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Oil moving from transmission to Engine

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So my friend's bike (crf250 2007) just had a bottom end rebuild he put it back together with out replacing seals and Oil was moving from side to side ..So he repleced the seals and rode it. The bike ran perfect 10 hours then he was checking his oil then he saw that his oil was moving from Transmisson to engine

What can cause this ? Maybe a cracked case ?

-Thanks .

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I understand that he replaced the seals, but there's only 2 ways for the oil to transfer to the engine side from the trans side. Balance shaft seal and right cranks seal. There are post on here from time to time that folks have had the same problem with new seals and they replaced the them and everything is fine now.

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its either the counterbalancer seal(s), right side crank seal, or a center case gasket, hope he did every bearing while it was apart too, if he didnt, might as well split it again and do it right.

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yep its either the right crankshaft seal or counterbalencer seal.. he may have installed the seal wrong as well.. and i dont think it would be a cracked case, it is possibile..but not likley

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When installing the new seals, use some blue loctite around the outside edge of the seal. They have to be in straight or you're going to have problems.

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wel i think the right crank seal is on the right crankshaft , behind the nut and 2 sprockets .

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Did you use an OE crank or aftermarket? I have seen the same problem with the crank causing it. I got involved in one that was rebuilt with all aftermarket parts, then split again for your same problem on just 2 hours of run time. Replaced all the seals with OE and the same problem at about 6 hours. Then he brought it to me and I inspected everything and took a stab at it and replaced the crank with OE and all the seals again and all is ok now.

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