026 piston diagnosis

[Al Smith]

I think too it might be the piston .I have detected both taper and ovality on the large Macs .With the Stihls I was unable to determine between that and piston wear due to the fact they were old half worn out to begin with .Plus the fact the Stihls are cut skirt type and the Macs full skirt .Not much area to measure on a cut skirt .

Really though because these pistons are so small any difference would be very slight .I would take a guess most likely below 1 thou on real small ones if any at all .

On automotive type pistons if I'm not mistaken this method was to eliminate or lessen the effects of piston slap .On a 2 cycle because of the cooling effect of the fuel / air being transfered from the crankcase it might not be needed at all .

Very interesting method was used in the days before CNC stuff .It was all done with a "cat head " turner that cut on an elipse for ovality and a wedge for taper .Those old machine tool designer used their heads for something other than a hat rack .

 

[Magnus]

Even if it is round when you measure it cols, as it gets hot it will expand more on exoust side, change shape from round to sort of oval.
The pistons that are used and older are almost never exactly round...

 

[Al Smith]

I'm sure the piston face expands and distorts at least some under running condition but I have no idea how much .I will say though on a badly worn out engine the rings can really look odd .

 

[Timberwolf]

Yes, when I was saying round I was talking new pistons, never checked a used piston other than to measure clearance. No doubt a used piston will be out of round.

 

[Magnus]

I know Parner and Jonsered had various shapes at one point, a long time ago.
Husqvarna had different shape too and marked the kits so it would not turn ugly. A and C could not be mixed, but B you could have in all.

 

[Al Smith]

I think that most likely was due to the fact the machining methods were not as precise as they are today ,neither was the plating .A slight difference in bore sizes made it neccessary to use different sizes of pistons .

The early plated 81 cc McCullochs were the same way but later in production they only offered one size .Evidently the manufacturing process had been refined by that time .

 

[Magnus]

I really doubt that. The precision and quality of casting in 70's and early 80's was higher then than now on Husqvarna at least.

 

[Al Smith]

I'm not saying Husqvarna or in fact that McCulloch did anything incorrectly with the manufacturing process .It was just the fact that the processes at the time were not as refined as they later became as the technoligy of mass production improved world wide .

During the 80's for example it would not have been possible to have consistant 4-5 -10 micron size differentials as it is today .When we are talking 1/ 1000 of a millimeter ,that's tight now .

Logically thinking were it not for the fact that Husky nor Mac could hold the precission they had no other choice than offer three piston sizes to match the cylinder bore size .They didn't do anything wrong as it was just the practice of the time period .

 

[Magnus]

Acc to Husqvarna the reason was to keep track of different sizes, test different expansions and material mixes.
They marked them to make sure it was not mistakenly mixed.
Royal pain to sort thru...

 

[Al Smith]

Just strictly for reference I'd like to know what the factory set clearances are for modern saws ,bore to piston skirt .I don't recall ever hearing the subject being discussed anywhere .

My only reference would be on old Mac cast iron lined engines which is approx 4.5 thou ,which like I afore mentioned are a tad more on the kart versions .Those however were 2 1/4" bore .I suspect that with a smaller bore and plated cylinder it would most likely be less .

 

[Timberwolf]

For what it is worth when I have had custom Nikisil bores done I have spec'd 0.0020 from 50-70mm and 0.0015 for 44 or 45mm pistons.

 

[bill22]

i know this is gonna sound really stupid,but i have seen similar damage on a piston from a guy trying to use a punch through the plug hole trying to hold the piston while removing a clutch on an old 038.Maybe a spark plug electrode busted off in there?

 

[Al Smith]

Well anybody ignorant enough to push a center punch through the plug hole for a piston stop deserves to get a screwed up piston .

 

[bill22]

I agree Al.

 

[Magnus]

It would look a little different. I doubt even if he tried he could not get same random pattern to the sides.

Electrode damege also look a bit different. Fewer marks and it usually is stuck some were...

 

[Magnus]

It would look a little different. I doubt even if he tried he could not get same random pattern to the sides.

Electrode damage also look a bit different. Fewer marks and it usually is stuck some were...

