Learning Center
Plans & pricing Sign in
Sign Out



									                                CAM TIMING
Several people have asked me for information related to cam timing of the 2.3 SOHC
engine as fitted to our race Viva GT so here I hope is some useful information. Please
note the basic timing principal described covers all of this range of engines from 1.6
to a full race spec 2.6 and is an industry standard for most similar single cam engines.

Do you need to check or reset the timing?

If the engine is to be fitted with a modified cam or the block, head or cam box has
been machined then it is certainly possibly the cam timing may not be correct when
the engine is reassembled.

As this type of engine is not fitted with an adjustable cam pulley as standard a suitable
adjustable pulley will have to be sourced. We are actually using a modified standard
pulley fitted with an offset dowel centre hub.

The first job is to calculate the actual position of TDC, as the production crank pulley
mark is not necessarily that accurate. With the head removed from the engine,
position the dial gauge to measure the travel of No1 piston (Fig 1). Fit the timing disc
to the front of the crankshaft, a simple pointer for the disc can be fabricated from wire
and bolted to the front of the block (Fig 2). Turn the crank until the piston is at its
maximum height then zero the dial gauge. You will find that there is approximately
10 degrees of crank movement where the piston is at is max height (dwell period).
True TDC position is at the centre of this dwell period. To calculate the actual
position of TDC rotate the crank to just before TDC giving a reading of lets say
0.020” on the dial gauge. Record the angle on the timing disc shown by the pointer.
Now rotate the crank in the opposite direction stopping after TDC at 0.020” on the
dial gauge. Record the angle from the timing disc a second time. The true TDC
position is the centre point of these two angles. Rotate the crank to this centre point
and then adjust the timing disc so the pointer is set to zero. This is a good time to
check and remark the crank pulley. I tend to repeat this exercise for confirmation.

Fig 1                                              Fig 2

It is still possible to check true TDC position if the head is fitted but more difficult as
piston movement has to be measured using an extension on the dial gauge through the
spark plug hole.
With the head fitted but before fitting the cam and housing assembly, rotate the
crankshaft to 90 degrees after TDC. This ensures that the pistons are half way down
the bores preventing any contact with the valves when the cam box assembly is fitted.
After fitting the cam box position the dial gauge so it will read the lift of the inlet
valve of cylinder 1 from the top of the valve spring cap. Rotate the cam until the dial
gauge shows the valve is at full lift (valve fully open). As with the crankshaft there is
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 and zero the gauge. Now rotate
the crankshaft to the specified full valve lift position for the cam that is fitted, this
figure may vary by several degrees depending on the cam choice (we are using a
GT5X cam that has a timing of 107.5 degrees after TDC). Fit the timing belt and set
the tensioner.

Now rotate the crank back (opposite direction to normal rotation) about 20 degrees
and then forward (normal direction of rotation) this will make sure the timing belt is
tensioned as it would be under normal running conditions. Slowly rotate crank until
number one inlet valve is just off full lift position with a dial gauge reading of about
0.005”. Record the angle on the timing disc shown by the pointer. Then continue to
rotate the crank forward until the valve has fully opened and then closed by the same
distance as previously used ( 0.005” ). Record the angle on the timing disc again. The
actual position of full lift is the middle of these two figures. If this figure is not the
same as specified by the cam manufactures (In our case 107.5 degrees) then
adjustment is required. After any adjustment re-check the timing again using the same

Once the timing is correct it is a good idea to re-mark the cam pulley. With the engine
set to the standard timing check position, crank at TDC and cylinder one valves
closed, place a straight edge between centre line of cam and auxiliary shaft and re-
mark edge of cam pulley in line with the straight edge (see fig 3-4)

Fig 3                                               Fig 4

Straight edge between centre line of shafts        New mark on cam pulley (orange)

To top