USING THE DEGREE WHEEL by Mark Bergfelt
It does not matter if your power plant is of the
two stroke-cycle or four stroke-cycle variety. Both types belong to
the family of reciprocating engines. These engines utilize a piston
that slides back and forth or up and down, in a cylinder. That type
of motion is called reciprocating motion. Karts have wheels that
need to be rotated. Unless the reciprocating motion of the piston can
be changed to rotating motion, it is absolutely useless to us. Fortunately,
some mechanical genius many years ago figured out how to connect a piston
to a crank with a device that we call a connecting rod. The crank was
connected to a shaft that could turn, an object that we call a crankshaft.
The rotating energy supplied by a crankshaft is very practical for many applications,
most importantly for turning a sprocket and chain and ultimately a kart's
drive sprocket, axle, and wheels. The rotating motion of the crankshaft
is very important to us if we are going to discuss the performance characteristics
of engines. One commonly accepted means of describing and measuring
rotation is in degrees. Some where along the line, in a math or science
class the reader should have learned that when something turns one complete
time from the starting point back to that point it has revolved 360 degrees.
Obviously one half of a turn or revolution is 180 degrees of rotation and
one fourth of a turn is 90 degrees. READERS WARNING; do not read this
article fast. If you do it won't make much sense and it may give you
a headache. It was meant to be read slowly. When the procedures
for using the degree wheel are outlined, simply do one step at a time and
everything will work out fine, I hope.
Pistons do not revolve. They move in a straight
line, ie., linear motion. The amount a piston moves can be conveniently
measured in inches or millimeters. It is very easy to accurately measure
the distance a piston travels by attaching a 2" or 3" travel dial indicator
inserted into the top to the cylinder over the piston and held in place with
some type of bracket. Turn the crankshsft until the needle on the dial
stops turning and reverses direction. Turn the dial face to zero at
the exact point where the needle stops. You can then measure piston
movement distance very accurately.
There are many events that occur during the operating
cycle of an engine that allow it to run and make power. Examples of
some of those events are valve opening and closing, port opening and closing
and ignition. All of these things must occur at precisely the right
time to get the desired performance characteristics.
Since the piston and crankshaft are positively linked
via the connecting rod, crankshaft position can be accurately described by
measuring inches of piston travel or piston position can be measured in degrees
of crankshaft rotation.
The stroke of an engine is determined by the distance
from the center line of the crankshaft to the center of the crankpin.
The actual stroke is two times this measurement. This is precisely the
distance the piston moves from the very top of the cylinder to the very bottom.
When the piston is at the top of the cylinder, it is customary to say that
it is at Top Dead Center (TDC). When it is all of the way down we say
that the piston is at Bottom Dead Center. Additionally, the letter
A (after) or B (before) may be inserted in front of TDC or BDC, ie., ATDC
means After Top Dead Center and BTDC means Before Top Dead Center.
Manufacturing tolerances make it impractical and
expensive for two engines that are the same model to have exactly the same
stroke. If you were to measure the stroke of any two Yamaha or Briggs
engines and compare the measurements, more likely than not they would be
several thousanths of and inch different, sometimes more than that.
For that reason, measuring where engine cycle events occur using inches of
piston travel is second best. Since ALL crankshafts revolve 360 degrees
in one revolution it is most accurate to describe engine operation events
in degrees of crankshaft rotation. In order to do so a degree wheel
must be used.
A degree wheel is nothing more than a full circle
protractor. All of the commercially available models that I know of
are printed on an aluminum disc and as far as precision tools are concerned
they are inexpensive. They have a hole in the center to make attaching
to some type of hub and the crankshaft possible. They are usually about
seven inches in diameter but bigger ones are available. It is easier
to be very accurate with the larger diameter degree wheels, but I have found
my 7" wheel to be quite accurate enough and I happen to wear very thick glasses.
They (degree wheel not the glasses) are available from many kart shops but
the ones from a good auto parts store will work fine too.
They are necessary for degreeing in automobile cams. Sound familiar?
The old McCulloch American Racing Engines owners manual included instructions
for making a degree wheel from a small plastic 180 degree protractor.
The manual also included instructions for using the tool to set the ignition
timing on the old Macs. My first degree wheel was one of those.
It worked for it's intended purpose, measuring an event close to TDC, but
dealing with an event near BDC was out of the question.
