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 Cam Timing

(another article adapted from an article that I don't remember where I got it from!)

FINE PRINT: Once again the information presented here is merely meant as an opinion. Anything that you might do, or any actions that might result from using this information is your own blooming fault. There is no way that this information is guaranteed to be 100% correct anyhow.

To avoid lots of swearing and unnecessary damage, check the timing gears slide neatly onto their respective bosses. If tight, first check for any high spots in the gear bores and key-way slots. Clean out using with fine emery cloth, medium Wet 'n' Dry paper, or some such. Remove the Woodruff keys then dress the bosses using abrasive material as mentioned previously. Re-check fitment before re-fitting keys. Check the key-ways and keys too before re-fitting. Get rid of unwanted sticky-out bits on the keys that would inhibit a slide fit. Clean out the key-way and de-burr top edges. You may want to file a slight 'flat' across them to give plenty of clearance to the gear key slot. If the pulley is a slack fit on the key, turn it anticlockwise before nipping the bolt up. ALWAYS fit it like this. It's imperative to fit cam (and followers in the case of solid wall blocks) following manufacturers instructions precisely using a good quality cam lube. 

Sit the block/crank/rods/pistons/front plate/cam and cam retaining plate assembly on something that will allow full rotation of the crank without fouling. The gearbox is ideal, but needs to be stabilized by wedging with a thin sliver of wood or some such. Fit the flywheel retaining bolt and washer to facilitate crank rotation. Use something reasonably long as a lever to turn the crank with - jerky motion is to be avoided for accurate settings. 

For those confused about which end is which of an engine, the FRONT is the water pump end, thus number one piston is nearest the water pump. Worth remembering as most engines are like this. 

When timing cams in, two terms are continually confused - ADVANCED and RETARDED. To explain/clarify her are these terms applied to a common manufacturer recommended 106 degree setting as an example. 

ADVANCED cam timing means that the valve events are occurring BEFORE the recommended point. Your measurement will therefore compute a reading LESS than 106. Generally this increases bottom and mid range outputs, but losses a little peak output. To correct this the cam will need turning counter-clockwise (anti-clockwise). 

RETARDED cam timing means valve events are occurring AFTER the recommended point. Hence measurements and computations will give a value MORE than 106. Generally this increases peak outputs, but will lose bottom and some mid range output. To correct this the cam will need turning clockwise. 

In both the above, I have stated just the CAM needs turning. To achieve this offset keys (available in one-degree increments from 1 to 9 degrees, or use a vernier cam gear (for the big Healeys this is available from Denis Welch Motors in the UK). 

The reason for taking readings either side of full lift as depicted in the methods is to get a true full lift point. A certain amount of 'dwell' exists at the cam lobe peak, so is difficult to ascertain true maximum lift point. The points used could be anywhere between 0.020" to 0.003". The nearer 0.003" the better as some cams have asymmetric lobes. These will cause inaccurate settings to be established if points further from 0.005" where used. 

Determining Cam Timing


REMEMBER: 2 crankshaft degrees = 1 cam degree

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 1 is at the bore top (TDC). Fit timing gears 'dot-to-dot'. Set up DTI with stem resting on number one piston centre with a little preload. 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 0.005" 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 0.005 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.