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Discussion Starter #1
well i tore into the bike tonight, this is what i found...








 

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So what caused the failure? Bore clearance too tight,lean-out(don't see burn/melt evidence),piston material flaw?

I learned something about cast pistons after a buddy of my almost destroyed my CR450 engine. If you don't know CR450s are kinda rare and parts can be expensive and hard to find.

Well,mine had piston slap(needed bored/new piston)and I told my friends this and to NOT RIDE IT as it was about to blow up. It was out at a friends house where I kept it,so I couldn't keep an eye on it 24/7.

Well,of course they didn't listen and one of them took it out and sure enough the exhaust side piston skirt snapped off,got caught between the crank and the engine case. Steel crank versus thin aluminum case,guess which one won that battle! The skirt was blown through the case and I had to split the case,have it welded and re-machined.

I replaced the piston with a forged Wiesco so this hopefully wouldn't happen again.

Sad part was I sold the bike to another friend a few months later and he blew the trans out of it which blew out the case near the output shaft rendering that side of the case useless and he sold it for parts.
 

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I'd say it started to seize and snagged a ring in the exhaust port. Is the piston top really that white?

And quit drinking that cheap crap. Unless you're saving up for new pistons...
 

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quote:Originally posted by UngaWunga

I'd say it started to seize and snagged a ring in the exhaust port. Is the piston top really that white?

And quit drinking that cheap crap. Unless you're saving up for new pistons...
I've seen that also where the whole or most of the top outer part of the piston was ripped off. I will NEVER use any cast piston on a two-stoke engine that I have rebuilt.

It may not stop a serious failure,but I'm confident that it will at least limit some of the damage by the forged piston(a high quality one)holding together better than a cast one. I've seen too many cast pistons break into pieces and cause more damage than if they had stayed together.

Of course a lean-out may still hole even a forged piston,but they takes a lot more stress before they let go versus a cast one.

If it snagged a port as UngaWunga brought up,was the port chamfered enough? Was the exhaust port ported too much?

Did/was the head gasket(copper?) sealed/torqued properly or could the head/barrel surface have any flaws which could cause an improper head to cylinder seal at that point?

Something else also...what about your exhaust pipe to cylinder seal?
Some times if the seal is bad the pipes will "bounce/move in and out" with the exhaust pulses. Springs wear out or seal/collars don't seal correctly sometimes. You of course know from racing/wrenching you don't want cold/cooler outside air to get sucked into a hot exhaust port.

Let us know what you find out Tyler.
 

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Discussion Starter #5
um guys, these are forged wiseco pistons....not cast. i honestly think that i could have had a piston failure, but it is really strange to me why the one side is perfect and the other is so lean. i dont have allen's to get the carbs off and i need a big cresent to get the axle loose to take the chain off, then i'll have the whole motor out. the motor is all stock. i have a boost bottle on it and the wiseco pistons in it. i did nothing else because i was trying to make this bike a reliable daily rider...so much for that! i torqued the head down to specs and checked all the surfaces, and they still seem fine. now the exhaust pipe seal is a good point. i have new springs on it, but i really don't think they are tight enough....so that is a possibility.
 

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Discussion Starter #7
its kentucky, we dont wear shoes. oh yeah, on the beer thing, i knew i needed to work up a good drunk, and i wanted to do it cheap....i had goose island with my supper. but thank you for the concern! LOL!!
 

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Did I mention crank seals? Have you checked them? Look for oily residue on the outside of the left crank seal. Leaking left crank seal will suck air and run lean leading to seizure. Leaking right crank seal will leak oil in and out so it pressurizes the trans and blows trans oil out the vent and will suck trans oil into the right cylinder causing it to be dark, oily and smoke more than usual. Look and sound familiar?

Check your crank seals, they might be 30 years old.

Ken
 

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Yeah, replace the crank seals everytime you take an engine apart. On the race RDs, I'd replace them once every 1-2 years. Did they need it? Not really. Did I want to blow up an expensive race engine? Not really.
 

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also make sure your intake and exhaust were tight. I seized an h1 motor like that a long time ago because the motor was sucking in extra air from around the intake manifold.

what the fuck is a boost bottle?
 

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also forged wiseco's (or seize-co's as they have been called) require a different tolerance and a good 5 minute warm up before you ride. If you just bored the cylinders to stock size then the piston would have likely siezed if it did not break. usually a lean condition burns a neat little hole in the piston or causes a full dome seperation, doesn't usually cause a chunk to break off.

I have wisecos in my T500 and she smokes like a bitch till she warms up.
 

