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Equal Lift on Overlap - the other cam timing method

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I've often advocated ELOO as an easy way to either check your cam timing, or to do a proper install of the cam.      I wrote it up for TRaction last year, and hope it would be useful if people can refer to it here too.        

“Equal lift on overlap”. A simple method of camshaft timing.

There are two conventional methods of setting a Triumph’s camshaft timing.  One way is to use the markings that Triumph provided on the sprockets.   Another uses a magic number given you by the cam maker, to advance the crank by that number of degrees after Top Dead Centre (TDC), set the No.1 Inlet cam to maximum lift and fit the timing chain.  Simples!  Your cam timing is set!   Fine, if you have markings on the sprockets, or know TDC and the magic number, but if you don’t then you are stuck.

That method may be necessary with exotic cams but most engines, including Triumph engines, have ‘symmetrical’ camshafts, with the inlet and exhaust cam lobes that are the same shape and set equally about the shaft.    With such an engine there is no need to know the grinder’s magic angle, if you use the method known as “Equal Lift On Overlap”.

What’s “Overlap”?   The four stroke cycle goes Intake, Compression, Ignition, Exhaust and back to Intake again.   At the end of the Exhaust phase, the piston reaches Top Dead Centre and begins to go back down again, to suck in more fuel and air.  As the piston approaches TDC and the end of a four stroke cycle, the exhaust valve will be closing, but the inlet valve will be opening too.   The valves are said to overlap, with the exhaust nearly closed and the inlet just opening, and at TDC they should be equally lifted by the cams.

So if you set the crank at TDC, and the camshaft at “equal lift” in this overlap part of the cycle, then join them by the cam chain, they will be correctly timed, for all parts of the cycle and for all cylinders. This sketch shows the position of both inlet and exhaust cams at Overlap.




Usually this procedure is needed when rebuilding an engine, so let’s look at the method without the cylinder head, but with the crank and cam shafts installed.   First, you need to find TDC.

Top Dead Centre is the point in the rotation of the crank where the con rod is exactly vertical and the piston is at its highest point.  The convention is that TDC on No.1 cylinder defines that position.   You can find it with a dial gauge by measuring the movement of the piston, but this is difficult as TDC is where the piston slows to a stop, then reverses.   There will be a range of several degrees of crank rotation in which the piston moves so little that even a dial gauge shows no movement.  TDC is somewhere in that range, but where?   A better method is to use a piston stop.

This is a homemade device to stop the piston at some point as it rises up the bore.   I use a length of square tube, angle iron would do, drilled to go over the head studs, with a bolt in the middle that will stop the piston about half way up the stroke. 


If the head studs are not in place, then a couple of bolts, hand tight will secure the stop, or else pop some tube over the bolts, so that you can use the normal head nuts.   A similar DiY device can be made from an old spark plug, for use when the cylinder head is in place.  Knock out the porcelain, drill and thread the body for a length of 10mm threaded rod.    Put it in the head instead of a plug, adjust the depth of the stop and away you go, as below.



Fit the timing disc to the crank.   Some large washers will help grip the wheel so that it will only turn with the crank.   Arrange a length of wire so that it acts as a pointer, as close as possible to the disc.   It’s some way from the front face of the block, so a pointer makes it easier to read the correct angle.   Fit the piston stop, with the piston at the bottom of the stroke.   Now turn the crank until piston meets stop, and note the angle indicated by your pointer.  Turn the crank backwards the other way, until it meets the stop again and note that angle. TDC is exactly halfway between those two angles.   Here’s how:

Turning the crank clockwise, it reaches the stop at 100 degrees – OK, I set it there for simplicity!


And when turned anticlockwise, it is stopped at 31degrees. 


I’ve drawn the arc through which the crank was turned to get there.  You can see that TDC is going to be half way between those points, in the unmarked arc between 31 and 100 degrees.    That arc is 131 degrees, half is 65.5 degrees, starting from either end.


