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2004 yz250 broken piston

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Hey guys, this may or may not sound like a no brainer, but it's my first top end rebuild to any help is appreciated. Over the weekend I was riding and lost a lot of compression and my bike bogged out and wouldn't start afterwards, however it had free motion when I would kick it (no grinding sounds and it wasn't seized). So I tore it down and came to find out that my piston had broken along the side and I pulled some chunks of metal out of the cylinder. My question is, do I need t take a part the whole bottom end and look for metal or could I just drain the oil and it will all come out. The crank and connecting rod revolve smoothly so it doesn't feel like any displaced metal is hindering anything, but I don't know I've never done this. Cylinder bore is still smooth no gauges or nothing, but the head looks peppered a little (tiny dots everywhere) would I need to get it re finished? Thanks in advance!

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Yes should be split and cleaned.  New crank bearings and seals atleast.  But play in big rod bearing can cause piston to break so check crank for up and down play:thumbsup:

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You should also try to figure out how/why it broke,

preventing a sooner than expected rebuild on your new top end if the problem is still present.

(air leak, wrong jetting, injested debris from air filter etc) 

 

Note that 'the oil you drain' is gearbox lubricant, not engine related at all on a 2-stroke.

Before buying any parts, you should first acquire a service manual. 

Edited by mlatour
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There isn't any up and down play, I'd like t avoid splitting the case if there's a chance that I can just desire oil. But I also wanna do it right so if I gotta split the case I guess you gotta do what you gotta do 

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As Motox367 suggested, 'doing it right' involves a complete teardown and inspection.

There are new threads monthly on this forum about people taking shortcuts after an engine problem and then wondering

why their 'freshly rebuilt' engine (partially rebuilt in their case) blew up again after simply slapping in a new piston and calling it good.

 

These aren't lawn mower engines but rather high revving MX race bikes with a huge HP per cc output.

Edited by mlatour
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5 hours ago, anthonyquags12 said:

There isn't any up and down play, I'd like t avoid splitting the case if there's a chance that I can just desire oil. But I also wanna do it right so if I gotta split the case I guess you gotta do what you gotta do 

Are you the original owner of the bike? How many hours were one the current piston? Do you feel you have the mechanical ability to do a complete rebuild (I am not questioning your ability, but some folks will honestly tell you they don't feel comfortable splitting the case)?

Where you mention that the top of the piston had little dots on it, that was (broken) metal bouncing around between the top of the piston and the combustion chamber. Some of it could have exited the exhaust, however you probably have some in the crank area. Plus, it could have (probably did) do damage to the crank bearings etc....

IF it was my bike, and IF I were going to keep it, I would just bite the bullet and do a complete rebuild. I would spring for new seals, gaskets and bearings throughout so you will KNOW that it is done right.

Check out this link to a very thorough disassembly of a yz250 engine

The above link is part one of two. He also has assembly videos as well. You can also find some good videos by RockyMountain.

Like the above posts mention, you can find lots of posts where people wasted a lot of money doing a partial rebuild and wondering what went wrong. 

Good luck.

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5 hours ago, anthonyquags12 said:

There isn't any up and down play, I'd like t avoid splitting the case if there's a chance that I can just desire oil. But I also wanna do it right so if I gotta split the case I guess you gotta do what you gotta do 

In case you missed it a few posts above - you can't drain oil out of a 2 stroke crank case (which is where there is most certainly debris still hiding).  The oil in a 2 stroke is in the gearbox only - and is a separate and sealed compartment where there will not be any piston debris.

Do it right, or there's a good chance you'll be doing it again soon.

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In case you missed it a few posts above - you can't drain oil out of a 2 stroke crank case (which is where there is most certainly debris still hiding).  The oil in a 2 stroke is in the gearbox only - and is a separate and sealed compartment where there will not be any piston debris.
Do it right, or there's a good chance you'll be doing it again soon.

You can't just pull the motor and fill the case with diesel and rinse it out repeatedly? (with the top end off)

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22 minutes ago, HeavyRotation said:


You can't just pull the motor and fill the case with diesel and rinse it out repeatedly? (with the top end off)

I never would recommend that. You would never get all the metal out of bearings.  Split,clean, replace is the right thing to do.  Maybe that's why all  of the engines do always run :ride: easy  $$

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57 minutes ago, HeavyRotation said:


You can't just pull the motor and fill the case with diesel and rinse it out repeatedly? (with the top end off)

Yeah, maybe...but I'd say still pretty risky.  I guess if I had the piston in hand, and could find all the pieces to see they were all accounted for...

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