[Tony Smith]

It sometimes occurs when installing new valves in conjunction with hardened valve seats. The new seats have a seating surface higher than the old one and the new valve has a thicker head, resulting in the lifter to stem clearance being larger than spec, particularly if the lifters and cam lobes are worn. Naturally, good practice is to cut the seats deep enough first time round, but with cutting seats, measuring stem clearance, grinding stem, remeasuring clearance, lapping seat, remeasuring clearance and possibly grinding once again and doing this 16 times, well, some machinists will take short cuts. Your local corner mechanic who's not familiar with the intricacies of Sidevalves will possibly take too much material off the stem and need to build it up again. It's much simpler to get adjustable tappets, cut the seats, lap seats and adjust ONCE ONLY.