I have a rm 125 and I might need to split the case to make some repairs and I was wonderin if you have to replace all of the bearings and seals when you do it
i would recommend it because you dont want to be back in there for a long time. the bearings are obsorbing air and gas all the time, to deteriorate them. seals will shrink terribly if it has set in storage a long time. any snowmobile mech. will tell you that. the seals have to be in great shape, or it defeats the vacuum effect to such air and fuel into the bottom end. with worn bearings they will try to seize when hot. and without the seals in great shape, it will never run at optimun level. and youll think you have coil problems all the time. you will have to takethe crank, to a shop with a press, the bearings have to be pressed on.
No Change the bearings if they appear worn. Change the seals if they leak. The clutch cover has to be removed to change the right hand crankshaft seal, so it would be a good idea to change it while the engine is apart. The other seals can be changed while the engine is assembled, so no need to change them if they're not leaking at this moment.
I always replace any seals that I have to mess with when I'm in there - so the kickstarter shaft seal, the clutch push rod seal, and the countershaft seal (if I remove that shaft from the cases). I also replace crankshaft main seals/bearings. If it's a low hour machine, that would be it. Although I'd check all the bearings just because I'm paranoid. If it's a hi mileage bike, and it's got a crank main bearing going out or something, then yea, I'd replace all the seals and most of the bearings. Normally the smaller bearings pressed in the cases are ok and don't need to be replaced unless you've had a tranny or clutch failure that's thrown a bunch of metal shrapnel inside. You will need to replace all the gaskets though, no matter what.
No, you don't HAVE to replace all the bearings and seals. When we do bottom end work we split and clean the entire bottom end. Then we decide what seals and bearings to replace. For example, on an RM that was split due to a transmission shift fork problem we would split the cases, remove the gear set, measure all the transmission surfaces and forks, replace what is bad, check the output shaft seal, replace if torn, or leaking, check out the crank bearings and replace if there not smooth and check both crank seals, if they look good under a micro-scope and it ran good when we received the machine then we don't replace them, if they look bad or torn then they get replaced. To answer your question, No, you don't have to but don't be upset when you have to tear it back down cause you missed replacing a $2.00 seal.
