Before attempting to adjust valve timing it is necessary to check the manufacturers crankshaft timing marks.
These marks are usually located on the crankshaft pulley and corresponding marks located at the front of the engine eg. the front timing chest on a Ford/Lotus twin cam engine.
Alternatively, the timing marks may appear on the flywheel and have a corresponding pointer located in the bell housing eg. the gearbox bell housing in a Lotus Turbo Esprit.
It is necessary to check these marks because of the manufacturing tolerances that are involved when machining these components.
These small or insignificant tolerances can accumulate and add up to several degrees of error when measured at the crankshaft.

To accurately set or check valve timing, it is necessary to use a protractor bolted firmly to the crank nose and a dial gauge registering motion of an appropriate valve.

But first of all, the protractor needs to be zeroed at T.D.C.
The most accurate way of doing this is to use a modified spark plug, with a fixed probe in place of the electrode.
The fixed probe should be just long enough so that it stops piston motion a small distance before T.D.C.
Rotating the crank slowly backwards then forwards will be enable accurate zeroing between the two points at which the piston is stopped.
After this, all further timing checks must be carried out in the correct direction of rotation to ensure that all valve train slack is taken up in the normal way.
This process is some times referred to as “Crankshaft Indexing”

As an example, a camshaft has been selected with the following characteristics:

Inlet 40 degrees B.T.D.C. - 72 degrees A.B.D.C. Valve lift 0.400 in.
Exhaust 76 degrees B.B.D.C. - 36 degrees A.T.D.C. Valve lift 0.380 in.

Method 1
Set valve clearances to a known figure of 0.010”.
Turn the crank slowly and watch for a 0.005” reading of the valve lift and note the protractor reading.
Continue to turn the crank through the opening and closing phases of lift until the dial gauge returns to the same reading and note the protractor reading again.
Repeat the procedure with the other valve and then convert the protractor readings to their appropriate ‘before and after T.D.C. and B.D.C. status.

This method is not always satisfactory due to the fact that uncontrollable valve train movement takes place.
This is caused by wear in rockers, shafts, tappets and guides etc. This in turn tends to give a slightly distorted indication of start and finish of valve motion.

This method is quick and is recommended for single camshaft engines such as the Ford Cortina or Escort.

Method 2
If the valve lift at a given crank angle is known, say at T.D.C., then the timing can be set accurately by the following procedure. Install the camshaft on the standard timing marks with the valve clearances set to the correct running.
Zero the dial gauge with the chosen valve on its seat.
Now turn the crankshaft to T.D.C. and note the valve lift.
If it is not correct then disconnect the cam drive and rotate the camshaft alone until the correct lift is achieved.
Re-couple the drive using one of the methods discussed below to accommodate the need for a small amount of angular change.
Twin cam engines will need a lift figure for both inlet and exhaust valves.
Always start by adjusting the camshaft that comes first in the driveline and always ensure the drive is held at running tension while readings are being taken.

This method is applicable to asymmetrical camshafts installed in a twin cam engine.

Method 3
This method of setting the camshaft timing is referred to as “Valve Full Lift Position” or “Maximum Opening Point” (commonly abbreviated as MOP) and is used when setting valve timing on symmetrical camshafts.
This is the most accurate method and requires that the full lift position of the inlet and/or the exhaust valve be determined in the following manner.
Referring to example camshaft characteristics above, the inlet timing is:

40 B.T.D.C. - 72 A.B.D.C.

From these figures we can say that the total timing duration is:

40 + 180 + 72 = 292

The full lift of the valve will occur half way through this period, which is 292 divided by 2, or 146,
So, if full lift occurs 146 after the start of the valve motion, and the valve motion itself starts at 40 before T.D.C. then full lift must occur at 146 minus 40 or 106 after T.D.C.
Similarly, the full lift position of the exhaust valve can be calculated to be 110 before T.D.C.

The cam timing can now be set by the following initial procedure described in “Method 2” and then turning the crank to the appropriate position.
In the case of the inlet cam, this should be 106 degrees A.T.D.C. At this point, the drive can be disconnected and the camshaft rotated slightly until the inlet valve is fully open.
Having ascertained the point where the inlet valve is fully open, the drive should then be reconnected.

Do not rely on visual estimation of the dial gauge to decide the actual position of full lift.
The velocity in this area is so low that there is an apparent period of dwell (no movement).
It would be inaccurate to try and guess the centre point of this period.

To be absolutely sure, take a point either side of full lift, say 0.020” before and after, note the protractor readings at these two points, add the two readings together and divide by two.
This will give you the true angular mid-point position.

Determining the two reference points at 0.020” before and after valve full lift should only be carried out by turning the engine in the normal direction of rotation.
The normal direction of rotation for a Lotus engine is clock-wise when viewed from the front.

Do not be tempted to make an angular reading at 0.020” before full lift by turning the engine in an anti-clockwise direction, as errors will be introduced through slack or inherent hysteresis in the valve drive train.
However it is possible to obtain an accurate reading by first turning the engine anti-clockwise, moving sufficiently past the reference point and then rotating the engine clockwise (in the normal direction to take up the valve train slack) to make an angular reading at 0.020”

Having checked and corrected the timing, it will probably be necessary to use an offset dowel; key or vernier sprocket to couple up the drive in the correct position.

Some sophisticated engines are built with an integral vernier cam adjustment.
The Rover 2000 P6 engine is a good example of this.
The cam timing is easily adjusted by changing the spline engagement on the camshaft relative to the driving sprocket.

Summary
At a later date, it may be necessary to repeat the valve timing adjustment for a number of reasons and this may include –

a)    A worn or stretched timing chain discovered during routine servicing. This will result in the valve timing being retarded.
b)    A cylinder head and/or cylinder block having been skimmed. This will move the camshaft to crankshaft centre distances closer together and will result in the valve timing being retarded.
c)    New camshaft(s) being installed. This will certainly mean the valve full lift position will be different to the camshaft(s) originally installed.

The process of “dialling in cams” is probably the most cost efficient way of realising some additional horsepower by correcting manufacturing marks and establishing the correct operating position for the camshaft(s).

WARNING
This procedure needs to be carried out with the upmost of care so as to avoid the possibility of bending valves.