Degreeing Cam By Stock Timing Marks

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Your new camshaft can be the latest, trick profile in the world, but it will not run correctly if it is not phased properly. Cam phasing is usually checked by the procedure known as "degreeing the cam". This month's Tech Topic will describe a simple method for checking one cam lobe's relationship to the crankshaft using the stock timing marks. This will help assure that your cam is "degreed " right !

You will need an assistant, a 15/16" socket, 3"-extension, and breaker bar. You should use a dial indicator with an adjustable magnetic base, however, it is possible to use a dial caliper. You will be measuring the maximum lift position of the #8 exhaust valve.

1 Remove the passenger side rocker cover and all the spark plugs.

2. Set up the dial indicator with the base on the flat surface at the rear (firewall side) of the passenger side head. Position the adjustable base and dial indicator so that it reads the lift of the #8 exhaust valve at the retainer. The actual amount of lift is unimportant to this procedure.

3. Rotate the engine with the socket & wrench on the harmonic balancer bolt until  #8 exhaust is fully open. If you have hydraulic tappets, rotate the engine to just before full #8 exhaust open and wait several minutes for the tappet to "bleed down". Stop rotating the engine just when exhaust is fully open. There will be some dwell at full lift, so the exact position may be a little tricky to determine. Repeating the procedure several times will allow you to zero in.

4. After you have determined full lift, stop exactly there and note the relationship of the timing marks and TDC line on the harmonic balancer. The phasing of the exhaust can be determined from this listing:

Timing Mark                Exhaust Lobe Phasing
TDC                                90  Before TDC
2                                      92
4                                      94
6                                      96
8                                      98
10                                  100
12                                  102
14                                  104
16                                  106
18                                  108
20                                  110
22                                  112
24                                  114
26                                  116
28                                  118
30                                  120

To extend the timing marks on the stock timing cover, mark another line 1" circumferentially clockwise from the TDC mark. This new line is 17 BTDC, so just add 17 to whatever mark it lines up with. Example: the new line is across from the 14 mark on the timing cover, therefore the actual timing is 17 + 14= 31, which corresponds to 121 BTDC exhaust lobe phasing.

5. For your information, here is the exhaust lobe phasing specification for the popular Pontiac and H-0 camshafts.

Camshaft Exhaust Lobe Phasing

HC-01 068HL     116 BTDC       
HC-02 744HL     120
HC-03 041HL     118
HC-55 (9779068) 119
HC-56 (9785744) 118
HC-57 (9794041) 115

6. To calculate the exhaust lobe phasing -given only the valve timing events, use this formula: (opening minus closing plus 180)/2.