Return to Technical Article Directory

Return to BRE Home Page

BRE Logo


     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: