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Camshaft Timing

1) Determining True Crank TDC Position - Head off block

Although the crank pulley will have a mark to show TDC position, this point may not be accurate due to manufacturing tolerances. It is always recommended to calculate the actual position of TDC rather than rely on the mark on the crank pulley. Fit the timing disc to the front of the crankshaft. A pointer for the timing disc can be made from a piece of bent wire secured under a suitable front cover bolt Position a dial gauge to measure the travel of No I piston (see figure). Turn the crank until the piston is at its maximum height and zero the gauge. You will find that there is a dwell period of approx. 10 degrees where the piston is at its maximum height. True TDC position is at the centre of this dwell period. To accurately measure the position of TDC, rotate the crankshaft and stop at a figure just before TDC, for example 0.020" on the dial gauge. Record the figure on the timing disc from the pointer. Now rotate the crankshaft and stop at the same figure (0.020") after TDC. Record the figure on the timing disc again. True TDC is positioned in the middle of these two figures. The TDC position can be calculated by adding the two figures together and dividing by two. Adjust the timing disc so that it reads zero on the pointer at true TDC.

2) Determining True Crank TDC Position - Head on block

If the head has not been removed when changing the cam, it is still possible to measure the true TDC position. The procedure is the same as above but the movement of the piston is recorded by using an extension resting on the piston crown (such as a socket extension or push rod). Access to the piston crown is made through the spark plug hole.

3) Timing in the Camshaft

Rotate the crankshaft clockwise to 90 degrees after TDC. This will make sure all the pistons are half way down the bore. Now position the dial gauge so that it can read the lift of the inlet valve of number 1 cylinder from the top of the valve retainer (see fig. 4). Rotate the cam until the gauge shows that the valve is at full lift. As with the crankshaft, there will be a dwell period where the valve is at full lift. True full lift is at the centre of this dwell period. Roughly position the cam at true lift position. Now rotate the crankshaft clockwise to the full valve lift position (as specified on the camshaft data sheet - this figure is typically from 100 to 120 degrees after TDC), fit the timing belt or chain and set up the tensioner. Now rotate the crank clockwise until the inlet valve of number one cylinder is just off full lift position (such as 0.005" or 0.15 mm). Record the figure on the timing disc from the pointer. Then continue to rotate the crank clockwise until the valve has fully opened and then closed by the same distance as previously used (0.005" or 0.15mm). Read the figure on the timing disc again. The position of full lift is the middle of these two figures. The full lift position can be calculated by adding the two figures together and dividing by two. Adjustments can then be made to the camshaft timing, using an adjustable cam pulley or offset dowels, if this figure does not agree with the one on the data sheet. Check the timing again after adjustments using the same.

Having timed the camshaft, check that there is no piston to valve contact. Minimum clearance is 0.060" (1.5mm). This can only be checked by dummy building the engine with a piece of plasticine placed on the crown of the piston. As the engine is turned, the valves will indent the plasticine. The clearance is then measured as the thickness between the piston crown to the bottom of the valve indent.

Before starting the engine, turn the engine over by hand to ensure that it turns freely. Prime the oil system and check that everything is set to ensure that the engine starts straight away. The engine must not be turned over for any length of time on the starter.

Once started do not allow the engine to idle for the first 20 minutes and keep the revs to a minimum of 2500 rpm. This will ensure adequate lubrication of the cam and followers and reduce the contact force between the cam and follower. If any adjustments need to be made within the first 20 minutes, then shut the engine down. DO NOT ALLOW ENGINE TO IDLE. Please note that new hydraulic lifters may in some cases operate with excessive noise for a few minutes before they are fully charged with oil.




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