Engine experts - I need your help!!

[bstuart]

I blew a hole in my stock piston. There is a fragment missing that I believe disentigrated. I have several questions. #1 Can the cylinder be honed at all or do any scratches warrant replacement? #2 The missing piece of the piston apparently wore on the top of the rod, is this something to be worried about? #3 Should I be worried about any fragments in the crankcase. I can't see any evidence of metal in the oil, wondering if just flushing the case with diesel and a few quick oil changes will work? Here are some pics of the damage.http://img212.imageshack.us/img212/3869/piston004ya4.th.jpg (http://img212.imageshack.us/my.php?image=piston004ya4.jpg)http://img479.imageshack.us/img479/390/piston006pv5.th.jpg (http://img479.imageshack.us/my.php?image=piston006pv5.jpg)http://img476.imageshack.us/img476/7504/piston002wb4.th.jpg (http://img476.imageshack.us/my.php?image=piston002wb4.jpg)

[stevo3176]

I'm not engine expert by far but I would be worried about that chunk of metal. I once pulled apart a blown engine that did not seem to have any metal in and found that the oil pump gear had the piece of metal jammed in it. No matter what I used to flush that metal was't coming out unless the the cases were split. If it was some crappy motor I would probably do the flush but in your case I would tear it apart.

[1fst400]

Is that a yfz motor? If so I have taken them apart many times. The cases arent that hard of a job to split, and re-assemble. not that it realy maters but im 17 and did it.

Looks to me like the conn rod got a little hot from what ever was rubbing it. It could have done somthing to weaken the metal which could cause the rod to snap later on.

That cylinder looks a little to bad to be just honed, I havent had a whole lot of experience tho. you can always have it bored out just a tad though.


I dono, its your call on just flushing the cases. This might be a no no. but could you do somthing like submerge the cases in some liquid. ( desil oil or somthing) and spin everything, It would seem if everything spins freely then it would be fine... right?


But then again, it takes all of about 20 minutes to take the cases apart inspect, and throw it back together.

[bstuart]

Yes, its a YFZ. I have a race this weekend. I was trying to see if I could get away without having to pull the motor out of the frame and split the cases. If thats what it comes to, I'll do that and just put an 06 crank in as well and cover two of the concerns. But, I'm trying to see if its worth the risk to slap her back together for this weekend. (Last TT race of the season)

[CannondaleRider]

I wouldn't just throw it together if you can avoid it.

You can risk SERIOUS damage by just throwing it together. That rod can snap and the piston can totally annihilate your valves/head....stressed/overheated parts can screw you over if ignored

I'd split the cases, make sure everything is PERFECT, and put in the new parts it needs.

[BSTURDIVANT]

Mark on piston is actually the piston material melted to the rod! Clean the rod real good along with flushing the crankcase. Hone the cylinder to see if the aluminum material will hone off- may take a little work by hand directly on the area before honing!Should hone ok to finnish the last race, but make sure the lean or overheat condition is repaired before racing!

[bstuart]

What do you mean "by hand". What would you use to remove the aluminum from the cylinder wall? Like emory cloth or fine grit paper? I have a little sponge that I use to get a little "hillbilly crosshatch" on my little 90cc two smokes. Is that allright to use on one of these types of cylinders? -- Thanks

[1fst400]

doesent the yfz have a nicicel (sp) coating?

[BSTURDIVANT]

Yea the nicasil is very hard and you can work that area first with emery or 320 grit then deglaze the rest of the cylinder. looks like the stock piston cracked through the deck surface( very common), then fired through onto the rod. Rod small end is gaulded some but with a little clean up you can make one race then rebuild this winter!Be carefull to clean everything thoroughly with brake clean, hot soapy water then repeat till bore is clean!

[bstuart]

Thanks Bro, that info is worth a ton to me.

If you every need any help with 90cc CVT quads, let me know. Thats where I've spent the last 3 years researching and experimenting. That whole time I've raced this stock motor, spending all my free time on the kids quads. BTW - I don't think she went lean, running a 182 main. Sounds super rich, but thats the way it was setup when I bought it and it runs great (I was leading an A class race when she popped) with a good color on the plug.

[BSTURDIVANT]

Probably just from a lot of time on a stock piston!

[bstuart]

Thanks again Brent. Congrats on your finish in Ohio.

[dork]

thats not an uncommon piston failure on the yz/yfz motors. my yz426 bike's piston cracked like that(but it had a cut down deck for a stroker). you're going to need to replace the rod at least, or put in the 06 crank like you said.

[bstuart]

Heres an update. Cleaned up the melted aluminum on the rod, honed the aluminum off the cylinder walls. Wasn't sure how much to take it down, just went till it felt smooth and right. Put a stock piston in and took it to the race track. Went thru two 15 minute break in cycles, changed the oil and raced. Won the Pro payout class and finished fifth in my other class. It seemed at least as strong as it was before, so I am assuming that the piston sealed up against that cylinder just fine. I had alot people tell me that it wouldn't without being recoated or at least honed with a special diamond hone?? Anyway, it seems to be running fine.

[BSTURDIVANT]

Good Deal!!!