Singer 99 Series, Service Manual

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To Time the Sewing Hook
Remove the oscillating hook slide, presser foot, throat plate, bobbin case,
and feed dog. This will give a clear view of the point of the hook and the needle
for the purpose of timing.
Turn the balance wheel over toward you until the needle bar connecting stud
(G, Fig.6) which has upon it two timing marks, moves down to its lowest point.
When the stud is in this position, the upper mark should be centered on the
top of the finger of the timing gauge (H, Fig 6) which is fitted around the needle
bar bushing. In some cases it may be necessary to raise or lower the timing
gauge to bring the top of its finger in line with the upper mark on the stud
when the stud is at its lowest point.
Then turn the balance wheel over toward you until the lower timing mark on
the needle bar connecting stud is centered on the top of the finger of the timing
gauge (H, Fig. 7) when the needle bar is on its upward stroke. When the needle
bar is in this position, the point of the sewing hook should be directly behind
the center of the needle as shown at L, Fig. 7, if the hook is correctly timed.
In case the sewing hook is not correctly timed, turn the machine back on its
hinges and loosen the oscillating hook clamping screw (O, Fig.8) and turn the
hook until its point is directly behind the center of the needle, then securely
tighten the clamping screw, leaving just enough end play to permit the hook to
turn freely.