Degreeing A Camshaft

THE WHY:

A four-cycle engine’s components work together in a very precise relationship with each other. [ For example: crankshaft to a connecting rod; crankshaft/connecting rod to a piston; valves to a camshaft ] One of the most critical relationships in an engine is the camshaft to piston/connecting rod/crankshaft timing. This instruction manual will show you how to find and adjust this relationship for the best performance results.

THE EQUIPMENT:

The tools you will need are:

1- Large degree wheel 8″ to a 10″ diameter

1- Adjustable magnetic base with a 1 inch travel dial indicator

1 – Short length of wire to bend into a pointer (Coat hanger or electric fence wire)

1 – 1/4 x 2 x 6 Aluminum plate to make a pushrod guide

THE HOW:

The simplest way to degree a camshaft is when the crankshaft assembly is easy to rotate. You can accomplish this with only the # one cylinder piston with rings and connecting rod installed. Having the crankshaft disconnected from the clutch is helpful. Doing this procedure while the engine is out of the chassis will speed up the process. Install the camshaft and the # one cylinder lifters into the block with light oil on the bearings, lifters and # one cylinder camshaft lobes. Install the cam gear(s), idler gear(s), crankshaft gear(s), etc. to line up the camshaft and crankshaft timing marks. Mount the degree wheel centered on the nose of the crankshaft so it clears all other engine components. Rotate the crankshaft until the # one cylinder piston is very close to TDC (Top Dead Center). Rotate the degree wheel on the crankshaft nose until the TDC marking is near the top in a position easy to read. Snug tight the bolt holding the degree wheel to the crankshaft. Bend the pointer wire and fasten it to the block. Adjust the pointer wire so it points to the degree wheel at the TDC mark. When making measurements and calculating midpoints and center-lines, always rotate the engine components in their normal direction of rotation. This will ensure that the measurements reflect what the engine components are actually doing.

To find an approximate piston TDC, set the dial indicator on top of the block so the dial indicator plunger is perpendicular to the piston top. Make sure enough dial indicator plunger travel is remaining for the piston to travel up slightly, but more importantly for the dial indicator plunger to follow the piston down approximately 1/4 inch. Using light thumb pressure on top of the piston to take up bearing clearance, rotate the crankshaft forward in normal rotation. Continue rotation until the piston reaches the end of travel at the top of the block. Set the dial indicator needle to zero. Rotate the crankshaft opposite normal rotation and then forward again to recheck your piston TDC dial indicator reading. Align the pointer so it points clearly and squarely at the degree wheel TDC mark. This is the approximate TDC setting.

To find the exact TDC location, rotate the crankshaft in reverse of normal rotation until the dial indicator drops about .200 inch of travel. Slowly and smoothly rotate the crankshaft in normal engine rotation until the dial indicator reads .100 inch of piston travel before TDC. Record the degree wheel pointer reading. Example: 15 BTDC (Before Top Dead Center)

Continue to rotate the crankshaft in normal rotation until the dial indicator reads .100 inch of piston travel after TDC (the piston came up to TDC and started back down). Record the degree wheel pointer reading. Example: 25 ATDC (After Top Dead Center)

The exact point of piston TDC will be ½ of the distance between the two recorded points. [ Example: 15 BTDC and 25 ATDC ] To find this midpoint, add the distance between 15 BTDC and 25 ATDC. The distance between 15 BTDC and TDC is 15 degrees. The distance between TDC and 25 ATDC is 25 degrees. Example: 15 + 25 = 40

This is the number of crankshaft degrees between the two recorded example points. Dividing this number by two (2) finds the midpoint. Example: 40  2 = 20

To locate this calculated point, rotate the crankshaft in reverse rotation past your first recorded number. [ Example: 15 BTDC ] Rotate the crankshaft in normal rotation to your first recorded point. [ Example: 15 BTDC ] Continue in normal rotation another 20 crankshaft degrees (½ of the distance between the two example points). The wire pointer will be at 5 ATDC on the degree wheel. This is the example’s true TDC. Without rotating or moving the crankshaft, loosen up the bolt holding the degree wheel and rotate the degree wheel until the wire pointer points precisely at the TDC mark. Re-tighten the bolt holding the degree wheel to the crankshaft. As an option, realign the pointer to point precisely at the degree wheel TDC mark. Either method is OK; use whichever is easier.

