Are you tired of squeezing the feeler gauge into the rocker boxes in an effort to adjust the valve? Only to tighten the nut and find that the gap is incorrect. The following procedure can help!
Most British Bikes have 26tpi (threads per inches) cycle thread valve clearance adjusters. This means that rotating the adjuster 26 times would move it one inch. If we approximate this to 25 turns to the inch we see that one full rotation equals 1/25th of an inch, which is 40 thousandths or 0.040 inch. Therefore, one flat of the square head equals ten thousandths or 0.010inch. One-eight of a turn equals five thousands or 0.005 inch and so on.
To adjust your valves using this technique, first, remove the valve covers. Then, rotate the engine in position to adjust the appropriate valve. Loosen the lock nuts on the adjuster. Then, hand tighten the adjuster of the valve until it makes contact with the valve and no more, then back off as much as necessary to give the correct clearance. For example, if the adjustment is suppose to be 0.020, then you need to back off the adjuster 1/2 turn. Tighten the valve nut. Continue with the remaining valves.
This is far easier than trying to manipulate feeler gauges in a restricted space. It is also more accurate because often valve caps have a dent in the top caused by constant battering. A feeler gauge will bridge this gap, thus disguising it and yielding a false reading.
Another problem that can not be over come with this or a feeler gauge is that the adjuster may have flat spots battered on it s surface giving unequal adjustment as you turn it. It goes from too slack to too tight in the space of a fraction of a turn. This is maddening and can only be over come by either setting by feel or by fitting new adjusters.
Give it a try. Adjust your tappets using the above method and then try a feeler gauge. Judge it your self, but beware of short comings!