400ex rebuild smoking

[jason_david1]

i just rebuilt the top end in a buddy of mines 400ex. he had let it go smoking for way too long and didnt keep the air filter as clean as it needed to be so im sure that didnt help with the original piston. now back to my problem. i had to have the cylinder bored. i sent the piston with the cylinder so clearances would be right. replaced all the gaskets and valve seals. before i put the head back on i turned it over several times to make sure there weren't going to be any marks on the cylinder wall from a ring not being in. well after about 20 minutes of riding it, it started smoking again. he then informed me he was using synthetic oil to break it in. he drained it and went back with gn4 non-synthetic. continued to smoke. i took it back apart tonight only to find the intake skirt on the piston was really scored and some on the cylinder wall. i assume this is the smoking issue but what would have caused it? the exhaust skirt looks fine and the rings moved freely in the piston locations. any help would be appreciated so when i put it back together this time i dont have the same issue.

[on the rocks]

20 mins doing a synthetic brake is sounds like a great way to have a glazed cylinder. Not sure about the smoke. Maybe the fine synthetic could get past the rings before they were seated and its still burning off that oil or the glazed cylinder is allowing oil to pass through.

[jason_david1]

i dont think the synthetic oil helped to seat the rings but im stumped on what would have caused the damage to the piston?

[DragonGunner]

I have a KTM motor that was rebuilt and runs full synthetic....however I learned you must use motor oil for the break in period, then after that go to full synthetic.....so I would say this was the problem.

[jason_david1]

i could see that being the case on the smoking until i got it apart and saw the damage to the piston. the rings didnt show any wear from where i might have have one that didnt stay in and nothing was broke. maybe i am missing something but synthetic oil wouldnt cause piston damage would it? or is there is heat issue?

[CJM]

Lets backup a second and consider the following:

1. When you assembled everything how tight was the piston in the bore? Shoulda taken effort to put it in, it shouldnt have went in easily.

2. Are we sure the rings didnt break during assembly or after while running and cause damage?

3. Are we sure the bore was good and properly lubed upon assembly? YOu just dont slap the piston in there without some sorta lube.

4. Is the crank possibly bad and causing the piston to move erratically?

5. You put the piston with the arrow facing the exhaust port?

6. The wristpin didnt move/come out?

7. It wasnt running super lean (far out guess but possible) causing overheating damage.

Im outta ideas..