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Replace a two-stroke crank seal | How To

Crank seals in two strokes can wear and start to leak, creating problems. If a crank seal is leaking it can allow gearbox oil to enter the crankcase and causes the bike to run rich.

A typical sign of this is an oiled plug or oil dripping from the exhaust pipe. Gearbox oil has a very distinctive smell when it burns so it is easy to tell it apart from the two-stroke smell.

1 CLEAN AWAY

Start by cleaning off any dirt or foreign matter from around the clutch cover. You don’t want any of this falling inside your motor. Then remove the brake pedal and kickstarter and set them aside.

Step 1

2 DRAIN COOLANT

Drain all the coolant and oil from the engine and discard it. Always use new oil and coolant when reassembling.

Step 2

3 FLICK THE CABLE

Remove the coolant hoses from the pump and disconnect the clutch cable from the actuator arm.

Step 3

4 REMOVE BOLTS

Remove all the bolts from the clutch cover and set them aside. Make note of where each bolt came from as some are longer than others and will need to go back in exactly the same places.

Step 4

5 REMOVE COVER

Remove the clutch cover. This may be stuck to the gasket and may need a small tap with a rubber mallet. Be careful and don’t hit it too hard.

Step 5

6 DUMP CLUTCH

Remove the clutch from the engine by removing the pressure plate and then undo the clutch nut. Slide the clutch off the input shaft and set it aside.

Step 6

7 GO NUTS

Remove the nut from the end of the crank and then the primary drive gear.

Step 7

8 PICK YOUR SEAL

Use a seal pick to remove the old crank seal. Make sure the engine e cases are clean and then grease and fit a new seal in place. Be gentle and don’t damage the new seal.

Step 8

9 REFIT CLUTCH

Refit the primary drive gear and clutch, making sure everything is tightened to manufacturers torque settings.

Step 9

10 NEW GASKET

Refit the cover using a new gasket and tighten all clutch cover bolts.

Step 10

11 ADJUST MIXTURE

Refit the brake lever, kickstarter and coolant hoses and then refill the engine with coolant and oil. Then start the bike, you may need to adjust the mixture and idle to get the bike idling correctly.

Step 11

LEAKS’ TRAIL OF DISASTER

  • A leaking crank seal can result in a lean idle, a flat spot at low revs or an engine that will not rev out.
  • In some cases, the motor will not idle at all.
  • A typical two-stroke engine will have seals on both sides of the crankshaft but here we are replacing the right-hand-side one located behind the clutch.

DO

  • Use a new gasket when refitting the clutch cover.
  • Use new oil and coolant when reassembling the engine.
  • Be gentle when fitting the new crank seal.
  • Use grease when fitting it.

DON’T

  • Reuse any old, damaged parts.
  • Over-tighten any clutch cover bolts.
  • Don’t use excessive force to remove any parts or they may break.
  • Over tighten coolant hose clamps.

Technical Editor Mat Boyd