Measuring & setting up cam timing

 In Engine - Modified, MGA-MGB, Midget

To ascertain what type of camshaft you are running  (or at least get a feel for it)

Camshafts can be identified by the opening and closing points of the valves, in conjunction with the amount of lift that the cam has.  The actual valve opening and closing points are in an area of the cam action known (surprise surprise) as the opening or closing ramps (resp) or just ramps.  This area is one where the cam is lifting very slowly so as to avoid hammering the valves, but the slow action makes precise measuring of the opening and closing points relatively vague.  For this reason, cam timings are usually specified at a nominal “timing clearance” which at a lift clear of the ramps and which is significantly greater than the normal tappet running clearance.

A common and popular timing clearance for after market camshafts is 0.050″.  However, as BMC specified their cam timing at a lift of 0.016″ off the base circle of the cam lobe, that is the relavant figure to use for MG engines. At the valve end this translates to .023″ on the std B series rockers, .021″ on std A series rockers.

Doing this job fully is a fairly involved job, for which you will need a degree wheel and dial indicator.  However, it is possible to get quite good results by using feeeler gauges and  vernier (or digital) calipers instead of a dial indicator if you don’t have one.

Feeler guage method:

Valve timing:  Select a thin feeler blade, such as 0.003″.  Add the relevant timing clearance at the valve (for example, 0.021″ A series) to the feeler thickness and set the tappet clearance of the lobe being measures to this figure.  Insert the feeler and detect when the engine just nips (valve opening) or releases (valve closing) the feeler.  Measure off these opening and closing points from the degree wheel.

Important:  Don’t forget to re-set the tappets to normal running clearance afterwards!

Cam lift: the best place to take this measurement is at the pushrod, as the rocker ratio will increase the actual lift at the valve and it’s really the cam lift you need to identify the cam.  Measure the total lift from the back of the cam – you can tell this when its opposite number (“rule of 9”) is fully open.

Dial indicator method:

I prefer to  measure off the top of the pushrod, which means releasing the tappet adjustment completely and pushing the rocker to one side (you may have to push the valve down to achieve this.)   Zero the gauge on the back of the cam (when the opposite valve is fully open) and note the degree wheel at the timing lift of 0.016″.   The amount of full lift is of course a direct measurement off the gauge.  With this method you can also measure the position of full lift, by noting the degree wheel 0.005″ either side of the peak and taking an average.

So, what do these figures tell us?

The characteristics that affect a camshaft’s performance are the duration, lift and overlap.  Long duration and overlap figures make a camshaft perform well at higher speeds but lumpily at idle and poor torque at low speeds.  The duration can be calculated by adding 180 + opening degrees before TDC + closing degrees after BDC.  (This is for the inlet lobe, exhaust lobe is similar but of course TDC & BDC are reversed).

Total duration is 252 deg for the standard MGA/B/C cam, 230 deg inlet and 252 deg exhaust on the Midget camshafts, 252 inlet and 268 exhaust on the Metro.   Race camshaft durations of over 300deg duration will be very sluggish at low revs. Fast road camshafts typically have durations from 265 – 290 deg, the more duration the lazier it will be at lower engine speeds (under 2000 – 2500 RPM) At 265 deg you’d expect torque not too dissimilar to the standard cam, (depending on how it is timed in) and idle would be still reasonably smooth. At 290 deg, idle will be reasonably uneven and torque noticeably sluggish under 2000 RPM.

Theoretically too much lift can cause loss of low speed performance but in practice in the BMC engines we deal with there is no such thing as too much lift, as they get better and better the more lift they get. The downside of extra lift can be shorter valve gear life for very high lift cams, but there is certainly a lot of scope to increase the lift over standard camshafts without significant extra wear.

Keeping things simple

An quicker alternative method to get an approximate feel for the type of camshaft fitted is to measure how much valve lift there is at TDC.   This can be done using either a dial indicator or by setting the clearance to zero at TDC and measuring the tappet’s clearance when back on the base circle.

The standard MGA/B/C cam gives 0.055″ valve lift at TDC (no 1 inlet). the standard Midget cam gives between 0.025″ and 0.030″ valve lift at TDC.  (If the camshaft is incorrectly timed or the chain is worn, that won’t be the case of course, so this is a bit of a limitation.)  Race camshafts will be up to 150″ open at TDC, fast road somewhere between that and standard.   If the valve lift is significantly less than .055, then it probably indicates retarded cam timing which would result in poor low end torque.

To fit a replacement camshaft to the correct timing:

Using a degree wheel, find the position of peak lift of no. one inlet as described above.

Calculate the difference between this figure and that specified for camshaft you are fitting. (If it is not specified, assume it to be halfway between the opening and closing points.) This is the amount (in degrees) you need to offset the camshaft relative to the timing gear.  This can be done by fitting a vernier timing gear, or less expensively by using an offset (stepped) key.

When fitting an offset key in the camshaft, position the offset as follows:

  • If the measured figure is greater than the spec, fit the key so the part which protrudes is offset in the anticlockwise direction (viewed from the front). This rotates the camshaft clockwise relative to the gear, thus making the cam lobe engage a little earlier relative to the crankshaft. i.e. the camshaft has been advanced.
  • If the measure figure is less than the spec, retard the camshaft by fitting the key the other way around.
  • And ALWAYS recheck after you have fitted the offset key in case it got muddled and the wrong way round along the way.

Offset keys are available ready made, or you can make your own if you remember that 1 (crankshaft) degree corresponds to .004” offset on the A and B series cam keys.

Finally, assume nothing! Check the timing of the reassembled timing gear just in case! It’s much cheaper than bent valves & damaged pistons!

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