anatomy of a burndown

[bwalker]

I know this is a little off topic, but as discussed in a earlier thread I head a burndown caused by bad fuel this weekend in my sled. There is a lesson to be learned from this. Use good fuel from a reliable source.


Here is the head. The damage was caused when the ring groove melted away causing the rings to snag the exhaust port and break.

 

[bwalker]

Here is the piston itself. Notice the detonation damage.

 

[bwalker]

Heres the inside of the exhaust. Notice the white color that is painted on the inside. Thats melted piston.

 

[bwalker]

Here is the cylinder. Its not seized too bad. Most of the marks you see are melted aluminum that was deposited during the meltdown.

 

[bwalker]

This is the piston pin boss area. Notice the crack caused by the rapid spikes in pressure that occur when a engine detonates. also notice the "death ash" on the exhaust side. of the domes underside.

 

[bwalker]

Here is the other cylinder. Notice that the crosshatch is still present, there are no deposits in the exhaust tract and none on the piston dome. This is the result of good synthetic oil and lots of it.

 

[oakman]

ben,
how do you know it was bad gas? was it old, or have water in it? just curious as to how you determined the cause of failure.

 

[bwalker]

Mechanice helper.

 

[treeclimber165]

So, in a twin cylinder sled you had one cylinder destroyed by "bad fuel", yet the second cylinder was perfect because of "good synthetic oil, and plenty of it"?

How are you feeding two different fuels to the two separate cylinders, and why? Sounds like a contender for the 'Tall Tales' thread.

 

[bwalker]

Beginner, Its a snowmobile. Each cylinder is 400cc, which is more than 4 times the size of a 066 mag.

Oakman, Got a tank full from a suspect station in BigBay, MI. If you are not familar with the area its basicaly one of the most remote areas of the upper peninsula. By process of elimination I rulled out the usual supects like airleaks, timing, etc. I ha s to be the fuel as it passed a leak downtest with flying colors and the timing was right on. The cylinder that melted was on the mag side. On this model motor this cylinder runs hotter than the other. The motor is stagger jetted to account for this so its not easy to se why this one fried first.

 

[bwalker]

So, in a twin cylinder sled you had one cylinder destroyed by "bad fuel", yet the second cylinder was perfect because of "good synthetic oil, and plenty of it"?

Brian,Always the skeptic,lol. Due to the coolant flow path one cylinder runs hotter than the other on this model engine. It is common practice to stagger jet each cylinder to account for this. If you notice the good cylinders piston wash is non existant. The wash on this model should be at least dime sized. This cylinder was running hot as well, but didnt fry like the other because it is cooler by nature and has more of a saftey factor. Had I been able to stay on the gas longer It would have suffered the same event. BTW this motor was equiped with a digital egt gauge. It happened so fast the gauge never even showed high temps.

 

[bwalker]

Beginner, It was not piece of carbon, and the timing is dead on. Carbon could cause a scuff, but wouldnt melt a 1/8 of the piston!
BTW Fuel knock is a terrible term. Pre igntion is oftened called this. Detonation is a toaly differant animal than pre ignition. Much more damaging

 

[glens]

Ben,

About your p1010072.jpg, you call attention to the bottom of the wrist pin boss and state the crack was cause by pressure spikes hammering the piston down into the pin.&nbsp; Wouldn't stress cracks so caused appear on the <i>top</i> side of the boss?&nbsp; Wouldn't the fracture on the bottom side be indicative of what happens when, say, the ring(s) get caught in a port, thus momentarily halting the downward motion of the piston while the connecting rod had something different in mind?

I'm not familiar at all with the power plant, but it sounds as though you're saying the coolant circuit has the two cylinders in series and not in parallel.&nbsp; Am I reading that right?&nbsp; I've heard of uneven cooling/temperatures in fore/aft banks and inner/outer cylinders of inline banks of air-cooled engines (necessitating different jetting), but not much about the phenomenon in liquid-cooled units; but then I'm not well-studied in the matter.&nbsp; It's certainly interesting.

Overall I think you've done a fine job documenting your woeful event.

Glen

 

[mikek711]

Ben,
What does the EGT gauge usually read at normal operation?
Sounds like the ticket for spot on jetting of a two stroke.
I have an EGT sensor welded into the exhaust of my Turbo Eclipse.Are temperatures similar in our two stroke sled motors?
Sorry about your motor.......You can look at it this way though,at least you got a chance to ride it and blow it up.I'm in Northern Wisconsin with about a half inch of snow.I havent even taken the sleds out of the garage yet.
Mike.

 

[mikek711]

By the way....That old RM looks like its in pretty good shape from the picture!

 

[bwalker]

Wouldn't stress cracks so caused appear on the top side of the boss? Wouldn't the fracture on the bottom side be indicative of what happens when

Glens,The area you mentioned has shiny marks on it also. The crack could have been caused by what you say. Its really hard to say although with the harmonics involved in a detonation event cracks can appear anywhere.

Mike, It normaly operates around 1300 at wot. As for the bike. its a mint condtion 83 rm 250. My dad rode the bike twice, but layed it up because of a crash on another bike. he gave it to my uncle who sat it in has barn up until now. Put some fresh mix and cleaned the carb and it was ready to rock. BTW if you want snow you have to ride the UP.

 

[dbabcock]

Ben,
1300 is on the high side for my trail sleds. How far do you mount your EGT probe from the exhaust port(s). Also, are you running twin pipes or a single. I've seen twin piped twins run dramatically different EGT's even with supposedly appropriate staggered jetting. When I say different, I mean that at different RPM,load and OAT conditions, the spread between the two cylinders can be significantly different. If you have a single pipe, you have to watch out where you drill for the probes. It's quite critical and all sleds aren't the same. Do you have two EGT's or just one?

Tonight when I get home, I'll post a picture of a real nice "meltdown" for you.

 

[Tony Snyder]

Ben , Were you running Mobile synthetic?

(Just kidding)

 

[My2cents]

Ben, I looked at your pic, this looks like a cracked cylinder wall or a bad cylinder head gasket. Get that cylinder pressure checked.
the head mating surface looks good from the pic although. What happens is when the cylinder heats up the crack expands just enough to allow a little positive pressure in the water jacket thus restricting coolant flow. Did you experence any coolant loss prior to this event?

 

[bwalker]

Tony, A burn down like this has nothing to do with oil.

My@cents, It wasnt a coolant leak. A leak would never cause this type of damage.