Sep 28, 2003
I have a 1985 yz 125 that loses about 8 to 10 ounces of transmission fluid every riding session. I have rebuild the top end with being bored over and new piston and ring and bike runs strong. It seems like the trans oil may be burning thru the exhaust, is this possible ? Could it be possible that the bottom end needs rebuilding? Any info would be greatly appreciated.
Thanks Jim


Dec 3, 2001
All you should have to do is replace the right side crank seal. You might be able to replace it with the engine still on bike, but I'm not sure, with mine I was able to. The seal could of failed from age or from worn out bearings that let the crank move around and wear it out. So wiggle the crank around to see if there it any play up or down. If your bike suddenly started to foul plugs when you noticed the drop in oil and the plugs used to be good looking but now are all covered and dripping with ooze, then its probably your seal. Ooor, the crank seal could be just fine and maybe you have a leak on the clutch cover or somewhere in that area where oil could leak out. Look for any oil dripping, especially from gasket areas. You may just need to get a new gasket.


Dec 22, 2002
Agree on right side crank seal. Have a 99 YZ 125 that I had to replace this on. It will go through a lot of oil as it wears out. You can probably replace the seal without splitting the cases, but on an 85 your crank bearings are probably shot and the seal will wear quickly.
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