Russ: Here are the instructions from my F-20 instruction manual (printed 1939). Clearance of .009" is necessary between end of valve levers and valve stems when valves are closed--when engine is warm. To determine when valves are closed: Remove spark plug from No. 1 cylinder. [FRONT] Place thumb over opening and crank until outward pressure can be felt (#1 piston is moving toward upper dead center of compression stroke). Continue cranking until starting crank pin is in horizontal position. Both valves are then closed on compression stroke of #1 cylinder. Check or adjust No. 1 valves. Crank engine 1/2 turn--crank pin horizontal. Check or adjust #3 valves. Crank engine 1/2 turn--crank pin horizontal. Check or adjust #4 valves. Crank engine 1/2 turn--crank pin horizontal. Check or adjust #2 valves. [My note: Firing order is 1-3-4-2]
To adjust valves: Loosen lock nut. Adjust screw in valve lever so gauge will snugly slip between end of valve lever and valve stem. Tighten lock nut and recheck for clearance with gauge.
I presume you've done this before on other engines, and I may be telling you something you already know: the adjustment screw may slip and tighten up the clearance, so be sure to hold the screw while turning the nut down, AND then recheck your clearance. Valve levers too close can hold valves open a little bit during exhaust cycle and cause burning, so having them at the setting called for can be important. A tad loose might not hurt anything.
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