What should I replace before it goes...what's most common in bottom end to go?
Main bearings and the connecting rod bearing are the only real components to watch. Seals will fail periodically. They will let you know by putting premix in the flywheel side or transmission oil consumption. A crank may last for two to three main bearings. My main bearings have all failed on the flywheel side. Slight up and down movement made the failure easy to catch before any damage was done. Finally there is the actual failure of the crankshaft. During your regular top end maintenance check for any up and down play in the connecting rod bearing. You do want to replace a crank before it fails because it can send debris into the cylinder which will also require a replate.
I HIGHLY encourage you to inspect and resolve bottom end issues quickly. The price of repair jumps up tremendously as well as the down time of your bike and efforts required to get it back into ride ready condition. Monitoring how much trans oil comes out during changes can help tip you off to a leaky seal as well.
And leaky seals can tip you off to a worn main bearing as well.
Let me tell you that getting any issues with your bottom end fixed will save you a lot of heart ache and even more money. Trust me I know. Mine lets just say exploded, while my wife's friend was riding it a little to hard. Long story short the rod punched threw the case. Luckily I found a new case with the bottom end and rod for insanely low price 149. Witch is about a quarter of most prices you'll find. Needless to say take care of any bottom issues right away
yz250/125 cranks like to melt stuffers when run hard.
Heres a 08 125 (same problem as yz250)
here is how you fix it, any time in the future i will send my cranks to these guys, top notch work best i have ever seen
Check for crank bearing play on the flywheel under the left ignition cover. Even a tick of up and down play is to much.
Replace the top rod pin bearing every topend job at about 50hrs
every time you do a topend check to rod play, If you do rebuilds at decent intervals you will catch a worn crank bearing before it goes boom
3rd gear is a weak link. If you split the cases check all the gears for wear. I am in the middle of a complete tear down now because 3rd gear broke.
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