2001 Edition Yamaha F225A and LF225A Outboards Service Manual, Page 115Get this manual
Power unit 15Remove the driven sprockets19Measure the valve shim thickness with micrometer, and then note the measurement20Select the necessary valve shim by calculating its thickness with the following formulaNecessary valve shim thicknessRemoved valve shim thicknessMeasured valve clearance Specified valve clearance Example: If the "Removed valve shim thickness" is 2.10 mm, the "Checked valve clearance" is 0.30 mm and the "Specified valve clearance" is 0.20 mm, then the "Necessary valve shim thickness"2.100.30 0.202.20 mm 21Install the necessary valve shim into the valve lifter22Align the projection (wide one) on the camshaft sprockets to the alignment plate (gold) on the timing chain, and then install the camshafts to the cylinder head with new oil seals