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What's with these cracked pistons?

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  #1  
Old 12-08-2009
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Default What's with these cracked pistons?

Every 488 L/C fuji I've ever pulled apart had cracked or broken piston skirts. Today I went to replace the base gasket on my engine that only has roughly 600 miles on a rebuild, And sure as shit, cracked skirts. The pistons otherwise look great and the bores are well within spec. According to my micrometer they are .003 under factory spec. of 2.835.

Is there a design flaw with this particular engine? Are there any fixes for this problem other than Going with forged pistons?

I never hammer on a cold engine, And I've never heard any detectable signs of piston slap or pinging. I don't get it.
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  #2  
Old 12-08-2009
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If I remember right, the Fuji 500's had a huge piston to cylinder wall clearance. That might explain piston skirts cracking.?
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  #3  
Old 12-08-2009
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Maybe it's because you have your music turned up too loud when you're trailering it...
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  #4  
Old 12-08-2009
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I'll look into the clearance issue when The new pistons come in. Maybe a little research into what would be an acceptable tolerance. The only problem would be that I can't do a whole hell of a lot about it without sending the jug off to a machine shop with some over sized pistons.

What's odd is that when pistons break in anything else their is usually a very distinct and audible piston slap sound( That sound between a lifter tick and a loose wrist pin best I can describe).
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  #5  
Old 12-09-2009
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check the clearance and then also look into fuel quality, do you see any signs of detonation?
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  #6  
Old 12-09-2009
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Quote:
Originally Posted by Butcher_Boy View Post
Every 488 L/C fuji I've ever pulled apart had cracked or broken piston skirts. Today I went to replace the base gasket on my engine that only has roughly 600 miles on a rebuild, And sure as shit, cracked skirts. The pistons otherwise look great and the bores are well within spec. According to my micrometer they are .003 under factory spec. of 2.835.

Is there a design flaw with this particular engine? Are there any fixes for this problem other than Going with forged pistons?

I never hammer on a cold engine, And I've never heard any detectable signs of piston slap or pinging. I don't get it.
Under factory spec isn't necesarily a good thing. Metal expands when heated so if you are indeed running to tight a spec you may be hitting and cracking the skirt. But the question now becomes where is the rest of the skirt. I hope you found it.
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  #7  
Old 12-10-2009
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I will check the clearance when I get the new pistons in. What would be considered acceptable on this engine?
I run 91 non-oxy whenever possible but use 92 regular if I'm forced to. I don't see any signs of detonation.
The .003 differance I'd give to err on the caliper. It's not the highest quality instument I've used. I'll have to go dig out my good one.

The skirt didn't break, it has a hairline crack half way across the skirt.
I noticed also that one of the circlips came loose and hogged out the groove. This engine appears to have been within miles of catastophic failure. Thank you, seeping base gasket! lol
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  #8  
Old 12-10-2009
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Interesting I've replaced 2 pistons with broken skirts in my trail engine.
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  #9  
Old 12-10-2009
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is there any small scoring on the intake side of the piston? is it possible on the piston port to get thermal shock if the sled eats some snow?
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  #10  
Old 12-10-2009
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That's what I thought had happened to the first engine. This one inhaled a lot of water one night. Then the next day this happened. This baby is trashed. Name:  broken piston.jpg
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There are no score marks on these pistons. And as far as I know this engine never got wet. I have the foam sucked up tight to the hood and the plastic plate still in the air box. We didn't get enough snow last year for drift bashing.
Whats funny is that the crack on this piston is almost identical to the break on the piston in that picture. It was the same way with engine #2.

i find it very odd that all 3 engines I had all have the same problem. I can't be the only one to have this problem with these 500 fuji's. Unless I hit some sort of fuji lottery and got the only 3 bad engines they ever produced.

I never pin the throttle clear across a lake, red line the piss out of it or hold it wide open on the stand.

Perhaps it's time I just get something else and stop living in the past, But it's still going to bug me because I'm the type of person that just has to know why things fail.
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  #11  
Old 12-10-2009
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Now that i think about it. Could it be that these engines don't have a thermostat? I installed a temp gauge on this sled after the first engine failed. I notice that when riding on packed snow (groomed trails) it would get warm, not over heating by far, but warm. Then if I hit powder it would cool off quite fast. I'd say about a 50-60 degree swing on the gauge.
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  #12  
Old 12-10-2009
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tore down a couple 500s, scored, burned, but no cracks.
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  #13  
Old 12-10-2009
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Ive torn down a couple fuji twins, never noticed any cracks either. People say those 500 (well, like all of the fuji twin) fuji twin motors are pretty much bullet proof if i recall. Must have some bad luck with them..

When you switched motors after the first fucked up, did you use all of the parts off the old motor like the carbs, water pump, etc etc...?
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  #14  
Old 12-11-2009
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Quote:
Originally Posted by Butcher_Boy View Post
That's what I thought had happened to the first engine. This one inhaled a lot of water one night. Then the next day this happened. This baby is trashed. Attachment 7925

There are no score marks on these pistons. And as far as I know this engine never got wet. I have the foam sucked up tight to the hood and the plastic plate still in the air box. We didn't get enough snow last year for drift bashing.
Whats funny is that the crack on this piston is almost identical to the break on the piston in that picture. It was the same way with engine #2.

i find it very odd that all 3 engines I had all have the same problem. I can't be the only one to have this problem with these 500 fuji's. Unless I hit some sort of fuji lottery and got the only 3 bad engines they ever produced.

I never pin the throttle clear across a lake, red line the piss out of it or hold it wide open on the stand.

Perhaps it's time I just get something else and stop living in the past, But it's still going to bug me because I'm the type of person that just has to know why things fail.
Scoring indicates seizures which could be the result of excessive heat and/or tight tolerances.
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  #15  
Old 12-11-2009
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Carbs and pipe were the only things I re-used.
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  #16  
Old 12-13-2009
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well it's back together and running. everything is well within spec according to the book. I did notice that these pistons come way the hell out of the cylinder at BDC. The sleeve almost matches the crack line. But what's odd is they are cracking on the opposite side from the thrust load so it's not like they are getting hammered into the cylinder wall

but, what about sonic vibration? yeah, it would be like hitting a bell, if you hit one side the waves travel around to the opposite side and meet in the middle. I could see that causing a crack. maybe. I don't know.

I'm just thinking out loud. do I sound insane?
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  #17  
Old 12-13-2009
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If one part of a solid object vibrates the whole object is vibrating. Pushing on one side of the cylinder you're going to have a reacting force on the other side.
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  #18  
Old 12-13-2009
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crank straight?
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  #19  
Old 12-14-2009
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as far as I know. I took it out today for a little break in ride. I'll pull the cylinders off in another 500 or so miles and have a look see.
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  #20  
Old 12-17-2009
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Put 70 miles on today. it sounds just as happy as ever. I don't give a shit what anyone says, that 500 in the tight wooded trails will trump damn near everything. Excellent out of the corner, low slung, hook up and go. The lakes I get killed on, but we only have 10,000 of those in this state, so what are the odds? heh
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