At some point, you will wear your nitro engine out, and it will require a rebuild. The manual that came with the engine almost always tells how to rebuild it, but usually they are a little vague.
Tearing the engine downEdit
To tear the engine down, you need a few tools. Usually you need a 3mm hex driver to take the head and backplate off, a smaller hex driver or phillips screwdriver to take the starter off if you have one, and a 3mm or 3/32" (for american vehicles) hex driver to remove the engine from the vehicle.
We will leave the clutch assembly on the engine, and the glowplug in the head. First, remove the engine from the vehicle, following the vehicle instructions. Next, remove any starter that may be on the back of the engine. After that, remove the back plate and head. You need to do this in a criss-cross pattern, in an X for the backplate and 4 bolt heads, and in a star for 5 and 6 bolt heads. Loosen each screw a little bit at a time, until there is no pressure on the head or backplate, then remove them all the way. You risk warping the head or backplate if you do this improperly. Next, take the header off, this is usually held on by a spring or two 3mm screws.
There will be some discoloration on the parts, usually brown, but it can look similar to rust, or the inside of the engine may be blue if you use certain fuels. Don't worry about it. First, use an exacto knife, screwdriver, or something else, to scratch a small line on the top of the piston, at the same location as the pin for the sleeve is. This will remind you which way to install the piston, if you use it again.
You need to remove the sleeve from the engine now. On some engines this may be difficult, but sometimes you can just turn the crankshaft a few times and it will move out slightly, then you can just pull it out. If the lip of the sleeve is not sunk into the block, you can often get a small flathead screwdriver under it, and move the sleeve out. If the lip is sunk into the block, and it wont move out by turning the crankshaft, you will need to put a ziptie in the exhaust port, and turn the crankshaft. It will be pushed out slightly after this. Remove the ziptie and see if you can pull it out by hand. If not, spray WD-40 around the edge and let it soak in for a few minutes, then try to move it out with a screwdriver under the lip, or needle nose pliars on the edge. If you use pliars, do not squeeze the sleeve, just use them as a tool to push up on the lip. Never do anything that might scratch the top or inside of the sleeve, these are the parts that matter!
After you have the sleeve out, rotate the crankshaft until the rod is where it can be pulled off the crank without hitting the block, this is usually at top dead center (TDC). It may be easy to pop it off with your fingers, or very gently with needle nose pliars, but what really works great in dental floss. Get it behind the rod and pull it off. The piston and connecting rod will now fall out with the engine upside down.
Next, put some after run oil on the sides of the piston, and push it as far as you can into the sleeve, without using too much pressure. It should go in most of the way and stop. If it gets to the top, or past the top of the sleeve, the piston and sleeve set is worn out, and the engine needs a rebuild.
Before you decide if you will rebuild or replace the engine, you should take the clutch and flywheel off. You will usually need a crankshaft locking tool of the type that goes in where the backplate goes to do this. If you have a pneumatic impact wrench, this will work well for removing the clutch nut, and you will not need a locking tool.
If the flywheel collet (split beveled cone) is in good condition, it will be very difficult to remove the flywheel. Automotive battery terminal pullers cost 3 US dollars, and work very nicely for removing the flywheel. There are also wrenches made that grip the flywheel pins and allow you to twist the flywheel off. A rubber or plastic hammer, tapping from behind, will also work. Or if you ruined your flywheel from some other method, just hit it from behind with a small section of steel pipe, this always works!
You will now be able to push the crankshaft out the back of the engine, it should push out with a little effort. Put the connecting rod on the crankshaft journal, and check how much play it has. It should be fairly tight, if it isn't, you will need a new connecting rod. If a new connecting rod feels loose on the journal, then you need a new crankshaft. Stick your finger in the rear bearing and turn it a bit. It should feel smooth. Turn the front bearing with your finger tips, this should also feel smooth. If the bearings do not feel smooth, they should be replaced.
If the crankshaft is fine, use some fine sandpaper to take the shine off the journal, and then polish it with chrome polish. This will make the connecting rod last longer.
If your crankshaft is bad, then it is usually cheaper to just replace the engine. Some manufacturers like Traxxas offer engine exchange plans, where you can send the old engine in, and get a completely new engine for cheaper then you would normally. Depending on promotions, etc, it may even be cheaper to replace the engine if it only needs a new piston and sleeve assembly.
When you rebuild the engine, you #1-flush the engine with alchohol, #2-replace the worn parts, usually just the piston and sleeve, and #3-put all new gaskets in.
Assuming the engine is dissassembled, get a can of denatured alchohol, or if it's available, methanol. By methanol, we mean the pure stuff they use for race cars Not what's available at gas stations in your area, as this is mixed with gasoline, and should not be used for cleaning. Take the carburetor off the crankcase, use alchohol and a tooth brush to remove as much dirt as you can get from the outside of the engine. There will most likely be some burned on stuff that won't come off, that's ok. Then pour the alchohol in the crank case, splash it around, pour it out, repeat a few times. Let it air dry a while, then put a few drops of afterrun oil (3in1 or synthetic motor oil) in the bearings and turn them a few times. Rinse the parts you will be putting in the engine with alchohol also, and cover the crankshaft in after run oil (3in1 or synthetic motor oil). It is very important that no dirt get in the engine. Even the smallest spec of dirt can cause engine failure.
If you will be reusing the connecting rod, use an X-Acto knife to pop the G spring out. It will most likely fly away, and you should have bought spares for it. The wrist pin should easily slide out the side that the spring came out. Put the new piston on the rod in the same direction the old one was, put the wrist pin through it, and put a G-Spring in. It is quite difficult to get the spring in, expect to lose a few.
Put the crankshaft in through the back of the engine, and put a drop of oil on the journal. Then, put the connecting rod/piston assembly in through the top of the engine, noting the direction that it originally came out. The manual should help you with this step, and it is very important that the rod and piston face the right direction. Slide the rod over the journal.
Cover the inside and outside of the new sleeve with lots of afterrun oil (3in1 or synthetic motor oil), noting the direction the slot should go in to mate with the pin, slide it slightly into the top of the engine, with the piston at bottom dead center. Turn the engine upside down and rotate the crankshaft as close as you can to top dead center. The piston should now be in the sleeve. Push the sleeve in the rest of the way.
Coat any new o-rings with after run oil(3in1 or synthetic motor oil) and put the head, carb, backplate, starter, flywheel, clutch, and header back on, with new gaskets, in the same way they came off. Get the head and backplate as tight as you can with an allen wrench or hex driver, but never any tighter then this.
Check the engine manual for the baseline break-in carburetor settings, and set the carburetor to these. Install the engine into the vehicle, and break-in the engine.