How to Rebuild Two Stroke Engines

Written by tom lutzenberger
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How to Rebuild Two Stroke Engines
Installing a piston and cylinder on a two-stroke engine is one of the final rebuild steps. (cylindre et piston image by Unclesam from

Rebuilding a two-stroke engine properly can avoid costly mistakes. The process involves building the system from the bearings and internal core parts outward. Each part connects in a specific sequence to be assembled correctly as a whole. Understanding how to follow the steps correctly saves you heartache and frustration as a vehicle owner and also saves your wallet from having to pay someone else labour costs to perform the same work.

Skill level:

Things you need

  • Large wrench socket
  • Propane torch
  • New engine bearings
  • Hammer
  • Circlip tool
  • New oil seals
  • New engine case screws and studs
  • Crankshaft puller
  • Metal punch pin
  • Clutch-holding tool
  • New gaskets
  • New nuts and washers
  • Piston bearing
  • Gudgeon pin
  • Circlips
  • New engine oil
  • Socket wrench and sockets
  • Crescent wrenches
  • Rubber mallet
  • Exhaust nut and bolt
  • Screwdriver

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  1. 1

    Position the main engine case on a workbench with the engine bearing sockets inside the frame facing you. Place new engine bearings in a freezer. Wait 30 minutes and then heat up an engine case bearing cavity with a small propane torch. Use a large wrench socket as a drift and hammer the cold bearing into the hot bearing cavity until seated. Perform the same process for all the engine bearings. Use a circlip tool to install any bearing securing circlips needed.

  2. 2

    Position new engine oil seals so that the inner seal spring faces the bearing in the bearing seal cavity. Use your thumbs to carefully squeeze them into place next to a given bearing. Carefully tap a seal with a small hammer if stubborn, Avoid hitting the engine case or it will crack. Insert the engine case screws and cylinder studs into the case if missing.

  3. 3

    Position the crankshaft assembly into the middle of the main engine bearing installed in Step 1. Use a crankshaft puller to pull the crankshaft into place. Tap the crankshaft carefully into place with a metal punch pin and a hammer if the puller is unavailable. Insert the transmission cluster into the lower part of the engine case where its axle sits in the transmission bearing. Tap the cluster axle into the bearing centre.

  4. 4

    Place a woodruff metal slot key in the clutch-side crankshaft arm. Install the clutch assembly on clutch-side arm, pushing it into place. Hold the clutch from spinning by locking it to the engine case with a clutch-holding tool. Tighten it onto the crankshaft using a clutch tab washer and clutch nut and a socket wrench.

  5. 5

    Insert the rear axle through the inside of the installed rear wheel bearing. Tap it home with a metal punch and hammer. Slide the gears onto the axle in the order of small (4th) to big gear (1st). Confirm the teeth mesh with the transmission cluster. Use the circlip tool to install the gear circlip onto the gears to lock them into place.

  6. 6

    Insert the kickstart star gear onto the transmission cluster axle top. Put a healthy blob of engine grease on the inside of the engine cover and squish the kickstart star gear spring into the grease opposite the transmission cluster. Insert the kickstart gear into and through the engine cover and secure with the kickstart pedal on the outside of the engine case to secure it.

  7. 7

    Place a new engine gasket on the main engine case. Slip the engine cover with the kickstart pedal onto the engine case and squeeze the assembly together. Turn the kickstart until it meshes with the transmission cluster and the case cover closes. Bolt the engine case studs shut with new washers and nuts. Tighten them with a socket wrench.

  8. 8

    Attach the piston to the crankshaft end sticking out of the engine case using a piston bearing, gudgeon pin, and circlips to secure it. Oil the cylinder with new engine oil. Place a new cylinder gasket on the front of the case and insert the cylinder over the piston onto the cylinder studs. Push it tight together as the piston enters the cylinder. Tighten the cylinder cap with a socket wrench. Screw in a new spark plug.

  9. 9

    Connect the exhaust to the cylinder bottom. Tap it into place with a rubber mallet and secure the support arm with a socket wrench and nut and bolt. Connect the electrical generator to the engine case. Insert the flywheel onto the crankshaft arm sticking out of the engine case after inserting a woodruff key in the crankshaft arm. Tighten the flywheel nut down with a socket wrench. Attach all remaining engine outside covers (fan, cylinder, engine bottom) onto the engine, securing them with screws and screwdriver.

Tips and warnings

  • If you've never worked on a two stroke engine before, have an experienced friend help and guide you the first time. Then you will remember the steps better since even the best guidebooks don't cover every detail.
  • Make sure not to have any flammable sources nearby when you work with any kind of solvent to clean an engine part. The fumes can catch fire. Make sure to work in a well-ventilated area when doing such cleaning.

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