One of the critical steps in setting valve timing or just in checking the ignition timing on a car you have never worked on before, is to determine the position of TDC and make sure that the mark on the crankshaft pulley lines up with the reference pointer. A special tool called a dial indicator makes this measurement easy; it screws into number one spark plug hole and measures the position of the piston to determine when it has reached the highest point of its travel. This position should occur when the pointer lines up with the mark ‘P’ on the pulley. However, in using this method we have to remember that it may not be very accurate. This is because that, for a given angular change in the position of the crankshaft (say 5 deg) the position of the piston doesn’t change much near TDC whereas it changes a lot half way through the stroke. Thus at TDC, 5 deg rotation of the crank moves the piston about 0.009 inches whereas at the 90 deg position of the crank 5 deg moves the piston 0.150 inches.
Now, how accurately do we need to measure the angular position of the crank and can we do it with the dial gage alone? The width of the reference marks on the cam sprocket and on the Camshaft bearing cap is about a degree of camshaft rotation, similarly one hole on the camshaft sprocket vernier changes the Camshaft position 1.5 deg. So one degree accuracy for the camshaft position is about right and that corresponds to two degrees at the crankshaft. Now the relation between piston travel and angular position of the crank is complicated and different for different engines, but for all four cylinder Alfas you are likely to work on and for the old 2600 six, two degrees crankshaft rotation at TDC corresponds to about 0.0013 inches of piston travel. Since the dial gage can easily be read to 0.0005 the accuracy is just about right. But the message is: don’t get sloppy—read the dial gage with care and repeat the measurement several times. If you are really fussy use a degree wheel or put a piece of tape on the crank pulley with three equally spaced marks about half an inch apart. Measure them accurately and line up the center mark with the ‘P’ on the pulley. The dial gage should now read the same piston travel for the two outside marks; if it doesn’t the reference pointer must be reset.