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one of the most feared tasks involved with bike maintenance
is doing your own top ends. Ask yourself, “Why should
I pay somebody a lot of money to do the work, when I could
use the loot to buy new tires or plastics?”
For this example we’ll use a 2002
CR 125R. With that said, there are just a few important
things to know before you get started. Firstly, stick with
OEM (original equipment manufacturer) parts. However,
if you have an oversize bore, or prefer Wiseco, or Nik’s
Pro-X or any aftermarket stuff, that’s cool too. Just
make sure that you follow the instructions that the manufacturer
sends with the parts because sometimes there are specific
things to look for prior to installation (ie: ring end
gap or proper wrist pin fit).
Second, it would be a good idea to stick
with the same make of gaskets as the rest of the parts (plus
it’s easier to get the parts all at the same time!!).
Once you’ve decided what parts to use, then it’s
time to get down to business. I can’t stress enough
how important it is to start with a clean cycle. You especially
don’t want dirt and debris getting into the bottom
end once the cylinder is removed.
Once your bike is clean, remove the seat
and gas tank. Next, drain the coolant from the rads. The
drain plug is at the bottom of the water pump housing (see photo #1).
Some models might require removing the rads completely,
but you can work around Honda’s. With the coolant
drained, replace and tighten the drain bolt. Then it’s
time to start getting down to business.
Step One: Undo the rad hoses from the
head cylinder and push them out of the way. Then, remove
the pipe and exhaust flange – the metal gasket should
be reusable if it’s not too beat up.
Step Two: Remove the power valve cover,
actuating arm, cylinder nuts and head stay bolts. The head
nuts don’t need to be removed if you’re not
honing the cylinder. With nicasyl cylinders, they don’t
always need to be honed so get a mechanic to check it out
if you’re not sure. With the cylinder ready to slide
off, pay careful attention not to smash the rod or piston
on the case. The aluminum is really soft and can be dented
easily. Slide the cylinder up and off of the piston. QuickTip:
Cover the coolant elbow with your finger so the fluid doesn’t
spill into the crankcase. When the cylinder is off, remove
the old base gasket and scrape any old gasket off of the
cases or bottom of the cylinder with a razor blade. Again,
be sure not to drop anything into the bottom end.
Step Three: With a small pick, remove one
circlip from the piston and slide the wristpin out, then
remove the wristpin bearing. If the piston isn’t in
too bad shape, keep it for emergency use if you get in a
pinch on race day.
Step Four: I like to do as little work
over the crank as possible. Install the ring (with the
letter or number facing upward) and one circlip onto
the piston before putting it on the rod (see
photo #2).
Be sure the proper side of the piston is
facing forward. There is usually an arrow or letters on
the intake side. If ther isn’t, the ring-locating
pin faces the intake port on a two-stroke engine. Before
putting the new bearings on the rod, smear pre-mix oil onto
it (preferably the brand of oil that you normally use
or are going to switch to). Smear some oil on the wrist
pin bearing as well. Install the second circlip on the piston
once installed on the rod and recheck the first one to be
sure that it’s in place.
Step Five: Before putting the cylinder
back on, have a look at the power valves (see photo #3)
and make sure that they aren’t covered in carbon.
If they are, remove the power valve rod, valves and de-carbonize
with a sand blaster or wire wheel.
Step Six: Install the new base gasket onto
the case. Then smear a thin film of oil onto the cylinder
wall (It’ll smoke a little but I don’t like
putting motors together dry). Hold the ring in place
on the piston and slide the cylinder onto the studs. It’s
hard to torque all four cylinder nuts so unless you have
the proper tool, tighten them down as much as possible (if
you’re a human torque wrench, get someone to assist
that has experience!). If the head was removed, torque
the nuts down to 21 ft. lbs. in a criss-cross pattern.
Step Seven: Re-install the power valve
arm and cover. Usually you’ll need to replace the
gasket as it gets torn easily. Replace the head stay and
collant lines (I like to use blue loctite on the head
stay bolts because of vibration).
Step Eight: Put the exhaust flange and
pipe back on using high temp silicone to seal both. Fill
the rads with a 50/50 mix of coolant and distilled water.
If your going to use a premixed collant such as Engine Ice,
just pour in straight. I Prefer to leave the cap loose ‘till
after the bike has been run for a few minutes and the system
is bled.
Step Nine: Double check your work! Go over
all nuts and bolts, rad hoses and anything else that was
taken off to be sure everything’s tight. Re-install
the tank, seat and plastics. Start your bike and let it
warm up for a few minutes. Re-check the coolant level. You’ll
need to top it up once the air comes to the top. That should
cover everything you’ll need to do. Also, I like to
start with a new spark plug and a clean filter as well.
'Till next time, see you at the races!
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