YZ 125 04 GEARBOX OIL

it should be clearly marked on the engine gearbox casing or if on the dry sump system should have a dipstick on the top of the frame near the steering head main nut.

1.06 quarts w/o filter change > 1.27 quarts with filter change. 10w40 motor oil, any major brand. The gearbox drain plug is on the left case a little lower than the shift pedal.

1.06 quarts w/ no filter change
1.16 with new filter
1.27 if engine was torn down and is dry

The oil filter is under a cover plate just forward of the clutch and close to the top of the gearbox.

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Use 10w40 in the gearbox. Get the oil from your Yamaha dealer along with any filter your bike may need. Some bikes use an oil change as the "filter". To drain oil, get the engine warmed up and turn it off. Put a pan under the gearbox and remove the oil drain plug. Replace the oil plug when the flow becomes a slow drip. Change oil in late spring, the middle of summer and in early fall. Remember to lube the chain and adjust it when needed. Lubricate the cables with liquid graphite. Lastly, install a stock spark plug. Now go for a ride.

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Your bike is water cooled. Some of that water is leaking into the gear box. The cylinder gasket is the likely leak. I would let the shop take a look at it instead of trying to fix it yourself.

But be sure there is not an external source for the water. Do you wash the bike often? Ride the creek bed often? Leave the bike where the lawn sprinkler gets the bike wet? Leave it out in the rain? Change the oil and keep the bike dry. If the problem comes back then go see the dealer.

water pump / inner,seal needs looking into .