I have burnt 2 pistons so far, the first came that way i replaced the piston and sleeve less than a couple hours this one burnt as well. Obviously I am not getting oil to the piston. I'm not sure how oil is supplied to the piston any help would be appreciate. I do have the case open and the oil pump cover exposed....

Thanks!

 

Engine size, type of machine? Most engines have a hole in the con rod near the big end that squirts oil out as it goes up and down, spraying the bore and piston with oil. Crude engines have a scoop on the bottom of the con rod, as it dips into the oil in the sump it picks up oil and flings it at the cylinder wall.
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70CC (72cc marked on engine) Yamoto, Honda clone engine Late 90's I believe. So then I'm assuming the oil pump only oils the top end then?








Thanks Bob

 

Bye the way the piston was fine on the bottom but the top of the piston is where all the damage was.

 

It would be little end seizure first, this makes the piston drag on one side of the bore, hence the one sided seizure of the piston. Apparently early Honda Cubs were splash fed lubrication. On later engines Honda fitted the oil pump.

Looks like oil is quill fed from pump into centre of centrifugal clutch, through crankshaft, into big end and sprays out the sides of the con rod towards piston and bore. Either the pump isn't pumping enough, or you have a blockage, stopping oil getting to big end and spraying out. Note (4) orifice on drawing is a restrictor, so most oil goes to crankshaft and only a small amount to camshaft, if that was missing you wouldn't get as much going to crankshaft. However a blocked strainer or centrifugal filter, damaged quill, etc, are other possibilities.

 

Thank you very much, I now understand the oil flow. I'm going to check for blockages, do you think a High Volume oil pump would help as well?

Bob

 

Well a high volume pump couldn't hurt, especially if you are going to change pumps anyway. Being an afterthought, the pump sprocket is a bit inclined to wear though, so may not like even more torque on it.