Two Stroke Rebuild

[tigmaster]

This is one of the easiest things to do and all you two-strokers, this is now your time to shine. You don’t need to take your bike anywhere; you can do it in an afternoon. Obviously you will need to get your stash of parts first. Parts required are a new base gasket and a new head gasket or o-rings and possibly a new wrist pin and circlips. Whichever is required for your specific bike. Remember, starting with a clean bike is always best. Always oil all parts you are replacing in the engine.

Cracking into the engine is simple and requires no removal of the engine. Start by turning of the gas and disconnecting the fuel line from the carburetor. If you are crafty, you can remove the gas tank and shrouds as a single unit. Now drain the coolant. This is usually done by a bolt on the water pump housing. It is usually an obvious bolt with a copper washer on it. Remove the seat, via the two bolts at the back of it. Remove the rubber strap that holds the gas tank on. This is why we took the seat off. Next, remove the two bolts on each shroud. These are the bottom two bolts. One of them bolts to the frame and the other bolts to the bottom of the radiator. Watch for radiator braces behind because these will drop out now if you have them installed. If you have a front tank bolt, remove that as well. Pull the tank and shrouds off now. You now need to remove the expansion chamber. Two springs and a bolt on a rubber mount usually hold this on. Next is the carburetor. You can usually get away with just loosening the clamp on the rubber boot attaching it to the engine. Push the carburetor away from the engine. Now, depending on you bike, you may have an engine mount bolted to the head. Unbolt it from the head and from the frame. Remove the sparkplug wire and any coolant hoses. Four to six head bolts will get you in. Now, on the side of the cylinder will be a rectangular cover. Remove it. Usually it is four 8mm bolts. Inside there will be a linkage you will need to disconnect. Inside also can be a cylinder bolt and three more around the cylinder. Gently pull the cylinder off now with the piston as far down in the cylinder as possible when you start.

Now, this is where we will run into a few contradictions. Newer bikes feature Nikasil plated bores. This is a super durable, super conductive layer. This cannot be honed by any stone hone unless it is diamond tipped. You will not be able to hone it. Now I am always going to recommend you take you cylinder to a machine shop or dealer and have them check the runout. I am not going to go into mic-ing a bore in this tutorial because it is much too far out of scope for it. However, generally you can get away with scrubbing a Nikasil bore with a 3M pad and WD-40. Then wash everything with soap and water. This is basically de-glazing the bore. If you have an older bike or know for certain you cylinder is not Nikasil coated then you need to hone. Don’t take this lightly. You need to either hone you bore or have it honed. Obviously, you should have a manual which will list all of the specifications for the cylinder. There are many things that can be said here but this is just a basic tutorial, nothing more. Consult myself or your local machine shop if you have anymore questions.

Now to remove the wrist pin. There will be circlips on either side of the piston. Pop one of these out and press the wrist pin out from the other side with a deep well socket. I would recommend replacing the wrist pin and circlips but it is not always necessary. Insert your new piston, making sure the arrow on it is facing towards the front of the bike. Ensure proper cylinder to ring clearance, listed in your manual. Place the ring on the piston after coating it in clean motor oil. There is a notch in the ring groove where the piston ends sit. Make sure it is lined up. Apply a hefty serving of motor oil to the cylinder. Place the new base gasket on the crankcase. Gently work the cylinder back over the piston while compressing the rings. Apply the four cylinder bolts and turn the engine over to ensure proper working. Rebuild the rest and torque all bolts to specs listed in the manual. Fill with fluids and roll.

Obviously I have not even begun to touch on all the complexities of this but if you have any questions, feel free to PM me. I am always on. Read your manual and watch the videos. I have for sure missed something, let me know. Read on in this next tutorial about breaking your engine in after a rebuild.

The Real Break-In Procedure

[Hardtail]

Nice job Tig!!!

[tigmaster]

I know I am missing something.

[wr250f95]

some more rep for you!

[Hardtail]

Well like you said check the manual, wouldn't do it without one

[tigmaster]

That is very true.