How To: Rebuild a KTM 2-Stroke Top End

Posted By: KTM Talk

This procedure was done on a 2004 KTM 200SX, your machine may not look exactly like this, but all 2-stroke KTM’s are very similar. Also, this will detail the install after the teardown since I had already performed the tear down prior to writing this. The motor we are rebuilding here has already been torn down and the old piston has been removed, to perform the teardown just go in the reverse order. At this point your engine probably looks something like this:


Now it is time to install the piston back onto the rod, here are the parts you will need:

  • Piston
  • Ring(s)
  • Needle Bearing
  • Wrist Pin
  • Circlips
  • Gaskets

1. First, you will need to put the rings on the piston, if you have a two ring piston, put the bottom ring on first. Be careful not to stretch the ring too much or you will break it. Also, notice in the gap on the piston there is a pin that you should center the gap of the ring on. (Hard to see, but on the top ring look between the ring gap)

2. Next, put the needle bearing into the hole in the rod. I like to put a little 2 stroke oil on the needle bearing so it has some lube at the initial start up.

3. Now, note the forward direction of your piston, this piston was easy, it has an arrow stamped on it.

4. Next, insert the wristpin part of the way into the hole of the piston, just enough to hold it in there.

5. Set the piston on the rod, and line up the hole in the piston with the hole in the rod and needle bearing assembly. Make sure the piston is facing the right way.

6. Push the wristpin through the piston and needle bearing until it is flush with the piston.

7. A socket can be used to help you push, just find one that is the right size.

8. Now it is time to put in the circlips that hold the wristpin in place.

  • This is another good time to make sure that you have a rag in the opening to the lower end. These clips are springy and tricky to get in. You don’t want one falling into the bottom end!!!
  • Some people may prefer to install one of the circlips prior installing the wristpin. That way you avoid having to install two circlips over the open bottom end.
  • Notice how the open ends of the circlip is not lined up with the dimple in the hole of the side of the piston.

9. Install the 2nd circlip in the other side of the piston in the same manner.

10. Put a little premix on the skirt of the piston.

11. Also put some premix on the crank in the lower end. Just enough to cover it lightly.

12. Install the cylinder base gaskets, make sure the metal surfaces are clean and free of dirt and/or grease. (Note: Notice there are 3 gaskets in my particular application. This is to get the “X” dimension right. I put the thinnest gasket in between the other two.)

13. Now it is time to install the cylinder. (Note: An extra pair of hands comes in real handy during this particular step. Either to hold the cylinder, or to compress the rings. Especially, with a two ring piston.) Position the cylinder above the piston; try to have the cylinder lined up with the mounting studs so that after installing the cylinder over the piston you do not need to twist the cylinder to get it to line up with the mounting studs. Squeeze the ring(s) with your fingers so that they compress and fit into the cylinder. (Note: Make sure the ring gap is lined up with the positioning pin in the groove of the piston. Otherwise, if the ring is sitting over the pin, you will not be able to compress it enough for it to fit in the cylinder.)

14. Before sliding the cylinder all the way down, make sure that the powervalve actuating arm lines up with the receiving fork in the cylinder directly above it. Slide the cylinder down all the way.

15. Open the side door and verify that the powervalve actuating arm did in fact get seated in the receiving fork. Then put the side door back on.

16. Install cylinder base nuts.

17. Torque the cylinder base nuts down to the value specified in the owners manual. Torque the nuts down, tightening each nut a little bit at a time in order to achieve uniform torque readings. (Note: If you use a motion pro torque wrench adapter, make sure to adjust the torque setting on the torque wrench accordingly.)

18. Install the o-rings into the top of the cylinder. A little assembly lube will help to keep the o-rings in place. (Note: You engine may have a cylinder head gasket instead of o-rings.)

19. Install the head, and torque down the head bolts to the value specified in your shop manual.

20. Install the engine braces.

21. Install the cooling system hoses. (Note: When installing the hose that connects to the head, near the spark plug, make sure the face the tightening screw away from the sparkplug. This makes it easier to get a wrench on the sparkplug.)

22. Install the powervalve setting indicator gasket and breather hose. (Left side of cylinder)

23. Install a fresh sparkplug. (Aren’t you glad you left room for the sparkplug wrench?) Don’t forget to put on the sparkplug cap.

24. Add fresh coolant.

25. Put on the tank, shrouds, and seat. Turn on the gas, hold your breath, and kick her over!