I've disassembled the top end of a Scrambler 400 (1996) and found the top piston ring broken, a big chunk of the piston skirt missing and small chunks of metal strewn around the top of the crankcase. The walls of the cylinder are smooth and not scored (amazing, considering the other damage!) Questions: What caused the damage? Lack of oil or just metal fatigue after 6 years? Must I have the cylinder rebored even though it looks very smooth? If I don't rebore, should I get a stock piston or .010 over? There are bits of the piston skirt down in the crankcase. Is it possible to flush them out rather than taking the crankcase apart? Or, just leave them down there and hope for the best? Thanks!

 

PeterinAZ, Knowing the age, your piston suffered the all too common drastic fate of a broken skirt of a cast piston. I can bet it broke on the intake side as well. Your best option is to go ahead and bore it, then put in the good forged Weisco piston. Getting the remains out of the crankcase bottom is hard, but all of it has to come out. Use a liquid cleaner (kerosene works good) and flush it out and follow with compressed air. I had to do it a dozen times on my wife's wheeler before it looked perfect. Remember: Attention to detail!!! Hope this helps,Bubba

 

I had my 400 piston melt down last summer.The piston looked like a dries pile of cement.But the cylinder luckily did not need to be bored.If machine work is not required,it will not be expensive to rebuild yours.Leave that crap in there? Are you kidding.Get it out.I had mine done buy a professional and it only cost cost 280$ turnkey rebuild.

 

Bubba, you're right again. The piston broke on the inlet side like you said. I've taken the cylinder to be rebored and I've picked up a gallon of carb cleaner to get the shrapnel out of the crankcase. Yuck! I've got to turn the machine upside down and get a shower of cleaner, compressed air and chips of metal in my face. At least it's warm & sunny here in the desert so I can do it out in my wife's flower bed. When she's not home, of course.

Does anyone have the torque specs for the cylinder nuts, and the head nuts? Is there a procedure for filling the cooling system so there are no empty spots, or can it be filled from the radiator cap? Thanks!

 

The cylinder base bolts torque to 24 ft/lbs. You need to recheck them after a good warm up and cool down. Use a crow's foot on an adapter and extension. The head bolts will go to 18 ft/lbs, gradually and criss-cross. The stock head has a bleeder screw that you can remove to get all the air out before firing it up. Hope this helps,Bubba