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it needs ape hangers
 

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Discussion Starter #13
man hack, it took you FOREVER to reply to this...i figured you would have called already and offered me $50 for it!! I just bought a set of allens and a cresent wrench to get the rest of the motor apart. i think you guys are right on the crank seals, but my petcock was clogged. im letting it air out right now and im going to blow it out with the compressor and then kream the tank. that will fix problem 1. i am going to put new crank seals in it because i am with you guys, i think they are shot. they are 32 years old, bike has been sitting 20 years, i should have known better than to ride it without replacing them. i am also PROBABLY going to put knew crank bearings in and rods too if they are out of spec in the least bit. after getting out my frustrations last night, i am back in the game and want to see this thing on the road again. thanks for the tips geet on the piston. i usually let it warm up about a minute before i leave the driveway (don't want to annoy the neighbors) and then it is a 4 mile stop and go ride to the interstate, so i always take it really easy through there to build engine heat. but i'll have the rest of the motor apart in a few minutes. im not taking the bottom end out here at my house, im just going to take the motor out of the frame and then we'll split the cases at the shop. pics will be up soon.
 

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thanks Ty, I really appreciate you getting that thing righteous and perfect before you hand it over to me

see if you can find a big humpty king and queen captain america seat and sissy bar for it too

thanks man, you rawk
 

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"um guys, these are forged wiseco pistons....not cast."

Wow,well I'm sure you COULD have gotten a bad piston from Wiseco,but I doubt that would be the case,especially one of their forged ones.

Tyler,where the piston is broke did it break where the top ring/groove is? Also is the piston actually broke/snapped? Any signs of melting on the crown?

I'm pretty sure you had lean-out and I'm thinking THAT ring/piston got so tight in the bore that it seized,but maybe not enough to actually cause the piston to fail at that time/moment.

If it just seized and you didn't let it cool back down and tried to FORCE it to un-stick it that may have been when the top piece broke off? The top ring could have gotten so tight that it wouldn't move at all and something had to give. Well,the piston which is also hot(softer now due to the very high heat)gave way.

This maybe/could've happen(the forcing) if the bike had a combustion event at the moment it seized(either cylinder?) and the tight/seized ring flexed enough as the piston was being driven or pulled down that it broke the piston at it's weakest point(top ring groove).

Like when you open a Tupperware bowl with a lid on it. It's a lot easier to open it if you push up in one small area. Your fingers/thumbs are the top ring and the lid(piston top/groove)get pushed up rather easily with very little force needed because your only prying up in one small spot on the lid and the lid peels open once you get it started.

I've seen pistons that look just like that with the whole tops(outer edge)ripped off because of too much heat/piston/ring expansion and or the were set-up too tight in the bore(bore too small for the set-up used)to begin with.

The motor doesn't care or know it's about to seize and just keeps on a firing maybe even at the moment it sticks.Especial true on multi-cylinder engines. If the engine is powerful enough to force the piston down again the top of the piston edge all the way around peels right off easy as pie.

My first motorcycle ever when I was a kid was a Yamaha JT1 60 and on a hot day or if I was really thrashing on it putting it under load a lot it would seize up tighter than a drum.

I just put up with this uh...annoyance LOL. I would have to let it sit until it cooled down and then it would kick over and I'd ride it until the next time it locked up on me. Sometimes it would go for an hour sometimes just a few minutes if it was really hot out(Summer time heat)!:(

I know better now,but back then being a kid,not knowing much about bikes/engines and not wanting to tear open my engine,not having a dime to my name etc. I just put up with it. Of course this would not be acceptable on a street bike where it HAS to be dependable or else you get stuck out alongside the road in the middle of nowhere.

Bottom line and you know this,is if it was a lean condition/seizer you MUST find the true cause(s)or it will just repeat it's self.:(

If it's possible check the ring gap/bore size etc. on both cylinders. The "Good" side should hopefully still undamaged enough to get a good reading on. If the bore/piston/ring gaps are all withing spec for that set-up at least you know that wasn't a contributing factor.
 

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Discussion Starter #17
when i put the motor together i checked all my ring gaps and everything. honestly, straight out of the box, the piston and rings were dead on. i didnt have to file anything!! but i really think that starving the motor for gas and blowing out my crank seals is the culprit. i'll find out when i tear the rest of it down!! it would be sweet if the piston failed and i got a new one for free though, but i don't think that is the case. just for some info. when i was racing 250's on the half miles, when i felt the motor get tight (or if) i would pull the choke down the straight away to richen it up...no lie. my motor survived quite a few races like that...LOL
 

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the nearly perfect VM mikuni..........

with a cold start enrichener that doesn't actually work with the throttle open


too bad it has no actual choke that will work as you describe

doesn't even have a tickler

If I were building a two stroke of my own for the street


I doubt it would have VM's feeding it ;)
 

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(forged) weisco's have been very very good to me over the years. my only failures (two in a row) i traced back to a bad ignition (dry) side crank seal. have had the same piston in the same (re) bore since for three seasons and it's still got super compression and hasn't worn the bore oval (yet) more than .001". doubt weisco is your problem. me thinks you got a bad intake spot (leak) or a fuel starve (clogged jet?). my 2 sense.
-parks
 
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