I picked up an 02 YZ 426F yesterday that I found on Craigslist for a good price. The PO had it lock up on him a few times, but it always freed up again and would run fine. Took it to a mechanic who pulled the side covers and valve cover off and found nothing obvious and figured the piston skirt had broken off and was the source of the problem. The PO decided he'd rather off-load this than fix it, and thats where I stepped in The bike as I received it: One hours worth of time got me here: and another 30 minutes got me here: So far the only issue with the chassis I have found is a rusted/stuck swingarm pivot bolt: I was able to drive it out without even breaking out the BFH but managed to booger up the threads while driving it out so I need to find a die to chase the threads. Anyhow back to the motor- the head and valves look good, but the center intake valve appears slightly burned: Is this a normal condition or not? Onto the piston and cylinder- they look perfect. That's the good news. The bad news is the lower rod bearing has failed, which is why the motor would occasionally lock up. I know this photo isn't the best but: Are those heat marks on the crank adjacent to the rod normal or not? Now I get to self-educate myself on how to split the cases, and decide if I'm going to replace the crank assembly or just the rod and bearings. I'm hoping to keep the cost as low as possible on this because there is a strong possibility I'll sell it when its fixed. However I have toyed with the thought of keeping this one and getting rid of the 450 because I've had such a love/hate relationship with it (more hate than love for the most part). Anyhow, if anyone has advice on how to proceed from here I'm all ears. If the cost of rebuilding the motor gets too high this will end up being parted out.