Singer 66 Series Service Manual page 15

The presser bar should be set so that when the presser bar lifter is raised,
there will be a clearance of about 5/16 inch between the presser foot and the
throat plate. When the presser bar is lowered, the presser bar will be parallel
with the feed dog.
In case the presser bar is not correctly set, raise the presser bar lifter, the
loosen the presser bar bracket screw (M, Fig. 7) and raise or lower or turn the
presser bar until it is set in the correct position as instructed above then
securely tighten the screw (M).
To Adjust the Presser Bar