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yz125

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It's not that bad, you can do it with the engine in the bike.

You have to drain the oil, pull off the main clutch cover and pull the primary drive gear off the crank.

If memory serves, there's a metal tab with a bolt that holds the seal in place, remove that tab.

Then you need something to pull out the seal. You can pierce it with an awl and thread in a wood screw.

Then you can yank on the wood screw with pliers.

Two holes and two screws, one on each side of the seal will allow you to pull it out straight.

Check for radial play in the main bearing while it's apart.

If you can feel or hear any radial play at all, your main bearings are shot and the new seal will probably fail quickly.

A few years ago I started my '02 with the silencer plug in it. The bike coughed and popped once. Pressure ruined the right hand main seal. There was a ton of motor-oil smoke when I removed the plug and started it again. So I didn't ride that day.

I replaced it using the method I described above, and I'm very careful to remove my silencer plugs first now.

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There should be some premix oil in the bottom end when you tear down your engine, that's normal. That is the oil that coats the the engine parts while it's running. When you shut the bike down most of it drips off the parts and pools in the bottom of the crank case.

If there's transmission oil, it will look and smell different, when sitting in the bottom end, and when being burned by the engine.

If the bike was smoking very badly and smelled like a worn out, smokey four stroke, there's probably a bad seal in there.

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The only way to tell for sure if the seals are bad is to perform a vacuum or pressure test on the crankcase. If you have a hand vacuum pump (like the 25.00 automotive kind used for bleeding brakes) it is fairly easy to make a test rig for this out of a few dollars worth of parts from Home Depot. Basically, you remove the exhaust pipe and plug the opening with an adjustable rubber plug and make a PVC attachment that fits in place of the carb. Then you pump it up and watch the gauge to see if it holds pressure or vacuum. I tested my YZ125 this way. I did have to crimp the power valve vent tube shut. There are several detailed threads with pictures online somewhere . Let me know if you need the links and I'll look them up.

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my top end is allreddy off

If you want to do the pressure test but aren't ready to put the top end back together yet you just need to bolt the cylinder and head back on with the old gaskets temporarily. It only takes about 10 minutes. You don't even need the piston installed. But in your case it sounds like you don't even have a seal issue.

Edited by rjpjnk

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