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Yz 250 Piston Blew Apart

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Hey guys, first off i'd just like to say that i've been reading the forums and enjoying the site for a long time so thanks.

 

I ride a 2002 YZ 250 and a few days ago my bike locked up as I was climbing a hill. Got the bike back to the house and tore it down to find that my piston skirt had broken off and there were pieces all inside the crank and head. I had the cylinder inspected yesterday and amazingly its totally unharmed. I plan on splitting the doing a full top and bottom end rebuild, but I have to wait till the parts and my new tools come in. This is my first time splitting a case so i'm not the most well versed as what is going on inside there.

 

So today I decided to clean the head up as much as possible and pull the power valves all apart. Well when I get the PV cover off everything in there is really dirty and gunked up. And I see that the end of the rod connecting the valve is worn and rounded over. Just spent the last 3 hours pulling the pieces out and cleaning them as best as I can. And I get it back together and it kind of sticks open when its turned all the way out. It catches on the rounded off part of the valve. I'll post pics to help show what i'm talking about. I am thinking maybe the PV was sticking open and that's why its so dirty in there. Also, the valve feels like it is catching in the middle of the movement as well, so maybe it wasn't opening fully. 

 

I'm trying to figure out was it this power valve problem that caused my piston to blow apart? And where can I find a replacement that isn't going to cost me an arm and a leg. The parts for the rebuild and the tools are already setting me back a lot so this is really hard to deal with. 

 

Also if the PV didn't cause the piston to break, what are the other options as what caused it?

 

Anything you guys can tell me is appriciated. 

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The powervalve on the YZ is a 2 stage movement. The front flap opens first and when higher in the RPM the side ports open too..... Its supposed to kinda stiff because the side port pins slide on channels, first straight then angle, that slight "stick" is the powervalve pin changing direction on the channels....

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The piston is fairly new. Less than 30 hours. Sorry forgot to mention that. Also the piston skirt broke off on the exhaust side. And the stick happens in the middle of the first stage and not during the transition between stages. The transition also sort of sticks but I assumed that was normal.

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The only way the power valve would cause a piston failure is if it physically caught it. Pull the main valve and see if there arenmarks showing the piston caught it. Typically dropping a skirt is due to piston to bore clearances being too large.

As for the pv sticking, you'll just have to pull and inspect everything. The wear in the top of the main valve looks like theres a lot of time on that bike.

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The piston is fairly new. Less than 30 hours. Sorry forgot to mention that. Also the piston skirt broke off on the exhaust side. And the stick happens in the middle of the first stage and not during the transition between stages. The transition also sort of sticks but I assumed that was normal.

Is the piston OEM, or something else? Like everyone has mentioned, the common reason for failure of the piston skirt is that it has been run past it's service limit, or that the piston to cylinder bore is excessive.

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I just had the cylinder wall inspected and measured yesterday and it's within the range that is listed in the manual. I had it related last year. As for the wear on the main valve, would you say that is from some malfunction or just to many hours?

The owner of the local shop thought maybe the piston may have had a problem before I put it in and it broke from getting a little to hot. It was a vertex piston btw.

Also I'm replacing the main crank beating regardless, but the crank itself seems to be on good shape with no vertical movement in the rod. Can I get away with reusing it as long as get the pieces of piston out of? it

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Sorry, I had the cylinder replated*. Also, I meant main crank bearings*. I'm on my phone, that's the reason for the grammar mistakes

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Okay so a cast piston then with 30hrs on it seen that happen before and vertex has different sizes too small of piston used for the bore or bad crank. Sounds like there was excess piston slap or bottom of piston was hitting the cases or crank lobes just barely.

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I'm having the worn valve repaired by a machine shop and I'm just going to send the cylinder to milleniun. At this point saving money doesn't seem possible. I don't want this happening again so I'm not taking any risks. Total engine rebuild.

Thanks for all the input guys. It must have been a combination of the high hours, worn cylinder walls and maybe a manufacturing defect. Hopefully replacing everything fixes the problem

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BTW I found the reason the power valve was sticking. There was some gunk and carbon build up that I missed down in the head. Cleaned that and put some 2 stroke oil on the parts and everything moves smooth now.

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BTW I found the reason the power valve was sticking. There was some gunk and carbon build up that I missed down in the head. Cleaned that and put some 2 stroke oil on the parts and everything moves smooth now.

Good stuff. When you test it, be sure to do it with lots of 2 stroke oil on the parts, so it's tested in a real way.  Hopefully that gunk restriction hasn't worn the end of the valve shaft so bad that it catches. 

 

If you use a typical JASO FD (ISO EG-D) rated oil, then the PV parts should stay nice and oily and spotless from build up. So if the carbon gunk build up happens again I'd guess you've got some abnormal wear on the PV slide or more likely the cylinder alum casting.

 

With the piston replacement, check the new skirt/cylinder clearance next time. And your cylinder stamp letter.  I find I get a pretty good measurement using feeler gauges.  For the longevity and reduced chance of physical breakage, I like the idea of forged pistons, but there are mixed opinions on the topic. With my 295 kit the forged Wossner or Wiseco pistons seem the same in design and quality.  My skirt/cyl clearance is 2.2 thou new and the motor idles quiet enough to keep me happy.  The previous piston wore out to 5 thou, and it had a loud rattle.

 

With your crank, after the old piston broke up, the bottom end is most likely still fine. The beauty of 2 strokes.   You could try washing out the case (with some diesel would do) then hand feeling the main bearings for play and smoothness, and with the rod big-end bearing check the vertical play of course, but more meaningful is to check bigend bearing wear as per the manual, which is a wobble test when measuring side movement at the *small* end. I use a small ruler held against a cylinder stud as I rock the small end left/right. Very easy. I think new is something like 0.8mm and the service limit is 2.0mm.  Mine was 1.7mm when I bought the bike (used), and was 1.9 after about 90 more hrs. So it wears slowly, but I've got a new ProX crank assembly ready to go in with the next top end rebuild.

Edited by numroe

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Yeah thanks a lot all that info helps a lot. I'm having the machine shop repair the worn and rounded off end of the PV. And yeah, when I tested the power valve the first time I didn't think to put 2 strike oil on it, my mistake.

I'm getting a wiseco piston this time and Ill take your advice and check the clearances. As for the bottom end, I think I'm just going to replace the crank bearings at the least and I check the movement of the bearings and recheck them at every top end rebuild.

But I wonder is it okay to replace just the crank and not the crank bearings day next year, out should I replace it all if the crank needs replaced? Because I'd rather do it all now rather than replace the like new parts next season.

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If it was me and knowing what these cost to rebuild..........I would do it now, Mains, bigend and little end. 

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If it was me and knowing what these cost to rebuild..........I would do it now, Mains, bigend and little end. 

"It's better to do it correctly the first time, then to half-ass it twice."

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AbleTyler, Consistent with what the other boys wrote:

 

- You'd always replace the main bearings if they are anywhere near as old as the crank you are replacing.

 

- You should not keep using an old crank assembly as-is, if the big end bearing is past it's service limit.  I know these 2 strokes are tough, but eventually it'll start to hammer and then seize or worse.

 

If your motor is generally tuned well, these parts get a good work out.

Edited by numroe

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