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)

[intruder]

Looks like someone was running lean. The cylinder needs replaced or replated. The rod doesn't look very good either and you'll need a new piston.

[bstuart]

You really think I'd have to replace the piston?? I was thinking JB weld might fill that hole. hehe (kidding)

Seriously, can YFZ cylinders be bored or not? I've heard both yes and no.

[K_Fulk]

They can be bored but it will still need replated also.

[bstuart]

Thanks, now I think I understand. Doesn't the bore & replate cost almost as much as a new cylinder?

[spanky101]

proboly not far off... i would get a wr crank and piston and rebuild that motor if it was mine

[Piker450]

lean? the top of the piston was black. looks like an oil problem. id try to exchange the rod and crank assembly, or buy a new assembly. its about as cheep to buy a new cylinder, and quicker. and take jb weld and 500 grit sand paper to the piston to smooth it out, add some to the top of the piston to add more compression,then youll be flying!!!!!!!!!!!!!!!

[Ride1Rob]

Actually, I think that's an unknown problem with YFZ's. Took mine apart for a rebuild and this is what my piston looked like as well. Of course I had well over 400 hrs on my motor so I was lucky.

http://img91.imageshack.us/img91/2764/img1424qg6.jpg (http://imageshack.us)

http://img91.imageshack.us/img91/969/img1425xa2.jpg (http://imageshack.us)

http://img91.imageshack.us/img91/6582/img1426pz3.jpg (http://imageshack.us)

[motomanmike]

I would definetly pull the whole motor apart and disassemble the oil tank filter (actually in the tank, flush the lines and tank), what happened it seems is the wrist pin was starved for oil and tried to seize up, a very common yfz problem addressed in the new 07's. I've seen them rip the pin right out of the pistons, and some just crack the pistons as some of these pictures look very typical of. While the engine is apart send the case get the oil mod done, as Spanky101 stated go with the WR crank and piston, new crank main bearings and put new valve springs and cam chain in while you are at it. Definetly replace that cylinder too. Don't try to go cheap, you'll end up having it run another couple of hours and then the thing may grenade and cost you waaaaaaaayyy more than you want to think about. Don't cut corners, valve springs are cheap, and so is the cam chain. On top of it you'll know its fresh, and the WR parts bolt right in same as stock. You will be very pleased with the added pull in the midrange power. Its an incredible difference over stock.