From 100degrees, 65.5 degrees back is 34.5 degrees.     From the other end, moving forwards 65.5 degrees from 31 takes us across zero (marked TOP – ignore it for the moment!) also to 34.5 degrees.   So TDC is at 34.5 degrees. Take off the piston stop; rotate the crank to that figure and you have it at TDC.  Without moving the crank, loosen the timing wheel and realign the TOP, or Zero mark with your pointer.

Now set the cam shaft.  Following convention, we now have TDC for No.1 cylinder   In both four and six-cylinder Triumph engines, the pistons move in pairs, one firing at round about TDC  (+/- advance or retard) as the other is at the cycle end and the beginning of the next. So if No.1 is at TDC on the firing stoke, No.6 is at TDC in between cycles and its valves should be overlapping.  If we set No.6’s valves to overlap, and with equal lift, the cam timing for all cylinders will be correct.

     For this you need to be able to measure how much the cams have raised either the intake or exhaust valve cam followers.   There is no head on the block at this stage, so no push rods, rockers or valve gear, but that makes it less complicated.     Most cheap Vernier calipers include a depth probe but proper micrometer depth gauges can be bought on eBay for not a lot of money.  Owning such a lovely instrument can make you feel like a real engineer! 



But the instrument for this purpose is the Dial Test Indicator (DTI or dial gauge), that shows any movement of its probe as rotation of a needle on the dial.  Mounted on a stand with a magnetic base two of these make comparing the movement of the two valves very easy.


In the absence of pushrods, the DTI probe is too short to reach the cam, or even the cam follower, so a pair of wooden dowels, wrapped with insulating tape so that they run smoothly in the follower bore, are useful.





Fit the cam shaft, insert the cam followers for cylinder No.6, and fit the timing wheel and a pointer, which will help you to know where you are.   If you are using a depth gauge, measure the height of the cams, making sure that the probe always goes to the bottom of the cup in the follower.   Start with the follower as deep as it will go and then note how the exhaust cam rises, falls, and as it approaches the original depth, the intake starts to rise, in overlap.  Adjust the camshaft to find where the cams are at equal height in the overlap.   This is the tedious bit, because as one goes up the other goes down, and you will have to make many adjustments of the camshaft rotation, following the rotation on the wheel and pointer.   If you are able to use two DTIs at once it makes it so much easier, as you can see the movement of both at once as rotation on the dials.     Adjust the camshaft until the two cams are at equal heights.  That is the cam setting, so note that position and don’t move the camshaft again!

Now you need to fit the timing sprockets with the chain.  This is where the two DTIs are again useful, as once the chain is in position, you can rotate the crank and camshaft together and see that TDC coincides with Equal Lift On Overlap.


The Triumph cam sprocket.

In fitting the sprockets, sometimes you will find that you have to move the cam shaft to do so.   The cam sprocket has 42 teeth, each of which covers 8.5 degrees of the circumference, so that you might have to move the cam as much as half that to get the sprocket bolts aligned.    But Triumph designed the cam sprocket so that you can get much closer!

  There are four holes for the two bolts that fix the sprocket to the camshaft, and each pair is offset to the other, in a slightly different relation to the teeth.   From one position, turn the sprocket through 90 degrees to engage the second pair of holes and the teeth will be aligned half a tooth differently.   If that is too much, turn the sprocket over, and use the same holes to get a quarter tooth.  If you need more, turn it over and through 90 degrees for three-quarters of a tooth!  Thus you can get to within a quarter of a tooth of perfection, or only a tiny bit more than one degree!  If you require more accuracy and have deep pockets, Vernier sprockets are available, that allow infinite adjustment.


ELOO is not an eccentric method of cam timing, in fact if you have unmarked sprockets, it is the method recommended by the official Standard-Triumph workshop manual. Give it a try when you next ned to time your camshaft!


 Hope that helps!

Edited by JohnD

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Nice write up John, thanks. :biggrin:

Just one world of caution - cannot be used with asymmetric cam profiles (asymmetric lobes or different timings for inlet and exhaust).  Fairly rare in Triumph applications, but not unknown.  I have one in my Vitesse.......


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Thank you, Nick!

All standard Triumph cams are symmetrical, so no prob there.    But ask your can supplier if unsure.


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