SINGER 1725 - Manual (Page 32)

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Threading the upper thread________
This is a simple operation but it is important to carry out
correctly as by not doing so several sewing problems could
result.
Start by raising the needle to its highest point (1), and also
raise the presser foot to release the tension discs. (2)
Note: For safety, it is strongly suggested you turn off the
power before threading.
Lift up the spool pin. Place the spool of thread on the holder
with the thread coming off the spool as shown. For small
thread spools, place small side of spool holder next to spool.(3)
Draw thread from spool through the upper thread guide. (4)
Guide thread around thread guide (5) pulling thread through
pre-tension spring as illustrated.
Thread tension module by leading thread down right channel
and up left channel (6). During this process it is helpful to
hold the thread between the spool and thread guide (4).
At the top of this movement pass thread from right to left
through the slotted eye of the take-up lever and then
downwards again. (7)
Now pass thread behind the thin wire needle clamp guide (8)
and then down to the needle which should be threaded from
front to back. Pull about 6-8 inches of thread to the rear
beyond the needle eye. Trim thread to length with built in
thread cutter. (9)
Note:
If your sewing machine is equipped with the factory
installed optional automatic needle threader, instructions for
use can be found on page 26.