By Bob Krapp
Originally published in the Austin-Healey Magazine
When checking individual valve timing on a 100-4, the service manual (pages D4-"Timing" and D26 "Timing Marks") says to set the valve gap at 0.021 inches (21 mil) to observe the opening or closing point relative to the crankshaft angle. Then you must reset the gap to 0.012 inches (12 mil) for running. Did you ever wonder why you were instructed to set the valves twice or did you even notice? Is it really that important?
When I first read the 0.021 setting I thought it was a misprint. I figured it should be 0.012 but changed my mind after reading further.
I was investigating a running problem on a newly rebuilt engine with a reground crankshaft. Discipline prevailed, and I proceeded to check all opening and closing points with a protractor on the crankshaft pulley and a dial indicator on the valve. With the valves set at a 21 mil gap and using a criteria of 2 mils open, I found all valves to be within four degrees of the specified values. The camshaft was exonerated.
Then I got curious and checked all valves at 12 mil gap. Typical results are shown on the graph for No. 2 cylinder exhaust valve opening. Rocker tip motion is measured with a feeler gauge (before valve opens), and valve opening with a dial indicator. The 12 mil gap curve shows a "flat spot," probably due to oil film squeezing out and deflection in the valve train as the valve tries to open against the spring load. However, the design is forgiving. The Healey founding fathers had done something clever, and the valve "catches up" to where it should be rapidly.
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