Transmission Oil leaking into engine oil????

10 replies to this topic
  • Scott Chapman

Posted September 22, 2005 - 07:24 PM


We have a 2005 CRF250R and we are losing transmission oil into the engine oil. The engine level increases until oil blows out the crank case vent (we have the crank case vent hose routed down the back of the engine case and not into the airbox to prevent fouling the bike upon wipe outs).

We just replaced the counter weight shaft seal on the left side of the engine. We figured the engine was sucking transmission oil through this seal. But we still have the problem. We lose 500ml of oil in 20 minutes of hard riding. I'm afraid I need to split the case and go in deeper.

It's quite obvious the oil is moving over as the engine oil becomes very dark and is obviously contaminated with transmisison oil.

Any thoughs would be greatly appreciated??

  • Steve94c

Posted September 22, 2005 - 07:40 PM


I have benn going through a similar problem with a CRF 450 that I use for Dirt Track. We tried all seals and new center case gaskets. Recently I raced the bike and thought that I was being dillegent about oil levels.
I have now spent over $500 to replace the main transmission shaft, shift forks and 2 gears. BE CAREFUL.
We have now found that the cases were not sealing perfectly. Hopefully, this has now been remedied and I will reassemble next week. My advice would be to tear down the motor and look carefully at the cases. Also, Cometic makes very good gaskets for the center cases. Best of luck.

  • Bodyman

Posted September 23, 2005 - 03:25 AM


Rob, good instructions. Remember the gears have timing marks, pay attention to them.

That 7 dollar seal will solve your issues. I keep a spare with me.

  • Scott Chapman

Posted September 23, 2005 - 08:07 PM


Thanks for the info. I will try Rob's fix below first by replacing the main crank seal. If that doesn't work I'll tear it down.

  • greenbunch

Posted September 25, 2005 - 03:26 PM


Well it happened to me today at an Enduro today. My trans was down to 11 oz. of oil and my crankcase had 34 oz. Simple math, the trans oil moved into the crankcase. My friend saw a mess of oil leaking out of the airbox drain and it was coming from the breather hose. Its always something. I'm going order a couple of crank seals tomorrow and hopefully have her together by the weekend. Anyone know who has them in stock ?

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  • Bodyman

Posted September 26, 2005 - 03:24 AM


Shenandoah Honda. If not I have 2 spares.

If you just noticed this all should be OK, the low trans oil caused us a smoked clutch :lol: and then the valves went ...maybe from all that oil getting into the air box ad burning... :banghead:

Good cannot beat those honda gremlins (just feed them and live with them) :applause:

  • greenbunch

Posted September 26, 2005 - 05:17 AM


You got that right Bodyman, Only problem my wife has caught on and ever she is calling that RED thing a money pit. :banghead:

  • greenbunch

Posted September 27, 2005 - 06:53 AM


Well took the Red one apart last night and the seal fit very loosly so I am assuming it's worn out. Not a bad job, maybe took me an hour or so. The Clutch was OK even with running low on oil. I ordered two seals and a cover gasket, should be in Wednesday. Whats that clutch cover gasket made out of gold, I think it was like $ 21 dollars . Thought about replaceing the left side crank seal but skipped it. Hopefully that side is not a problem.

  • Scott Chapman

Posted October 06, 2005 - 03:18 PM


A bit of advice about the Head vent tube: We routed the Head vent tube straight down the back of the engine and not into the airbox. We DNF'ed at an important race when Jon high sided on a off-camber and oil blew off the head vent into the airbox with the engine still running. This fouls the carb up and the only way to start the bike is to bump start it. To prevent this we route it away from the airbox and just straight down the back. Be sure to plug the hole left in the airbox.

  • moto_x2005

Posted February 16, 2009 - 04:53 PM


kind of bring this thread back to life... haha.. was it the 04/05 models that had the bad seal? better to replace with a newer year seal such as 08/09?

  • BIKEsatori

Posted May 30, 2013 - 06:20 PM


Another super old dig.... my 04 crf250 is having issues with trans oil ending up in crank oil.... then excess being pushed out air box.
Any major tips or links to what needs to be done? Can the trans side crank seal be replaced without removing engine?

Pretty frustrated, but at least it's not the opposite way, contaminating my clutch with moly every time it leaks!

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