Included with this article is a full size degree
wheel cut out that you can glue to an aluminum, or masonite disc. Laminate
it with a sheet of clear adhesive plastic, drill a hole in the center and
you have made your own.
The first step in using a degree wheel is
attaching it to the crankshaft of the engine that you are working on.
I have made and collected a variety of small collars, hubs, nuts, washers
and screws to adapt my degree wheel to the various engines that I work with.
You can too. There are various methods of attaching the wheel.
Sometimes a little imagination is necessary.
You are also going to need some type of pointer. The pointer
can be made from a piece of stiff wire. A coat hanger will work.
Cut it about six inches long, the length is not critical. The engine
that you are working with will determine the length of the pointer.
Grind one end of the wire to a sharp point. At the other end bend a
loop. The loop will allow the pointer to be attached to the engine at
a convenient position near the degree wheel. The main consideration
about the pointer is that once the degree wheel is calibrated to top dead
center, the pointer absolutely must not move the slightest little bit.
A better pointer can be made using a piece of flat steel or aluminum.
Drill a mounting hole in one end and cut to other to a point. Bend it
so that when it is attached to the hole on the engine of your choice, it
is lined up with the degree wheel. Even though this method is best,
using the wire pointer has worked for me.
Once the degree wheel and pointer are mounted to
the crankshaft, the wheel must be synchronized exactly with the piston and
crankshaft position, ie., positioning the degree wheel so that the pointer
lines up with the TDC mark when the piston is at the very top of it's stroke.
There are two methods of accomplishing this, the positive stop method and
the dial indicator method. Before using either method, it is easier
if you initially place the piston at what looks like TDC and rotate and snug
up the degree wheel to line up with the pointer at this position. It
won't be exact yet, but it will make the next steps easier.
The positive stop method utilizes a piston stop,
a devise that is screwed into the spark plug hole of two stroke cycle engines
or attached to the deck of any engine, that keeps the piston from traveling
all of the way to the top. Chain saw mechanics use them for locking
the crankshaft for servicing flywheels and other revolving components.
Do not use a piston stop for the same purpose on a modern kart engine with
a built up type crankshaft. Holding the piston in this manner and torquing
on a flywheel or sprocket nut will twist the crankshaft out of alignment,
but the stop is ok if you are just trying to set up a degree wheel.
You can also remove the cylinder head and attach a small metal bar across
the cylinder and fasten it down with a cylinder head screw. It must
be tightly mounted so that it does not move at all. Lay a nut or similar
sized flat object on top of the piston or tape or glue the nut to your steel
bar. If you did everything right, the stop will positively stop the
piston from reaching TDC at the exact same place every time you try, regardless
of the direction that you revolve the crankshaft. Now we have to position
the degree wheel so that the pointer lines up with TDC when the piston is
at that point. To do that, revolve the crankshaft clockwise as far
as you can until the piston touches the stop. Look at the pointer and
write down the reading from the degree wheel and pointer. Now turn
the crankshaft all of the way the other way, counter clockwise until the
piston touches the stop. Write down that reading. TDC is exactly
in the middle of the two readings. Add the two readings together to
find out how many degrees the piston stop kept the degree wheel from turning.
Divide that number by two. The number you come up with is the position
that the pointer must line up with on the degree wheel when the piston is
up against the piston stop. Carefully loosen up the degree wheel and
rotate it so that it lines up with the pointer at the number of degrees that
you calculated. Rotate the crankshaft all of the way the other
direction until the piston touches the stop again. The reading should
be the same number of dergees from the TDC mark. If readings are not
the same, adjust the degrees wheel so that they are. Here's an example;
You've installed and tightened up your pointer
and guesstimated the initial TDC setting of your degree wheel but you havn't
tightened up the degree wheel retaining nut, but it is snug. You also
have installed some type of positive piston stop device. Now you rotate
the crankshaft in a clockwise direction until the piston contacts the piston
stop. Let's just say that the reading on the degree wheel is 39 degrees
BTDC (before top dead center) if you have the wheel mounted to the flywheel
side of a Briggs engine. See fig. 1. (It would be ATDC if the
wheel was mounted to the clutch side of the engine and you rotated it clockwise)
You record the reading. Next you rotate the crankshaft counter-clockwise
until the piston stops at the piston stop again. The reading this time
is 32 degrees ATDC (after top dead center). See Fig. 2. Add the
number of degrees the readings are from top dead center together. 39
+ 32 = 71 degrees. Divide the total degrees by 2. 71 / 2 = 35.5
degrees. Loosen the degree wheel retaining nut and rotate the degree
wheel so that it lines up with 35.5 degrees ATDC, assuming that the crankshaft
is still rotated all of the way counter-clockwise. See Fig. 3.
