Well I did it. Blew out my first top end.

[BlackRaptor]

Well guys last Monday I blew the top end out in my 2000 CR250. The funny thing is I’ve been all worried about my 91 CR250 because it's on it's stock top end still and this bike had one put in it a year ago. Anyways I've got some questions.
When the top end went a big chunk of the piston skirt broke off and ended up down in the bottom end. lucky for me the piston rod and jug were totally undamaged. See what happened was I heard it making some funky sounds so I finished that run went up on top of a big sand pile at the gravel pit I was at and shut it off. When I went to start it, it wouldn't turn over 100% hard locked. My guess is when I turned it off that crap fell down into the lower end and wedged it's self between the case and the piston rode.
Okay I told that long story to ask whats the best way to get that crap out of there without pulling the whole bottom end apart? I've been told just turn it upside down in a parts washer and flush all the chunks out. Is this what you guys do or is there a better way?

Thanks for all the help guys.

 

[Ol'89r]

I've been told just turn it upside down in a parts washer and flush all the chunks out. Is this what you guys do or is there a better way?

QUOTE]


Raptor.

Do yourself a favor and take the whole engine apart. There is a good chance that you already have pieces of piston in the big end bearing and in the oil holes that lead to your main bearings. Plus, you could have damage to your cases under the crankshaft or pieces wedged in there that may come loose later.

The only way to know for sure is to take it apart. :ugg:

 

[darringer]

Pull it apart. There could be damage to the crank that is unseen. Also, you won't know if all of the pieces were flushed out until it's up and running again, possible destroying the bottom end. Separating the cases really doesn't take that much time, and for the piece of mind and slight additional cost it is well worth it.

 

[BlackRaptor]

The bottom end has me scared to rip into.
How hard is it to get the timing reset once you pull all that stuff apart?

 

[darringer]

There are simply timing marks that need to line up on the stator and the crankcase. After separating the cases, the transmission can stay in one crankcase half intact. It is not as hard as it looks. With a manual, flywheel puller, some clean workspace, and patience it is pretty straight forward.

 

[reelrazor]

"How hard is it to get the timing reset once you pull all that stuff apart?"

Scribe a line from the case to the stator at EVERY hole before you loosen any of the screws that hold the stator on and you should be good. When you put the stator back on, just be real careful to get them lined up perfectly.

If you are too worried about it, still do that, leave the flywheel cover off the first time you start it and hit it with a timing light to verify.

 

[Rcannon]

I had this same thing happen on a CR 80 a few months ago. We were able to flush out the broken piston parts using kerosene.

We lucked out. The lower end was not taken down because the owner wanted to race the next day.

Still, it is a risk. For us, it was worth taking.

 

[reelrazor]

Sometimes you can flush 'em and it pays/makes sense to do so, sometimes not.

What I mean is, if you can find two big chunks, that correspond to what is missing on the piston, well, hell, slosh some gas around down there, hit it with a blowgun, oil the crank bearings and refresh the top end.

If you are finding smaller chunks, chewed up, and bunches of shavings/aluminum gravel...well, pull that bastid apart all the way..

Ask yourself "why did I break a chunk off the side of the piston?". THAT is the REAL question you NEED to know the answer to before you put it back together regardless of what you do to clean the crankcase.

Usually skirts break because the piston-bore clearance is excessive. Sometimes they break because wrist pin bearings are either worn out in general or they have overheated and seized.

Did the intake or exhaust side skirt break(or both)? Does the opposite side of the piston show wear? Is it the dusty satin look or scrape/scoring wear? Is the wrist pin blued? Is there charred oil on the underside of the piston crown? Was there a decent oil film in the crankcase/cylinder wall?

All that stuff matters. True "material failure" is VERY rare. Noticing those things above(and more) while in tear-down and inspection tells a story of what exactly happened(and therefore what to try to rectify) in the engine's life to cause it to fail.

If it was just plain wear/high hours, that can be seen as well. It is a real good idea to get a good compression guage and use it on regular occasions from prior to breaking in a top end refresh, broke in, and regular intervals afterward.

The compression will rise until broken in, peak/plateau at a certain number for a while, then start to decrease. Refreshing the top end should happen as soon as that decrease is seen on the guage to prevent costly(both in money and down-time) catastrophic damage .

 

[BlackRaptor]

The intake side of the piston skirt is what broke.
I'm going to rip into it friday night. got all my parts in.

I wanted to thank all you guys for the help. without it i would have made a hand full of mistakes.

This beers on me. (Need little guy drinking beer icon) haha

 

[Rcannon]

Did you decide to rip into the lower end????

 

[darringer]

Good luck. Let us know how it goes.

 

[BlackRaptor]

Did you decide to rip into the lower end????

I think so. I'm going to really look it over close tomorrow afternoon and decide from there, chances are i will.
I'll drop a post in this thread once I finish and let you guys know how it goes.

 

[BlackRaptor]

Well It's 99% put back together. tomorrow is the airbox and little things like that.
Next question for you guys.
Break in. How should i break it in to get max power and power life? so i'm looking for the best of both worlds? Ride it hard let it cool? one of those deals?

 

[darringer]

I run it keeping it under 1/2 throttle and varying the speed under light load. Then I let it cool completely and run it through the same cycle again. I do this three of four times, each with the throttle opening a little more, but still light load. After that, ride at will. Never had a problem with this method, and always have a long service life of the engine.

 

[BlackRaptor]

It's 100% up and running.
And running great
Thanks guys for all your help.