Help: is this detonation?

[wrxsive]

Hey guys,

I'm new to sledding this year and purchased an '06 Arctic Cat M7 162 LE with the Speedwerx M8 kit (Stealth version, i.e. stock head with Speedwerx domes), reflashed AC ECU, and can. The sled has been ripping so far until last weekend.

I had done a bowl lap and come back down to park and about 200ft before I stopped I noticed the sled didn't sound right. I came to a stop and it bogged and died. I started it again and heard an awful rattling sound coming from one of the cylinders so I immediately shut it down. A buddy and I pulled the plugs and both cylinders were running very lean. We decided to leave it and came and got it two days later (that's a whole other story: picture two guys and a come-along winching a sled 100+ft up a hill; the tow out after that was easy, though).

Tore it apart last night. The piston and dome are shot and the cylinder wall has some galling on it. Note this is only one cylinder (rider's left), the other one looks alright. Haven't tore into the bottom end yet, that's tonight's project, but I'm not optimistic: a quick peak showed some small pieces of metal floating around down there.

So the question is, what caused this? The rattling, running lean, and piston destruction point to detonation to me, but I've never seen such large holes from detonation, just widespread pitting. The detonation could also put undue stress on the crank bearings, possibly causing the small metal pieces floating around in the crank case.
Additionally, the holes on the piston and the dome seem to match up fairly well. Does this represent the point of pre-ignition? This doesn't point to a foreign object in the combustion chamber, does it?

I've attached a couple pictures; sorry they're poor quality from my Blackberry. I'll try to post some better pictures later tonight.

Any help/advice is appreciated. Thanks.

Reid

 

[diggerdown]

It does not look like it. Usually you do not see that large of dimples caused by deto and yo would see some sign of it on the rest of the piston dome and head cup. It looks like something broke loose and was being hammered or possible water ingestion.

 

[M-Head]

Had the exact same thing happen on my m8. Double check the electrode and porcelain on the spark plugs. Mine went lean, the porcelain came off the plug and hammered the piston and head exactly like what you have.

 

[wrxsive]

I thought the dimples were too big for detonation as well. I didn't think water ingestion would cause damage quite like this??

I will double check the plugs first thing tonight.

Thanks for the comments so far guys, greatly appreciated.

 

[4Z]

Agree with previous post. The craters are from trying to digest something.
Hard to tell with the pics but it looks like there was some deto going on. Beyond the bomb craters, does it have a sand blasted type look around the edge of the piston, with higher concentrations at the exhaust port area? If so, that is a good indication she got some bad fuel, lean on fuel etc. If not, should find out what it tried to eat or it could happen again....

 

[Reaper]

Sure looks like a crank or connecting rod bearing is coming apart and took a trip up through the piston.

 

[RACINSTATION]

Agree with 4z. Look like water or bad fuel.

Metallica has been playing here.

 

[wrxsive]

4Z, there is no typical signs of detonation like the sandblasted appearance that I'm familiar with and that you mention.

Reaper, I agree, the size and shape are indicative of a bearing, but I don't see how a bearing from the bottom end could make it to the top end...the piston skirt would prevent that from happening, right?? The piston skirts are intact with some minor (relative to everything else) scuffing near the exhaust port.

 

[Reaper]

Its easy enough for bearing parts to travel up through the intake ports along with the air/fuel mixture to the top of the piston. The piston skirt isn't going to stop it. Im guessing a rod bearing.

 

[4Z]

Its easy enough for bearing parts to travel up through the intake ports along with the air/fuel mixture to the top of the piston. The piston skirt isn't going to stop it. Im guessing a rod bearing.

Yes, I have had carb parts make it through the reeds and make havoc.....

 

[diggerdown]

If you got V force reeds you may be missing a screw!

 

[rbblrsn]

Bummer Reid, if you want some help with tear-down/rebuild let me know. Looks like it chewed on something hard perhaps bearing, ceramic insulator from plug, or ??? Jug and piston wont set you back too many $$ hope you can make it out again soon.

Robb (MSU)

 

[907KB]

Is the rod connecting bearing still there?

 

[kane13]

To me it does judging by the scuffing in the piston pin and skirt area, the crown looks like detonation, looks like you caught it quick though fairly quick anyways. Jmo