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To ascertain what type of camshaft you are running (or at least get a feel for it).
To do it fully needs a degree wheel, and dial indicator to measure the timing points. BMC specified their cam timing at a lift of 0.016" of the base circle of the cam lobe. At the valve end this translates to .023" on the std MGB rockers, so set your clearance to this and measure off the opening and closing points from the degree wheel. Using the dial indicator, also measure the peak lift (best done by noting the degree wheel .005 either side of peak and taking an average).
Total duration is 252 deg for the standard cam. As duration increases, low end torque reduces. 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 reasonable uneven and torque noticeably sluggish
under 2000 RPM.
An alternative method which is simpler and gives an approximate feel for the type of camshaft fitted is to measure how much valve lift there is at TDC, using either a dial indicator or by setting the clearance to zero at TDC and measuring the
tappets clearance when back on the base circle. The standard MGB cam has .055" lift at TDC (no 1 inlet). 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. If the lift is significantly less than .055, then it probably indicates retarded cam timing which would result in poor low end torque. Race camshafts will be up to 150" open at TDC, fast road somewhere between that and standard.
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.
Fit an offset (stepped) key in the camshaft, with the offset positioned as follows:
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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. ie the camshaft has been advanced.
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If the measure figure is less than the spec, retard the camshaft by fitting the key the other way around.
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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