by Charles Reichard
a. Degree wheel. Can be any size, Larger is more accurate
b. Dial indicator, .500 or 1 inch (1 inch is usually cheaper)with a magnetic base or suitable bracket for attachment to the block or lifter bore.
c. Lifter, preferably with extension, or tool that holds the indicator and fits in the lifter bore.
d. Method for securing degree wheel to crankshaft. Ratchet adapter is best & easiest to use.
Adapter secured by balancer bolt is almost as good. It should allow wheel to be removed without disturbing the bolt. TDC must be reestablished each time that the wheel is removed.
e. Pointer: can be made up of metal parts or even stiff wire such as a coat hanger.
f. A positive stop to find TDC. Dial indicator can be used in several configurations such as a magnetic base or a U shaped bracket to position indicator over the cylinder.
g. Pen & paper to record your readings
Attach degree wheel to crankshaft and install pointer. Adjust pointer to approximate TDC on the wheel with piston at top of cylinder.
Install indicator to contact the piston near the center to minimize the effect of piston rock at TDC. Rotate crank through TDC in normal direction and 0 indicator at the highest reading. Continue to rotate crank in normal direction of rotation and record the degree wheel reading at .100 indicator reading before TDC. Continue rotating until you are at .100 after TDC. Record your degree wheel reading. TDC will be half way between these readings. You can use a point other than .100 as long as it’s the same point on each side of TDC. Adjust your pointer to read 0 at this point. Top Dead Center is now accurately known. This is a very important step because there is a point a few degrees before and after TDC when the piston moves very little. This method measures the same point on each side of TDC where piston movement is much greater per degree of crank rotation than at TDC. A positive stop such as a strap with a bolt to hit the piston can be used. The crank is rotated until the piston hits the stop and degree wheel reading is recorded. Rotate the crank in the opposite direction until the piston hits the stop and record the reading. Halfway between the readings is true TDC. If the heads are on the engine a stop can be screwed into the spark plug hole to contact the piston. Be sure to remove the rocker arms on that cylinder or you risk bending a valve if it hits the stop when open. Either method is accurate
It is very important that your dial indicator be perpendicular to the camshaft. The easiest way is to use a tool that holds the indicator and fits into the lifter bore. If your indicator is mounted separately be sure it contacts the lifter squarely. A good way to do this is to disassemble a hydraulic lifter and machine a piece of round stock to fit snugly inside the lifter and retain it with loctite. Make it long enough to extend above the top of the block. Make sure the top end is cut squarely to provide a good contact area for the indicator tip.
Next 0 the indicator on the base circle of the cam. The lowest reading is the basecircle.
There should be 0 runout on the basecircle, A reading variation of more than .002 indicates poor quality grinding or poor quality control making the masters If runout is consistent from lobe to lobe and variation is about the same then the problem was probably in the masters. Runout is more critical in hydraulic applications. Many cam grinders say even .005 or .006 runout is ok in mechanical cams. This is a copout for lack of quality control. If there is that much runout how do you know if it is above or below the basecircle design and how can you accurately set the valve lash. You certainly wouldn’t vary the lash .005 or .006 between cylinders.
Now we are ready to check the opening and closing points on #1 intake lobe. Begin by rotating the crank in the normal direction of rotation until the lifter moves up .050. Record your degree wheel reading. Continue in the same direction through maximum lift until you are .050 before the indicator returns to 0. Record your degree wheel reading. They should match the opening and closing points on your cam spec card. It is usually necessary to use an offset key or bushing to make the figures agree exactly. Repeat this procedure for the exhaust lobe. There can be considerable differences between brands of timing sets.
Now is a good time to mention that all degree Wheels are not marked the same way. The readings you are after are degrees before and after TDC and BDC. If your wheel is marked in 90-degree increments from these points you will use the readings you get. Many wheels are marked 180 degrees each side of TDC. You will have to realize that a reading of 160 degrees after TDC is really 20 degrees before BDC. A reading of 140 degrees before TDC is really 40 degrees after BDC.
Sometimes one side of the wheel is marked 0 to 360 degrees. This side is used to check duration of cams. When doing this rotate the crank until the lifter reaches your checking height. Record the reading. Rotate the crank through max lift until it is at the same point on the closing side. Record your reading. The difference is the duration at that tappet lift. .050 is a common, consistent height for lobe comparison. Advertised duration is commonly checked at .004 or .006 for hydraulic cams and .020 for mechanical cams. It is important to realize that not all cam grinder use the same point, In fact some use a different point on the opening side and closing side.
Another faster method of degreeing a cam is the centerline method. Here we are establishing the point of maximum lobe lift in relation to the crankshaft. This method may not exactly agree with the opening and closing figures but it will be close. This is due to the fact that almost all modern cams have an asymmetrical shape and the opening and closing sides are different. It usually isn’t too critical which method you use but don’t use one method for installation and another for changes. Be consistent. When you are looking for that last couple of horsepower on the dyno, use the opening and closing figures paying most attention to the intake closing point. It is the most critical. Verify your changes. Swapping a 6-degree bushing for a 3-degree bushing may not always be as accurate as you think.
With your equipment set up as before, 0 your indicator at maximum lobe lift. Reverse the rotation until the lifter has dropped about .050 before maximum lift. Slowly rotate the engine until the indicator is .020 before max lift and record the reading on the degree wheel. Continue past maximum lift until you are back at the .020-point on the other side of the lobe. If your degree wheel is marked properly you can add the 2 figures together and divide by 2. This will give you the intake centerline.
Popular centerline points are from 102 to 107 degrees after TDC. Other figures outside these are not rare depending on the specific application. Most cams are intended to be advanced and this advance is usually ground into the cam. A cam ground with a 108 lobe separation and advanced 4 degrees will have a intake centerline of 104 and exhaust centerline of 112. Advancing it another 2 degrees will result in a 102 intake centerline and a 114 exhaust centerline. Adding the intake and exhaust centerlines together and dividing by 2 will give you the lobe separation. The same cam installed straight up (with no advance) will have a 108 intake centerline and a 108 exhaust centerline.
Note that straight up refers to lack of advance. It does not mean, “ We lined the timing marks up and installed the gears”. When I ask where the cam is installed and get “straight up” for an answer then I know the cam was probably not degreed in properly.
ATDC after top dead center
BTDC before top dead center
ABDC after bottom dead center
BBDC before bottom dead center
Note that occasionally with short duration street cam, these figures maybe expressed as a negative number. Since most timing cards are preprinted, it is difficult to change these abbreviations. –5 BTDC means 5 degrees ATDC. Most timing programs in software are set up the same way so be sure you enter the figures properly.
Centerline: The position of maximum cam lift in relationship to the crankshaft. There is an intake centerline point and an exhaust centerline. When these numbers are the same the cam has no advance
Lobe Separation: The number of camshaft degrees between the centerlines of the intake lobe and the exhaust lobe.
There is much confusion over the terms centerline and lobe separation. Years ago lobe separation was referred to as centerline. This was probably started by some uninformed or misinformed magazine writer with good intentions. Centerline refers to the relationship of the maximum lift point on the cam lobe to the crankshaft. Lobe separation refers to the relationship of the lobes to each other A cam with 108 lobe separation typically has a 104 intake centerline. That is, it is installed 4 degrees advanced.
For more information on the effects of advancing or retarding your cam, check out “Tuning your cam.”