How to replace a 2 stroke crankseal

Written by tom lutzenberger
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How to replace a 2 stroke crankseal
The piston and crankshaft have to be removed to get to the crank seal. (piston de scooter image by Vetea TOOMARU from

The crank seal, otherwise known as the main bearing oil seal, sits deep inside the two stroke engine right next to the main bearing. Its job is to make sure that oil and gasoline don't leak out of the combustion chamber into adjacent sections of the engine case. In two stroke engines, replacing such a seal means performing an engine rebuild to get down to the area of the crankshaft. It is a lengthy process that will require both specialised and normal automotive tools and expertise.

Skill level:

Things you need

  • Socket wrench and sockets
  • Crescent wrenches
  • Clutch-holding tool
  • Hammer
  • Metal punch pin
  • New crank oil seal
  • Screwdriver

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  1. 1

    Use a socket wrench or crescent wrench to loosen and remove all the nuts and washers securing the cylinder to the engine. Pull the cylinder cap off by hand when loose. Loosen the exhaust with a socket wrench and remove it from the bottom of the cylinder. Pull the cylinder off the piston and engine case.

  2. 2

    Use a socket wrench to loosen and remove all the nuts and washers keeping the engine case cover secured and closed. Take the engine case cover off the engine. Open up the clutch side cover using a socket wrench, removing those bolts and nuts as well.

  3. 3

    Use a clutch holding tool to secure the clutch so it won't spin on the crankshaft or chain drive if one is involved. Loosen the clutch with a socket wrench and remove it. Use a hammer and metal pin to tap the crankshaft out of its seat in the mean bearing now that all the engine parts are clear.

  4. 4

    Catch the crankshaft as it pops out. Use a screwdriver to carefully and gently pry loose the old crank seal from the mean bearing socket wear it sits next to the bearing. Use a shop rag to wipe the area clean.

  5. 5

    Insert a new crank seal into the socket next to the main bearing. Press it into place with your thumbs. Tap it home with a small pin hammer if it is stubborn (be careful not to hit the engine case). Reinsert the crankshaft and tap it home into the main bearing again with a metal pin and hammer. Reverse steps 1 through 3 to reassemble the engine.

Tips and warnings

  • Using the round end of a crescent wrench and a smaller hammer works well when pushing the edge of a crank seal into place.
  • Never try to reuse an old crank seal once the engine case has been opened and the crankshaft removed. Oil seals should always be replaced with new ones to avoid oil leaks from the engine.

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