Repeat the TDC operation to confirm the accuracy of your measurements. Accuracy counts, so be sure to take careful measurements and smoothly rotate the crankshaft to get exact dial indicator readings. If you overshoot your intended dial indicator reading, back up and carefully approach the numbers again. The numbers given here are only examples. Your recorded numbers will be different, but the math operations work the same way.

The next step in this process is the camshaft timing. What you want to find is the precise midpoint or center-line of the # one cylinder intake lobe and where this center-line is in relationship to the crankshaft and piston TDC. To do this, you need a procedure to measure camshaft lobe lift. The following pushrod guide fabrication is used primarily for inline engines, but can be used for others if the camshaft lifters are inaccessible to the dial indicator. If the lifter is accessible, the dial indicator can be setup to read directly from the lifter. You will use the aluminum stock to make a pushrod guide, if required. Measure the pushrod diameter carefully and drill a hole .005 to .010 inch larger in one end of the aluminum plate. Install a pushrod into the camshaft’s # one cylinder intake lifter location. Find a bolt hole in the block deck close to the pushrod and measure the distance between the pushrod and the bolt hole. It is essential that you try to place the pushrod as close as possible to the OEM location to simulate how it would actually move. Layout this measured distance on the aluminum plate from the previously drilled pushrod hole. Accurately drill a hole in the aluminum plate large enough to insert a bolt through the plate into the block deck. Bolt the aluminum plate pushrod guide plate to the block with the pushrod installed in the camshaft’s # one cylinder intake lifter. Make sure the pushrod is free to move up and down in the aluminum plate without binding or wobbling. Adjust the pushrod hole diameter if necessary to eliminate binding. Install the dial indicator with the indicator plunger parallel with and straight into the center of the pushrod end. Carefully align the dial indicator so the pushrod is perfectly in line and free to rotate around the dial indicator plunger.

The next step will be measuring the camshaft lobe duration at .050 inch lifter rise on the opening ramp to .050 inch of lifter rise on the closing ramp. Rotate the crankshaft assembly in normal rotation to find the camshaft base circle. The base circle is the lowest point of dial indicator travel measured from the pushrod travel. Zero the dial indicator and continue to rotate the crankshaft in normal rotation until the dial indicator reads .050 inch of lifter rise. This is the camshaft’s intake lobe opening ramp. Record the degree wheel pointer reading.

Example: 20 BTDC

Monitoring the dial indicator revolution counter, continue normal crankshaft rotation until the lifter has reached maximum lift. The dial indicator will display the total camshaft lift, record this reading to compare with the camshaft specification sheet. Resume crankshaft rotation until the lifter is starting to drop. This is the camshaft’s intake lobe closing ramp. You want the camshaft closing reading at .050 inch before total lifter drop (the dial indicator will read .050 inch before zero). Record the degree wheel pointer reading.

Example: 40 ABDC (After Bottom Dead Center).

The exact midpoint or center-line of the intake camshaft lobe will be ½ of the distance between the two recorded degree wheel readings. [ Example: 20 BTDC and 40 ABDC ] To find this midpoint, add the distance between 20 BTDC and 40 ABDC. The distance between 20 BTDC and TDC is 20 degrees. The distance between TDC and BDC is 180 degrees (the number of degrees in half a circle). The distance between BDC and 40 ABDC is 40 degrees. Add up all the distances in order: Example: 20 + 180 + 40 = 240 degrees.

This is the number of crankshaft degrees between the two recorded example points. Dividing this number by two (2) finds the midpoint. Example: 240  2 = 120 degrees

To locate this calculated point, rotate the crankshaft in normal rotation to your first recorded point. [ Example: 20 BTDC ]

Continue in normal crankshaft rotation an additional 120 degrees (½ of the distance between the two example points). This example will give a crankshaft degree wheel pointer reading of 100 ATDC. This is the camshaft’s # one cylinder intake lobe center-line in relationship to the crankshaft and # one cylinder piston.