By the way, I'm very cautious about assuming anything. Carefully hold
everything in place and tighten up the degree wheel retaining device.
Turn the degree wheel all of the way clockwise and check the reading.
It should be 40 degrees BTDC. If it isn't, readjust the degree wheel.
The degree wheel must hit at exactly the same number of degrees from TDC
on both sides. When you have accomplished that, remove the piston stop.
Rotate the crankshaft so that the pointer lines up with TDC. Notice
that the piston is all of the way up. If it isn't, check back through
the procedure because something is wrong. If everything lines up you
are ready to measure engine operating events.
The dial indicator method is a whole lot simpler
than than the piston stop method. Install the pointer and degree wheel
to the engine the same as you did with the piston stop method. Mount
a dial indicator to the top of the engine so that it sticks down into the
cylinder slightly. Rotate the crankshaft so that the piston comes up
to the top of its' stroke. The needle on the dial will begin to rotate
when the piston contacts the dial indicator. The dial indicator indicates
TDC at the point where the needle stops and then starts to turn the other
way. TDC is EXACTLY where the needle stops. You may want to rotate
the dial indicator face to zero it at that point. With the piston at
TDC, carefully line up the degree wheel with the pointer and tighten everything
up. Revolve the crankshaft a few times to make sure the dial indicator
and degree wheel indicate TDC at exactly the same time. You can now
remove the dial indicator and you are ready to measure timing events.
By the way, always turn the end of the crankshaft that does not have the
degree wheel on it. Turning the crankshaft with the degree wheel can
cause it to slip out of sychronization with the piston.
Now that you have a degree wheel attached to your
engine and synchronized with the piston movement, there are many things you
can measure. If you are dealing with a four-stroke cycle engine, you
could check to see if your cam falls within the stock specs spelled in the
rule book. The rule book explains the procedure well. You could
also use it to degree in that new cam. To do that you will need some
type of fixture to hold a dial indicator over the valve that you are going
to deal with. Zero the indicator when the valve is closed all the way.
Some cam manufacturers provide the degree points on the dial indicator where
the valves just start to open or close. Others specify a degree reading
when the valves are at a certain lift reading on the dial indicator.
What ever the case, the valve stem must be ground or filed until it opens
at the place the cam grinder specified.
Port timing in a two stroke is alot like cam timing
in a four stroke. To measure when a port opens, turn the crank until
the top of the port just starts to show and thats when it opens. If
you want to raise the ports, remove the cylinder from the engine. Remove
the ring(s) from the piston. Coat the cylinder with machinist dye.
Put the cylinder back on the engine. Rotate the crank to the position
where you want the port to open. Scribe a line on the cylinder with
the scriber guided by the top edge of the piston. That is the line
you grind the port to.
If the degree wheel it attached to the pto (power
take-off or clutch side) end of the engine, it can be used for checking or
adjusting ignition timing. On a Briggs engine, check where the magnets
line up with the legs of the ignition coil. If you are dealing with
an old style engine with breaker points, such as a Mac or old STAR engine,
ignition timing can be set by disconnecting the condenser from the breaker
points. Then hook up an ohm meter or continuity light by attaching
one lead to the breaker point lead and the other to the engine block.
The spark occurs when the points open. That is indicated by the ohm
meter needle moving from zero ohms to infinity ohm, or the test light flickering
off.
When working with one individual engine it is convenient
to hook up a degree wheel AND a 2" or 3" dial indicator at the same time.
Taking the time to record the piston position in inches AND degrees for that
particular engine will save time when servicing that engine later.
It is far easier to hook up a dial indicator to the top of a cylinder than
it is to set up a degree wheel. Remember, those measurements are only
valid for that particular engine.
Using a degree wheel for the first time can be a
little confusing, but by taking the time to master the procedure some engine
measurements can be made very precisely. Getting things set in an engine
precisely every time will give a racer more consistency and that can be very
important to winning on the race track. Good Luck!
To comment or ask questions e-mail the author:
bergfelt@verizon.net