Can you install a main crank bearing backwards?

hi, i have a question, i am rebuilding my rm250 (1993) and i installed some new crank bearings and oil seals.  when i went to put the crank cases back together i couldnt get them to go together.  so i took it up to a motorcycle shop and they said i installed a crank bearings backwards..and they said i put on an oil seal thats to small..  the manuel dosent say that there is a right and wrong way to install one.  and the oil can that be to small when i know that i ordered the right one.


so can you install a main crank bearing backwards, and is the different oil seals for a 1993 rm250?? btw it was the gear shifter oil seal. their charging me $400 for this, and all they had to do was assemble the bottom end which means transmission crank and crankcases.


i have talked to some other people about this and they say you cant put a main crank bearing on backwards, and one of them mentioned that sometimes mechanics can rip you off.  is any of this stuff true???





Main bearings can go either way.


Oil seals are directional.  There are different schools of thought.  I always install mine with the spring side towards the crankshaft.

There are different schools of thought.  I always install mine with the spring side towards the crankshaft.


You need to know where the oil pressure is coming from. The side with the spring (might not even be a spring) goes against the pressure, which expands the seal.

No oil pressure on a 2-stroke.  The certainly is crankcase pressure, though.  That is why the open side goes towards the crankshaft in my book.

NO! You cannot install a main crank bearing backwards. The seal may be too small to fit the crank properly, but I would would doubt it after what was said about the bearing. You got honed.

so chances are they are just trying to get more money? they also said some other crap like something was bent in the engine and the oil seal for the gearshifter is apparently to small... 


lol they tell me i put a crank bearing on backwards yet i work on bikes and they think im dumb...okkk


there is no spring, i dont know what you mean by a spring.

As above, No you cant put the bearing in backwards.


I think the spring they are referring to is a tiny one you can see on the open side of the seal. It is part of the seal and is there to keep it slightly tighter grip on the shaft to give you a better seal. Hope that makes sense.



Unfortunately some shops make stuff up to try and earn a bit extra money but ultimately it ruins it for themselves when the word gets out.

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