You may have noticed that the checking height of the camshaft at .050 lifter rise (from our example) is an industry standard for comparing camshaft specifications. The distance between your previously recorded measurements will give the camshaft lobe duration at .050 lifter rise. [ Example: 20 + 180 + 40 = 240 degrees ]

The camshaft duration numbers always refer to degrees of crankshaft rotation.

Repeat the camshaft center-line operation to confirm the accuracy of your measurements. Accuracy counts, so be sure to take careful measurements and smoothly rotate the crankshaft to get exact dial indicator readings. If you overshoot your intended dial indicator reading, back up and carefully approach the numbers again. The numbers given here are only examples. Your recorded numbers will be different, but the math operations work the same way.

You may need to correct the camshaft intake center-line from the measured point to the specification given on your camshaft information sheet. This can be accomplished by moving the camshaft with offset keys, offset bushings, or by moving a tooth on the cam gear. The procedure for calculating the camshaft timing change by moving a tooth on the camshaft gear follows:

1. Count the number of camshaft gear teeth [ Example: 60 teeth ]

2. Divide 360 (number of degrees in a circle) by the tooth count of the camshaft gear.

[ Example: 360  60 = 6 degrees ] The result from the division will be the number of degrees change in camshaft timing by moving one tooth on the camshaft gear. An alternative option is to use a multiple keyway crankshaft gear. Which choice works the best depends on how far the camshaft timing needs to change.

Moving the camshaft further ahead in normal camshaft rotation will advance the camshaft and moving the camshaft opposite normal camshaft rotation will retard the camshaft in relationship to the crankshaft. If you move only the camshaft gear, the change to the camshaft center-line timing will be equal to the change in degrees of the camshaft gear.

Example: If you advance the camshaft gear four (4) degrees, the camshaft center-line timing will advance four (4) degrees.

If you move only the crankshaft gear, the change to the camshaft center-line timing will be ½ of the change in degrees of the crankshaft gear (remember the camshaft runs at ½ crankshaft speed).

Example: If you advance the crankshaft gear four (4) degrees, the camshaft center-line timing to the crankshaft will advance only two (2) degrees.

Install the camshaft to the specifications (as close as possible) given on your camshaft information sheet. Setting the camshaft intake center-line to within one (1) degree accuracy will give the best engine performance based on the information given when ordering a camshaft. If the camshaft timing is changed, follow this guideline: advancing the camshaft center-line results in a power-band at a lower RPM, retarding the camshaft center-line results in a power-band at a higher RPM. Changing the camshaft center-line will not generally increase the width of the power-band in RPMs, but will shift the power-band up or down the RPM scale. If your application requires a different width power-band, you may need a different camshaft grind. Make sure you understand the resulting effect before making any camshaft timing changes.

The valve to piston clearance is an important issue when installing a performance camshaft. It is critical to piston and valve train life that adequate clearance is provided. A rule of thumb for valve/piston clearance is, .080 inch intake and .100 inch exhaust. This should be checked by the individual assembling the engine.

While the camshaft is opening the intake valve, the piston is approaching TDC (start of an intake event). The intake valve/piston clearance will typically be the smallest after piston TDC. Advancing the camshaft intake lobe center-line will open the intake valve earlier, relative to the piston position, and will advance the exhaust lobe center-line The lobe separation (number of camshaft degrees between intake and exhaust center-lines) is a fixed parameter. The exhaust valve is starting to close as the piston approaches TDC (end of an exhaust event). Exhaust valve/piston clearance will typically be the smallest before piston TDC.

This instruction manual is only a guideline. Its purpose is to help the installer determine the relationship between camshaft and crankshaft position. For more information on the effects of camshaft timing, valve lift and duration, lobe separation and overlap, rocker arm ratios and valve lashes, consult publications available from camshaft vendors. The installer accepts all responsibility for the camshaft installation and accuracy of degreeing the camshaft center-line to the crankshaft.