How to do a "Top End Rebuild" on a 2-stroke motor.

Danny127

Aug 7, 2007

Cars

A step by step instruction/pictorial on how to change out a piston/rings etc on a 2 stroke motor.

Example of a top end rebuild

NOTE: This procedure was done on a 2004 200SX, your machine may not look exactly
like this, but it should be pretty close. Also, this will detail the install after the teardown
since I had already performed the tear down prior to writing this. To perform the
teardown just go in reverse order.
At this point your engine probably looks something like this:
(Note: Use rags to prevent dust from entering engine while the top end is disassembled)

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

(For the observant, yes, this is actually a 200EXC two ring piston instead of a one
ring 200SX piston)

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)

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.

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

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

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.

user posted image

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

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

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

NOTE 1: 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!!!

NOTE 2: 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.

NOTE 3: Notice how the open ends of the circlip is not lined up
with the dimple in the hole of the side of the piston.

There is a ridge on the inside of the hole in the piston that the
circlip seats into. I usually stick one end of the circlip in the groove
then squeeze it with my fingers to get it the rest of the way in. You
may need to push it in a little more with a flat blade screwdriver to
get it seated properly. You can see the ridge in this picture just on
the far side of the dimple in the hole in the side of the piston.

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

Put a little premix on the skirt of the piston.

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

Install the cylinder base gaskets, make sure the metal surfaces are clean
and free of diirt 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.

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. Especailly 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.

NOTE: Sorry no pictures of that last step since I did not have a helper
and both hands were occupied.

Here is a picture of the cylinder with the piston installed.

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

Slide the cylinder down all the way.

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.

Install cylinder base nuts.

Check the "X" dimension, refer to owners manual for instructions.

Torque the cylinder base nuts down to the value specified in the owners
manual.

NOTE: Now you are asking yourself, how do you get a socket on the cylinder
base nuts in order to torque them correctly? The way I did it was to grind
down my motion pro torque wrench adapter.

BEFORE:

AFTER:

I hated to do it, but it was the only way to get the nuts torqued correctly.
Torque the nuts down, tightening each nut a little bit at a time in order to
acheive 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.
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.

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

Install the engine braces.

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.

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

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

Add fresh coolant.

Put on the tank, shrouds, and seat. You are done!

Turn on the gas, hold your breath, and kick her over!

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44 CommentsAdd a Comment

Jacob Armstrong on Oct 16, 2017

Thanks we need more people like you, I feel like i get a better understanding this way. Instead of just reading a whole page to realize it doesnt answer the question, the pics help. Thanks again!