I have a yamaha 400 4x4 in fantastic shape except that somebody ran it low on oil until it died. The wrist pin in the rod seized and caused the piston to **** to one side and resulted in the piston seizing to the cylinder. After taking everything apart I see there is a 1/16 to 1/8 inch wide ring of build up scoring around the very center of the rod bearing where the wrist pin goes. The rest of the rod bearing is good. I also noticed the metal on the crank near the rod has turned blue from heat. Now, I know the "right" thing to do is to split the cases and put a new rod in (and who knows what the crank bearing will look like)... But it only costs $250 for a new piston and bore job. Would you slap a new piston in and hope for the best out of the rod? Or junk the whole quad because nobody is going to finance splitting the cases and buying new rods n cranks. Other than this, the quad is in new condition. The cam looks good, valves are good, rockers have slight wear on them. What would you do?

 

You could take the rod out and have it bored and honed for a bigger wrist pin. Rods are very delicate and need the perfect oil tolereances. That's why alot of engine builders hone them to 1/10,000 of an inch rather than 1/1,000 of an inch. For budget you could throw one in there and hope your wrist pin or piston gets proper clearance to oil.

 

That's the whole problem though,,, taking the rod out. Nobody wants to pay for that job and then pay for parts and/or machining. The whole quad would be junk if it required rod work.

We sent the jug off to be bored today. I'm going to use a dremel to carve a groove in the center of the rod to remove the built up metal from the wrist pin. The rest of the bearing is in good shape. I'm going to try not to touch the outer edges of the rod with the dremel tool, just the very center, about 1/8 wide. That's the only thing I know to do. A hone won't work because it would remove metal from the outer edges. As long as the wrist pin goes in easy and doesn't have any play side to side, it should be good, don't you think?