Cam timing - conventional method using DTI
Equipment required - one 0.500” minimum capacity DTI with magnetic stand, 360-degree protractor and a piece of reasonably thick gauge wire to use as a pointer.
DTI - Dial Test Indicator/dial gauge
Protractor - degree wheel
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.
Rotate crank so piston one is at the bore top (TDC). Fit timing gears 'dot-to-dot'. If using a chain tensioner, make sure you fit this. Whichever gears/chain set-up you are using, with the gears/chain fitted, rotate the crankshaft in the direction of running rotation (clockwise viewed from the coolant pump end) a small amount to take up slack in the keyways and chain/gears. Fit the cam gear retaining nut and pinch up tight. Set up DTI with stem resting on number one piston center with a little pre-load. Rotate crank backwards slightly (anti-clockwise looking at the front), then gently forwards observing the DTI, watching where the needle stops before descending. This is TDC. Repeat this procedure, stopping when the needle reaches its zenith. Zero the dial scale, and repeat until satisfied the needle shows zero on the scale.
Fit the protractor to the crank using the pulley bolt. Tighten so that you can just move the protractor. Fashion a pointer from the wire, held to the block by a water pump bolt and pointing at the scale round the protractor perimeter. Move the protractor to align pointer and zero, nip the bolt up to hold firm. Go back and repeat the last paragraph to ensure pointer is at zero on the protractor when the DTI indicates TDC.
Not essential, but to be super-accurate, reset the DTI - piston at TDC - with around 0.300" pre-load showing. Rotate crank backwards until needle on DTI is free of the piston then rotate forward until needle shows 0.200" BTDC. Note the degree reading on the protractor then continue rotating crank until DTI shows 0.200" ATDC. Note the degree reading. They should be the same although obviously either side of zero (TDC). The actual figure isn't important. If not, move the pointer in the relevant direction to correct this. Re-check by doing the 0.200" BTDC and ATDC again. Do it until the readings show exactly the same.
Pop a push rod down onto number one inlet lobe - second push-rod hole along from front. On small-bore engines make sure you fit the cam follower first! Reposition the DTI on top of the push rod, again with a little pre-load. Rotate the crank forwards and observe the DTI. The needle will swing round a couple of times, slow down, then reach peak reading (maximum lift) before falling away again. Zero DTI at this peak point. Rotate crank backwards and forwards a couple of times slowly until happy that maximum lift is showing with the needle at zero.
Rotate crank backwards until the needle passes the 0.025” mark. Stop and rotate it forwards very slowly, stopping when the needle reaches the 0.010" mark. Note the reading of degrees indicated by the pointer on the protractor. Continue rotating the crank forwards until the needle zeros and returns to the 0.010” mark again. Note the new reading from the protractor. Add these two figures together, divide by two, the result being where your cam is timed in at.
IMPORTANT NOTE –
Setting the cam timing at the push road does not give the same timing at the inlet valve. It is the timing at the inlet valve that is essential to get correct. From years and years of experience timing at the push rod, once the head and valve train is all bolted down in place, is retarded by 2 to 3 degrees depending on the set-up. So you need to allow for this when timing the cam in at the push rod.
The idea is to get this to where manufacturers recommend your particular cam be set to give optimum performance. If your reading differs, corrective measures will have to be taken. How this is done depends on your choice of cam drive. It should be noted that the cam manufacturers rarely do any real development work and testing to see what cam timing actually delivers the optimum performance for any engine type – hence you will find the cam timing they recommend tends to coincide with the LCA (Lobe Centre Angle), which gives 'split overlap' (inlet and exhaust valves open the same amount with the piston at TDC). The vast majority of A-series engines I have dyno tested have given best results with the cam timing advanced 2 to 3 degrees.