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How to Check the Left Crank Bearing & Seal on a 2 Stroke Dirt Bike


Checking the condition of a 2 stroke dirt bike's LH crank bearing and crank seal is something that should be done occasionally and this is true whether riding motocross, off-road disciplines or FMX. Checking the main bearing's condition and looking around inside the ignition cover for any signs of crank seal leakage is a fairly good way of keeping on top of routine maintenance in relation to the condition of the bottom end of the dirt bike's motor, as well as the crankcase sealing at the left crank seal.

It's often regarded as preventative maintenance to remove the ignition cover on a 2 stroke dirt bike during maintenance or after a ride as this allows a mechanic to inspect the cleanliness of this area for any signs of water or dirt entry, as well as for keeping tabs on the crankshaft main bearings and the LH crank seal's condition which we're going to show you how to do below.

On most 2 stroke dirt bikes, there's a plastic cover on the left side covering the flywheel and ignition system and this cover, (if still the OEM plastic unit) is frequently distorted and allowing water, dirt, sand and grit into this area during rides or while washing the dirt bike.

If your bike still has the plastic ignition cover, now would be a good time to replace this with a better sealing, replacement ignition cover.

How to Check the Main Bearings and Left Crank Seal

Occasionally, such as after washing a dirt bike, lubing the clutch cable or while the ignition cover is removed for other reasons, be sure to stop for a moment sometime to get an idea of what's happening with the crank bearings, as well as the bottom end's sealing capabilities.

With the Ignition Cover Removed...

  • First take a visual inspection inside the cover. If there is any water or dirt, this should be cleaned out immediately using an aerosol solvent such as “Brake Clean” and compressed air while also leaning the bike to the left to allow drainage of the water or solvent although be sure to pay particular attention to anything oily as there is more on that coming.
  • With everything clean, grab the flywheel by hand and feel for ANY movement.
Image showing procedure for checking crankshaft main bearings by hand

This is really as simple as it sounds and is an accurate way of checking the left side main bearing's condition. When holding the flywheel with your fingers as shown in the picture to the right, there should be absolutely ZERO movement when attempting to move the flywheel in an up and down, side to side or diagonal direction. Any movement at all would indicate that the bottom end is either long past it's needed maintenance or the flywheel is loose on the crankshaft and it would likely be safe to assume that the bottom end has been neglected for far too long and is probably worn out.

How to Check the Left Crank Seal

Image showing a clean ignition system indicating a sealed crankcase

Checking the condition of the left crank seal is another preventative maintenance technique which is quick and easy to perform as this only requires a glance to see if there is any oily residue seeping out from behind the stator plate or flywheel, or if the entire area is covered in an oily film.

If there is any oily residue, be sure to clean the area thoroughly with brake clean and compressed air, then run the engine to determine whether this is coming from a crank seal or the clutch actuator. If the clutch cable goes into the area under the ignition cover, there will be an oil seal which seals the clutch actuator arm shaft from leaking gear case oil inside the ignition cover and this seal's condition can be evaluated by visually looking for any signs of oil seepage or leakage which would need to be addressed before riding the dirt bike again.

If the Motorcycle Surges or Has a High Idle which can't seem to be corrected elsewhere and you're sure the carburetor is clean and there are no other vacuum leaks, remove the flywheel with a Flywheel Puller while in here and get a direct view of the LH crank seal while looking for ANY wetness whatsoever near the crankshaft, as any presence of oil here would indicate the likely need for a crank seal replacement.

Now provided everything looks ok, before you put the ignition cover back on, it's wise to ensure the drain hole is clear by using compressed air to blow through the hole (if accessible), then check the fasteners which secure the stator plate and ignition pickup, as well as checking the case screws for their tightness, then reinstall the ignition cover while paying attention to the condition of the ignition cover gasket, replacing it if necessary, as should you encounter any water or mud on the track or trail this will help to keep any
water or dirt out of this area.




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