CAM TIMING - Mechanical Method

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Terminology - TDC - Top Dead Centre, when piston is at highest point in bore BTDC - Before Top Dead Centre, before piston reaches highest point in bore ATDC - After Top Dead Centre, after piston has reached highest point in bore

Accurate piston position indication is the first goal. You need to know exactly when piston one is at TDC. Without engineering measuring equipment a ‘stop plate’ is required.

This you will either have to manufacture personally, or get one made. You need a piece of metal plate long enough to span the bore plus a couple of the head stud holes on either side - about 4.750" long by 1.375" wide will do the trick. It also needs to be thick enough not to deflect under light pressure. Drill a 3/8"/10mm hole at each end where the stud holes are and one around the middle over the piston centre. Put a bolt in the central one, head down with a couple of washers between it and the plate. Secure it with a nut on top. That’s your stop plate.

The pointer is worth making permanent as it makes a super-accurate ignition timing mark, so use a strip of metal or really stout wire. The pointer needs to be as close as possible to the pulley edge to avoid parallax viewing error, and where it can be seen with the engine in the car.

With the timing gears fitted 'dot-to-dot' and the crank pulley you're going to use bolted in place, position piston one half-way down the bore. Fix the stop plate across the bore, securing with 3/8"UNC bolts into the stud holes, stop bolt head over the piston centre. Rotate the crank forwards carefully until the piston is up against the stop bolt. Very carefully lightly mark the pulley exactly opposite the pointer. Now rotate the crank backwards all the way until the piston comes up against the stop. Again, carefully mark the pulley as before. Time taken here ensures the accuracy of the result.

Using a rule of some description, measure between these two marks and make a third EXACTLY half way. This is the datum point for your timing, so check and re-check, and be satisfied before finalizing the mark. Make it more prominent and permanent by nicking with a small saw blade. I always add white paint.

Assemble the cylinder head, rocker gear, etc. as you are going to run the engine. Torque it all down, set the valve clearances as suggested by the manufacturer, but do not fit the rocker cover.

Rotate crank until the rockers one cylinder one are holding both valves open a little. This is the overlap period and how you are going to set the cam timing. Look at the pointer and pulley. Rotate crank to set them so they are directly opposite each other. From this you know piston one is at TDC. Undo the rocker adjuster lock nuts and unscrew the adjusters clear of the valve tips. The valves are now shut. Screw the adjusters down until they JUST touch the valve tips then nip the lock nuts up.

Rotate crank one full revolution until the pointer and mark are aligned once more. Piston one is now on it’s compression/firing stroke. The large gaps showing between the rockers and valve tips represent the amount the valves were open during the overlap period - less the valve clearance. Measure these gaps as accurately as possible using feeler gauge blades stacked up. Record the total value for each valve. From this we can measure the cam timing.

The idea here is to get these gaps identical, the split overlap will then be equal and cam timing correct. A larger gap on the inlet valve means the timing is advanced, larger on the exhaust it is retarded. These details tell us which way the cam needs moving to achieve status quo. As with the first method, alterations will depend on the cam drive used.

This method initially restricts timing to the manufacturers recommendations. However, with patience and experimentation on a particular engine build a picture of how many thousandths of an inch represents each degree, facilitating alterations at a later date if required.

Article Date: Aug 31, 2000
Car Accociations: MINI,MORRIS,SPRITE,
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