THE ENGINE SHOP - 2012/06

When I left off in my last column, you had a tray full
of parts and (hopefully) a set of digital pictures to
help you put them all back in the right sequence.
Before we put “Humpty-Dumpty” together again, let’s look
at what could be wrong or needs servicing!
While you have the engine apart, there are a few things
you can look for. Most people reuse the old paper and copper
gaskets. Wherever possible, I replace them all.
The piston and ring are always worth scrutiny; look for
scratches and gouges. If you fi nd any, check the liner, because
something hard got in there. This will be the most expensive
thing to repair and you may decide to get another engine instead.
On the bright side, you will have spare parts that you can
use from the old one. I never throw away a damaged engine—
that is, unless it hits concrete at 100-plus mph!
The ring can often be stuck in the piston groove. Scraping
out the stick carbon/burned oil residue with an old, broken
ring is a good idea. I usually soak these parts overnight in 50%
denatured alcohol and acetone, and sometimes regular paint
thinner, to clean out the gunk.
It is important that the ring can “fl oat” to get a good seal
when the gases explode, and then relax a little before the
next bang. Remember that a new ring and/or liner will be like
having a new engine again, so use break-in procedures.
Look for play on each end of the connecting rod, or possibly
the wrist pin or crankshaft. Look closely at your crankshaft. If
the area under the main bearing is shiny, this usually means
that the main bearing’s inner ring has been slipping. This is
bad news. Most times you should change both of the bearings
and the crankshaft.
To check for bad bearings, press fi rmly with your thumb on
the center ring of the bearing and rotate one-half turn each way.
If you feel any roughness or a gritty feeling, change the bearings.
The bearings play an important role in the “rotating part,”
as well as supporting the shaft so that it does not touch the
crankcase. When the piston comes down it creates pressure
in the crankcase. This pressure pushes the gasses up into the
combustion chamber via the transfer ports.
What you don’t want is for the gases to escape along the
outsides of the crankshaft. The tolerance gap between the
crankshaft and the crankcase is critical and can be damaged if
the bearings allow the shaft to rub. Stuck rings and/or worn
casings could be the cause of excess fuel spraying out of the
carburetor.
It is important to check to see if the crankshaft is straight.
You can do this by rolling the shaft on a fl at surface (such as
a sheet of glass), leaving the fl ywheel hanging over the edge.
I keep a threaded die tool for each of the popular crankshaft
threads. It is a good idea to clean the threads while you still
have the crankshaft out of the engine.
You should also inspect your cylinder head. It is common
to fi nd burned oil in the fi ns. Generally the burned oil stain is
fuel that sprayed from the carburetor or muffl er.
Sometimes fuel leaks between the cylinder head and
the cylinder liner. You can check for this by looking for
discoloration of the head gasket and the cylinder head seat.
This usually means that the head bolts were either loose or
unevenly tightened. Check to see if the head is warped (on a
sheet of glass) and replace it if necessary. A new head gasket
will often take care of any minor problems, and as long as the
bolts are tightened evenly and accurately, the cylinder head
will usually pull back into shape.
If you have a 1/4 x 32 tap, it will do no harm to clean the
glow plug threads in the head. Remove any carbon deposits
using acetone. More stubborn carbon can be removed using a
wooden Popsicle stick.
I have one fi nal detail about the bearings: there are usually
two bearings in the modern two-stroke engine. Both bearings
have different functions/responsibilities. The rear bearing is
designed to handle the up-and-down action of the piston and
the associated forces from combustion. This bearing will not
stop the shaft from moving back or forth.
The front bearing (thrust bearing) is designed to stop the
shaft from pulling forward when the engine is running and
pushing backward when using an electric starter. When you
tighten the propeller, you pull the crankshaft hard up against
the inner ring of the bearing.
If, during your examination of the engine, you can push the
propeller shaft backward and forward, this means that the front
bearing is worn out. If you can rock the propeller tips back and
forth in opposite directions, this is telling you that the main
rear bearing is worn out.
After you have determined which parts you wish to repair or
replace, it is time to put the engine back together again. This is
where you will really appreciate your digital photographs.
I use a master list. I put this in my iPhone notes for reference
purposes.
Start from the inside out. I have provided a simple checklist
that you can tune to your own engine-reassembly needs.
• Bearings
• Crankshaft
• Thrust washer
• Drive washer
• Temporary spacer propeller nut and washer
• Don’t put the liner in fi rst
• Get rod the right way around
• Face piston the right way around
• Slip onto crankpin
• Slide in cylinder liner
• Head and gasket
• Backplate
• Carburetor/needle assembly
• Plug
Most of the engine reassembly is a
reversal of the process of disassembly.
I’ll share some tips about how to get the
harder parts back in correctly.
The piston, connecting rod, and ring
should be put together before they go
in the engine. You position the crankpin
(big end) at top-dead center as you did
during disassembly. Use your picture of
the original connecting rod orientation
to get the correct position of the piston
relative to the exhaust port.
Ensure that the phosphor-bronze
bearing chamfer is on the side that faces
forward to meet the fl ywheel portion
of the crankshaft. Slide the piston
forward on the wrist pin and push the
connecting rod in place with your fi nger.
You refi t the liner with the piston
already in the engine. Before you slide
the liner in place, position the piston
ring with the ring gap located at the
anti-rotation pin in the ring groove.
I usually put a fi ne-tip Sharpie marker
on top of the piston to help when fi tting
the liner. If the ring gap is not correctly
positioned, the liner will not go on. The
liner chamfer will compress the ring into
place as it slides exactly into the barrel
of the crankcase.
To prevent any unwanted warping of
the aluminum cylinder head because of
uneven bolt tension, a good method is to
lightly snug down the head bolts evenly
fi rst. Visually ensure that the head gasket
is aligned. (A trick to hold the copper
head gasket in place is to use a drop of
oil on top of the liner lip.)
With fi ve head bolts or more, you
should tighten in an “opposite” pattern.
Each time you tighten a bolt, move
the wrench across over the glow plug
hole to the bolt on the other side of the
cylinder. For a six-bolt head, the formula
is to tighten exactly diagonally opposite.
It is impossible to tell you everything
that you need to know about engine
maintenance in a couple of columns.
I recommend reading model airplane
engine books authored by others
knowledgeable in the fi eld.
If you use a smartphone or digital
camera to record everything you do,
then you will greatly improve your
chances of success as you attempt your
fi rst engine-maintenance project. Even if
you are an experienced engine person, it
never hurts to keep a record of what you
do when you take something apart. It
has saved my bacon on many occasions.
If you are unsure about taking apart a
relatively new or expensive engine, you
might like to consider buying old, used,
or beat-up engines at a local fl ea market
so you can cut your teeth on something
that you have not invested in heavily.
I strongly advise sending engines
that are still under warranty back to
manufacturers. When a warranty has
expired, then you have little to lose and
much to gain by servicing your engine
yourself. I fi nd it fun to take two beat-up,
old engines and make one good one out
of the pair—especially if I picked them
up for a song!