 

[sawcollector]

I don't know if I will put to rest the discussion on this piston but Brian sent it to me and I have studied it up close and personal and passed it around at work to some of the guys that work with me in the tech dept. and we all agree that it is FOD (foreign object damage) and not a carbon ball. I asked Brian if he had inspected the bearings really well and he said they look good. I tend to agree since there are no up and down scratch marks on the side of the piston or the cylinder wall where something got caught coming through the transfer ports, and usually if a bit of metal came through the carb or from a bearing cage it will do some damage in the transfer area. It also looks like the culprit didn't hang around long but made it's way into the muffler pretty quick since the damage is just across the front of the piston. I suspect that maybe a piece of flashing or some other small bit came loose in the muffler and due to the dangerous and lethal condition known as "suck-back" it got pulled into the combustion chamber, then spit back out again. (Sorry, but I couldn't resist mentioning suck-back since those yahoos at AS have several pages of discussion on it!)
Seriously though, I think it is possible that a hard bit of metal came in and went back out through the ex port. I think I have mentioned it before but at 12K RPM the spark plug is firing 200 times a second so a lot of damage can occur in a short amount of time.

Notice how the edge is peened out and was rubbing on the cylinder wall. It took something very hard to do that kind of damage. And I suspect the saw ran a bit after the event because some of the dents have a lot of black down in them. Brian why did you tear this engine down? Did it quit or lose power or something?
Anyway that is my opinion and that and a nickel won't get you a piece of bubble gum but it is fun to try to figure these mysteries out.
Eddie

 

[Timberwolf]

Saw was running fine, tear down was related to some ummmm.... performance enhancements.

I think I am going to hold out on my theroy a little longer and throw in a couple other ideas.

One reason I just can't see it being physical dammage from mechanical impact is that there are no sharp edges imprinted on the 026 crown and there is no cratering where metal is forced outward from the impact. My take on the crown is that the metal has been melted away.

Here are a couple pics of a piston that someone put in backwards breaking the ring in short order, the impacts show clear imprints of the shape of the broken object and the edges of the impacts are raised like a crater on the moons surface. Last pic is the 026 piston close up showing the difference between the two failures. The ring breakage piston also did not show any rubbing on the sides of the crown though the top of the crown was pressed down enough to stick what was left of the ring in the ring land.

On the 026 I still an thinking it was running on the edge of detonation likely at higher RPM, the crown heated up so much the little areas of dentonation actally started melting and blowing off small pockets of the piston. This then put more heat in the crown until it swelled enough to start to rub the bore.

026 will be seved very well by it's new cyl and piston which arrived on Friday.


I may well be off base, but I'm just seeing different clues in these two pistons.

 

[Timberwolf]

Thinking more... Another little difference is that where there was a known object on the loose there are impacts where the object(s) were caught between the port edge and piston edge as the port closed. On the 026 this is not the case and actually the very edge of the piston is preserved. To me the reason the edge of the piston is preserved is that it was being cooled by the cylinder wall.

 

[Magnus]

Top piston in these last two pic's is pretty typical for damage caused by part of a piston ring with its sharp edges in the grooves made.
From the carbon pellets you don't get that or the same constant prints as it changes shape until it escapes or get stuck.

 

[sawcollector]

This is what detonation looks like. The piston is melting away and splattered around and starts wiping up and down the skirt.

 

[Al Smith]

I guess I get a little confused when the term detonation is used .To me that would indicate auto or diesel ingnition .Most likely the result of lower octane rated fuels or with a lean condition .I suppose perhaps a hot spot caused by carbon glow could do it too .Good question whtever the cause .

 

[Timberwolf]

This is what detonation looks like. The piston is melting away and splattered around and starts wiping up and down the skirt.

I'm in agreement on that, no question. If an engine goes full into detonation the exhaust side of the piston melts down. What I was thinking on the 026 is it was just on the edge, a bit of mild detonation, just enough to warm the piston and soften the aluminium allowing little bits to be blown away. But it was never run quite long enough or hot enough for the detonation to go exponential.

One other little clue is looking at the markings on the piston under magnification, the surfaces of the two are different, the ones from impacts are bright and polished showing extrusion marks, the 026 piston aluminium in the dammaged area is rough and pore like, almost the look of erroded limestone.

Guess there is no way to know for certain, but interesting discussion.

 

[Timberwolf]

I guess I get a little confused when the term detonation is used .To me that would indicate auto or diesel ingnition

There seems to be a lot of poor information on this out there. Best I understand it, "auto or diesel ignition" is actually pre ignition and the fuel is being lit before or independently from the spark. If this happens too far in advance of the spark plug the cylinder pressure and heat run away exponentially, sustained for any length of time and pistons break apart if it does not fail due to swelling and seizing to the bore first.

Detonation is where combustion changes from normal flame front propagation to explosive combustion, can be due to flame fronts meeting as a result of spontaneous ignition of the charge remote from the spark plug.

In this way, I think much of the confusion is linked to the possibility that detonation can be caused by pre ignition.

http://www.hastingsmfg.com/ServiceTips/detonation_and_preignition.htm

A few other piston failures pics people are attributing to detonation.

 

[CurSedVoyce]

Piston Borer Grub