What would cause this?

[JMew03]

Blaster was running like crap. Had no power. So I decided to check the compression. Tested at front 150 rear 120. Took the head off and this is what I found.

It looks like the only damage was to the side/top of the rear piston and the dome. The oring came off when I removed the head.

The cylinder sleeve still feels smooth and doesn't have any bad marks like the piston. The crank still feels good with no slack and the bearings are still holding good. Could something have made its way in there some how and caused this damage? The only metal shavings I could locate were on the dome which I am guessing came from the piston. Lol

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[wanderer1]

Looks like detonation with a little seizure mixed in . too much compression/timing and not enough octane is one cause. cylinder could use a hone imo

 

[JMew03]

I will be upgrading a couple bore sizes before its put back together. Was talking to Maniac before this happened so now I have a reason to hurry.

I run 93 octane with 50:1 lucas oil and NGK plugs.

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[TheBuzzard]

crank bearing cage coming apart. take the whole motor apart and check it. thats what it is

 

[wanderer1]

My crank was just fine upon tear down once when I roached the rear piston. Not saying that his didn't let loose, just saying that damage looks like detotnation. Improper jetting, went lean, too much timing can all produce what is shown in the pics as well as a crank bearing cage. Always completely tear down motor and inspect when these symptoms appear.

 

[mybad]

X2......your crank.

 

[102nd7]

Possible Reed failure pulling intake and check those as well

 

[JMew03]

I will pull the engine the rest of the way apart this weekend. Didnt want to go to far if it was something obvious.

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[Myk]

crank bearing cage coming apart. take the whole motor apart and check it. thats what it is

X2

 

[kevbo]

Look for missing bearing retainer. It could be detonation it is usually more of a frosted appearance. Also bores this large tend to stick rings before piston tops look that bad.

 

[227]

Just for my own knowledge, half of you are saying detonation and half are saying crank bearings coming apart. To me, that looks like a tiny piece of metal got stuck on the top of the piston and caused the pitted damaged as it contacted the head, so I am leaning towards crank/wrist pin coming apart. Since the two ailments obviously cause similar types of damage, What is the main difference (In appearance) between detonation damage and a small piece of metal getting inside your engine?

 

[wanderer1]

With extreme detonation I've noticed more piston crown damage on the exhaust side of the piston, meaning more damage on one side than the other. Bearing cage failing, well ya pull the crank and voila, there are pieces missing or they have be "relocated". Just a guess, and maybe op can chime in, that the heavier damage on the crown in the pic is on the exhaust side. Just a guess. Also guessing that you were feeding pipe cooling jacket off of the heads rear water bypass.

 

[JMew03]

Once I pull the crank I will take some photos. The piston rods don't seem to have much play in them so we shall see.

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[TheBuzzard]

this is what detonation looks like on a forged piston . it erodes the piston away on the exhaust side. a cast piston comes apart in chunks and then the chunks beat the piston top up before it totally grenades. whenever there is steel stuck in the dome or piston, its the bearing cage coming apart. if you pull your head off often, sometimes you can catch the crank going out before it grenades everything. i was handed some WDK 1200 domes one time and didnt look at them before i threw them in my tool box. they had a few tiny sharp dings in one. lower comp domes were put in and the crank came apart big time a bit later. this piston was a 61x 84mm ported to the moon 701 dual 46s SJ held WFO way too many times, not by me.

 

[TheBuzzard]

Once I pull the crank I will take some photos. The piston rods don't seem to have much play in them so we shall see.

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it will be the main bearing cage. when a rod bearing cage or bearing let go its a totally different animal. the motor would clunk and rattle big time before it just stops and wrist pin bearings on 701s dont usually fail before a crank or rod bearing. had that top end been rebuilt before or was it all OEM?

 

[JT_Freeride]

I have seen very similar damage done from a broken reed pedal. I'm not saying that it's what happened here but it can't hurt to make sure. Just my .02

 

[Manny]

How are your domes ?? Are they also screwed up ? If so not detonation !

 

[Manny]

Here was mine

 

[JMew03]

Damn. That looks fun.

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[JT_Freeride]

Here was mineView attachment 247965 View attachment 247963

Hey your carbs are on backwards