The cam lift, on a normal MGB camshaft, is about 1/4". This is amplified through the rocker arm ratio (about 1 1/2 to 1) for a total valve lift of about 3/8". The cam guys will tell you that 0.010" wear on a lobe is too great. That works out to 0.014" at the valve. You cannot even begin to see such a slight difference. BUT, if you can see that one valve is opening less than the others, then the lobe is terribly worn. If the valve is an inlet, you will be underpowered at speed; if the valve is an exhaust, you'll burn the valve (and head) soon. It is VERY uncommon to see any difference in the lift simply by watching them bob up and down, with the valve cover off, at a very low idle -- but IF you can see a difference, it's cam time.