Transmission Oil Leaking

If the oil is traveling somewhere else besides the ground beneath the bike, then you have to replace some sort of seal on the crankcase...

You will have to strip the engine and find out. The seal kits are rather cheap (less than 40 dollars). My guess it would be the crank seal, which in fact, are the most probable problems with transmissions at least in my experiences... :nod:

Well can you see where oil could be draining out? It'll be caked all over your cases from dirt or if not you'll definately see residue. Check around your fillercap, around all case bolts, drain plug, and just all over the engine if there's oil you'll know it. You can also tell if your bikes burning the tranny oil because it'll smell really bad.

If the oil isn't leaking externally, it's the right side crankcase seal. The seal itself is cheap, but you need to pull the clutch hub in order to change it. Not everbody is up to pulling a clutch hub, but the procedure is simple enough. I suggest you have an impct wrench.

First question: How do you know you had half the oil when you returned? On my YZ I only get a Full / Not Full indication.

Second question: Was it using oil at this rate before you changed the oil?

Like 76 said, if it isn't leaking out where you can see it then it has to be leaking in along the crank, which means the crankshaft seal. The seal is cheap, ~8.00. Probably a few dollars more for the gaskets around the case cover. You might want to replace the clutch plates while you are at it, since you will be taking them off to get to the seal.

I would be surpised that the seal went from being fine to leaking half the oil in 3 hours riding time immediatly after an oil change. Either the old oil was a lot thicker and thus didn't leak, the synthetic oil somehow reacted with the seal (I certainly wouldn't expect that!), the timing of the seal failure was just right, or perhaps you didn't put as much oil in after the change as you thought you did.

I would certainly fill it back up and try again before I got too paniced about the leak. It is possible that you got a false "full" indication after the ride the oil settled into where it belongs.

If it turns out that the seal did go that bad that quickly then I would be afraid that there was a reason for the seal going bad. That seal has a pretty easy job: it has oil on both sides to keep it lubricated, shouldn't ever have to deal with dirt, doesn't have any sunlight shining on it to have UV issues, and it isn't that old. The most likely reason for such a seal to fail is that the shaft that it is trying to seal against is moving too much, which means the bearing is shot.

If you establish that it is the seal then after you remove it try to get a measure of the radial play in the crankshaft. If the crankshaft is wobbling around you will never get a seal to last.

Rod