistons and cylinders.

If you put a new one on and still don't fix the leak you will have a third fried top end.... the seizer is only the symptom the leak is the cause.

BobL said:

...but how does pressure generate bits of metal?

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The bits of metal are aluminum that melted and transfered from the piston. The line you showed in the combustion chamber is simply a casting line.

I dont think overheat lean air fuel mixture seize because that should swell the piston into hard contact all across the exhaust side. This is concentrated to a stripe. The top of the piston doesnt show damage or heat either. Is there anything that could cause localized pressure from the outside on that area of the cylinder. I have known cylinders to jam against the side of the case on the clutch side and on both sides on another saw caused scoring the piston. In both cases it was not real easy to spot the interference either. A casting flaw or a too long muffler bolt could be a thing to consider in the autopsy!

Grande Dog said:

Howdy,
Did you have any scoring 180 degrees from the scoring you've pictured?

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Here's picture 180º opposite of the piston.



Here's the barrel at 180º - the two shots are of the same side. Some rubbing but no deposited metal.


Thanks.

Thanks for all your help fellas, this very much appreciated - there are not too many people that can help here on the other side of the world

blsnelling said:

The bits of metal are aluminum that melted and transfered from the piston. The line you showed in the combustion chamber is simply a casting line.

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So are you suggesting that bits somehow came off (melted, broke ??) the piston? There don't appear to be any bits missing off the top or bottom of the piston.

Apart from setting up where I went over 14,000 rpm for a few seconds I always had the saw less than 13500 rpm. I was so paranoid about too lean a mix I dropped it right back to 12700 before cutting anything.

Erick, the piston never at any stage seized, it was running amazingly well. It was cutting as well as any new 066 I have used before. The one difference was it started vibrating more than usual at WOT, like the bar was loose? I even thought the cylinder was loose but I stopped and retorqued it and it was fine.

"Seizing" is a relative term. Just prior to seizing, it look just like yours...

Lakeside53 said:

"Seizing" is a relative term. Just prior to seizing, it look just like yours...

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OK.

Crofter, what you are saying is consistent with the scoring being on exactly the same place on the old cylinder/piston as the new one. It's on the clutch side of the exhaust. How long should the exhaust bolts be?

BobL said:

OK.

Crofter, what you are saying is consistent with the scoring being on exactly the same place on the old cylinder/piston as the new one. It's on the clutch side of the exhaust. How long should the exhaust bolts be?

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Not long enough to bottom and push a bulge on the cylinder. Any sign of the transfer bulge hitting the side of the casing over the sprocket area. I am just trying to think what could spot pressure or heat off on one side. Was the original cylinder that scored an OEM Stihl cyl.

Lakeside might know whether the typical scoring on a 66 is off center like that or not.

I rarely get them "a little scored". Most are run hard until they are completely toast.

I'd say that mostly it starts off to one side...

So the OEM piston and cylinder were lightly scored, and now the BB is also lightly scored in pretty much the same manner?

HMMM.... got me thinking....

Here's 4 066 pistons from my home bench (why don't I throw this crap out?????). At work it all goes into the recycling.

They all look "centered" to me.

BobL said:

Erick, the piston never at any stage seized, it was running amazingly well. It was cutting as well as any new 066 I have used before. The one difference was it started vibrating more than usual at WOT, like the bar was loose? I even thought the cylinder was loose but I stopped and retorqued it and it was fine.

Lakeside53 said:

"Seizing" is a relative term. Just prior to seizing, it look just like yours...

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:agree2:

If the saw was not run to the point of seizure the scoring is not as bad and will look just like your piston. It's not uncommon for a saw that has been seized tight to break free when it cools and start back up and run, it just wont run well. My guess is you probably made one cut that took a little longer than the rest, maybe that rip cut :dunno:, and the piston began to seize and transfer aluminum to the cylinder wall. The cut just didn't take long enough to do a lot of damage. I would pressure test the crankcase and look hard at the crank seals... seems (IMO) to be a very small leak probably under vacuum, pressure may not show it. JMO

Lakeside53 said:

I rarely get them "a little scored". Most are run hard until they are completely toast.

I'd say that mostly it starts off to one side...

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I think more heat is transferred at the corners for some reason, not sure why but I have seen them scored on both sides of the exhaust port and not in the middle before.

I suspect that the port has more of a cooling effect in the middle from the small amount of fresh charge that gets sucked out of the cylinder possibly cooling the piston more in the center. :dunno:

Here are the photos of the original cylinder and piston (OLD) alongside the (New) damaged BB cylinder and piston.





On another matter I used a new 54 mm base gasket but should I perhaps have used a different one?

The bolt on the clutch side of the exhaust is also very close to bottoming out - I'm definitely putting it back with a washer underneath next time. On the old cylinder the scoring is greatest around the area directly behind that exhaust bolt.

I will pressure test the crank case on the weekend which will help to narrow things down a bit.

Thanks again for everyones input.

Cheers

That is surprisingly suspicious! They look nearly identical. Could the rod be bent?

A bent rod should show a diagonal contact pattern or top one side and heels of skirts on the other. If you could get an accurate inside bore dia. measurements at various places it would be interesting but if it any distortion was bolt up related, not possible after assembly and might not show up when apart.

Crofter said:

A bent rod should show a diagonal contact pattern or top one side and heels of skirts on the other. If you could get an accurate inside bore dia. measurements at various places it would be interesting but if it any distortion was bolt up related, not possible after assembly and might not show up when apart.

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Thanks Frank, there is a bit of diagonal in the score pattern but is that enough? Should any bend in the rod be measurable with a straight edge or would something better be needed? I presume a bent rod would induce a fair bit of vibe as well?

One side of the scored area runs off the skirt and you really only see one edge of it. Because the piston is tapered smaller at the top than bottom it could give less than parallel border to it. Check the rod but I would expect the bearing to fail before a bent rod put that much force on the piston.

Check this link out. There is a few hours of good reading here if you chase down all the connecting links. This link is diagnosing lock ring failure and shows wear patterns where a bent or twisted rod is involved. A very informative site!

The power pulses and the piston trying to lock up could give alternating rotational forces that you might feel as vibration. It definitely was hurting.

http://www.kb-silvolite.com/article.php?action=read&A_id=54