Equipment required -
Stop plate - mechanical stop plate for setting TDC (see text).
Set of feeler blades
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.
Method -
Step 1
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.
Step 2
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 finalising the mark. Make it more prominent and permanent by
nicking with a small saw blade. I always add white paint.
Step 3
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.
Step 4
Rotate crank until the rockers
on 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.
Step 5
Rotate crank one full
revolution until the pointer and mark are aligned once more. Piston one is now
on its 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.
Step 6
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. 0.010”-0.012” is around a
degree. If you get them within 0.010” that's pretty good, and always bias the
larger gap towards the inlet. After each adjustment of the timing gears, you
need to go back and repeat steps 4, 5 and 6 as altering the cam timing will
alter the big gaps between rocker and valve tip.
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.