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Tillerman6

74 TY 250A PISTON & RING ISSUES

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While I was researching piston and crankshaft balance for a 2 stroke, I ran across this calculator.   Don't know anything about it, but this guy has put some work into balancing as far as doing the math and having all the right number crunching abilities.  Somebody let me know if you try it.  I think it's about thirty bucks.  Don't know if it's good or bad, just FYI.http://www.dragonfly75.com/moto/calculators.html

Edited by Tillerman6

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5 hours ago, Tillerman6 said:

For what reason?

Copemech, 

I don't mean to be obstinate, so please excuse my terse reply. Are you suggesting that B &J racing has some expertise in the area of balancing a 2 stroke engine? Or something like that?

 

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4 hours ago, lineaway said:

That analogy is flat out silly, I have seen brand new out of the crate bikes vibrate on the side stand!!!!!

I don't doubt that one bit.  It's much cheaper to build them that way and not worry about vibrations or out of balance conditions. On the other hand if you are trying to concentrate on your riding accuracy and balance it does not help to have your hands go numb or having the seat shake so hard it makes your ass and feet tickle.  I know there is always going to be some vibration and that is normal, but it's hard to measure and even harder to do anything about it.  I guess that's why they invented twin cylinder engines in the first place, But we are not there yet with trials bikes. After this next meet I will know more and make a decision.  Or someone may come up with a weight for the TY piston that is different than the 213 grams.  Since I have not owned this bike for more than a few years, it could have something else wrong or different about it that the previous owner did not pass on to me.

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14 hours ago, lineaway said:

 The weight really does not matter. What actually started the engine rebuild in the first place?

OK now I convinced myself.  Overnight I realized why the engine has to come apart again.  But it is not what I was thinking.  I think the crank is bent or out of alignment.  You can see a slight wobble  on the fly wheel outer face when the left cover is off and the engine is running.  It looks like maybe .005- .010" at the outer rim of wobble. There might be some excess play between the big end of the con rod and the insides of the flywheel halves too.  I don't have the specs on the 250 A model for that. And that measurement might be different around the circumference of the fly wheel because if the pin is bent, there will be less clearance at one angle than at another.  ( I did not measure that) 

So like you said, if it gets fixed now it will live on.  Does anyone have any ideas about who might be able to re-build the crank?  New bearings? Possibly a new rod?  I think I will try and get all the bearings before I tear it apart again.  If any of this stuff is  "unobtainium" I won't have the bike down for too long.

And I wondered why the exhaust pipe was wearing scratches on the plastic rear fender when they are mounted almost an inch apart! LOL!

 

 

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On 4/19/2019 at 8:50 PM, copemech said:

Look up B&J racing and give them a call.

Like he said.  I believe they are the US experts on this.

 

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Will do.  I was just waiting for Monday morning to roll around. After watching some videos on truing and balancing it looks like I am no where near equipped or skilled enough to tackle the crank straightening or balancing job.  Hope it's not too expensive.

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Any crank rebuilding shop should be able do the job quickly on a single cylinder motor but, if someone '' played '' around and bent the end of the crank trying to take the flywheel off, that's another story.

Guy

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Is the distance(measurement) from the piston pin hole to the piston crown the same for both pistons?

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11 hours ago, Bmonk said:

Is the distance(measurement) from the piston pin hole to the piston crown the same for both pistons?

Yes the piston tops  out exactly at the same spot on the cylinder. The skirt is a few MM shorter than the old one  The extra weight of the new piston was another problem.  I am back to the old piston, but today I sent in the whole engine less the cylinder, head and piston in to B& J racing.The vibration I'm getting isn't normal even with the old piston.  I took off the mag and backing plate and both outer covers to save weight. They will have a look and tell me what it needs. Probably be a week or so before it shows up, then another few days before they have a chance to look at it.

I took the time to put a dial indicator on the very outer edge of the magneto while it was still mounted to the end of the crankshaft.  I was getting 0.010" of in-out wobble. so I think the crank shaft is pulled out of alignment. I sure hope B& J can fix it permanently !!

 

 

If I had known B&J had the correct pistons, I would have let them do my cylinder work. That was a few months ago, and I was on here and could not find the correct one!

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On 4/21/2019 at 9:31 AM, mcman56 said:

Like he said.  I believe they are the US experts on this.

 

Sounds like it.  I explained what was going on with the vibrations and first thing he said was "sounds like you spun the crank!" So he's been there, done that! The lower end is in th UPS system now. On it's way to Tennessee.

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I recently rode a TY175 with the B&J full engine mods.  I don't know what all of the mods are but performance was extremely impressive.  More power everywhere, much better response yet still silky smooth and pulls to extremely low rpms.  I have ridden a TY with modified frame and another that was ported but they seemed at most a little different from stock.  This was a whole different animal.        

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Have heard that about their 250 engines as well 

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I wanted to see what could be accomplished by just straightening out the crank first. If that makes an improvement with the vibrations, I can always send in the head and cylinder next winter after the  riding season. I did not want to introduce too many variables all at once.

Today while I was stripping down the empty frame I found a broken off tang that was bolted to the right hand side of the upper engine boss.  I got it welded back on and am in the process of stripping off all the old paint and rust while I am waiting for the engine to come back from B&J Racing.

The rust is not too bad and the grinder motor is making short work of the rust removal. The forks I have on the bike are not stock and they don't have anyplace to mount a front fender bracket like the original. I might be able to fabricate something that will work soon.

 

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