Singer 15-91: threading, bobbin, oiling and timing

Is this your machine?

The 15-91 is Singer's full-size straight-stitch machine with the "potted" motor: an electric gear-drive motor bolted to the back of the arm instead of a belt and an outboard motor. It is a low-shank machine with an oscillating hook, it sews forward and in reverse at 6 to 30 stitches per inch, and most were sold in cabinets, which is why Singer's book keeps telling you to tip the machine back on its hinges.

The 15-91 belongs to Singer's Class 15. Singer's adjusters manual for the 15-91 says its stitching-mechanism instructions also apply to the 15-88, 15-89 and 15-90, so the timing section below holds for those too. The owner's steps on this page follow the 15-91 instruction book printed in the early 1950s.

Needles, the Class 15 bobbin and the right bobbin case

The 15-91 takes 15x1 needles. Singer's book: when ordering, ask for "Class and Variety 15 x 1" and give the size. Raise the needle bar to its highest point, loosen the clamp thumb screw, and put the needle in with its flat side to the left, pushed fully up. The needle threads from right to left.

1. Flat side of the needle shank to the left, pushed fully up into the clamp, then tighten.

ISMACS confirms the 15-91 takes Class 15 bobbins. Vintage Singer Parts sells metal Class 15 bobbins that list the 15-88, 15-89, 15-90 and 15-91 (their SKU 2996).

The bobbin case is where people go wrong. The 15-91 uses Singer bobbin case 125291, a backward-latch case whose finger points to 11 o'clock. Vintage Singer Parts lists it for the 15-75, 15-88, 15-89, 15-90, 15-91 and 15-125 and warns that it will not fit standard Class 15 machines or clones that need a 1 o'clock finger. A generic "Class 15 bobbin case" sold for modern front-loaders is the wrong part.

Threading the top thread

1. Raise the take-up lever to its highest point with the hand wheel. The hand wheel always turns toward you on this machine.
2. Put the spool on the spool pin on top of the machine and take the thread through the thread guide.
3. Bring it down, under, and from back to front between the tension discs. The thread guard steers it in.
4. Holding the spool tight, pull the thread against the take-up spring until it enters the retaining fork.
5. Pass it from back to front through the hole in the take-up lever.
6. Bring it down through the guide on the face plate and into the guide on the needle clamp.
7. Thread the needle from right to left and pull about 2 inches through.

Winding and loading the bobbin

1. Hold the hand wheel and turn the knurled stop motion screw toward you to free the wheel from the stitching mechanism.
2. Put the bobbin on the winder spindle so the small pin enters the hole in its side. Take the thread from the spool under and between the winder tension discs and through the hole in the left side of the bobbin from the inside.
3. Hold the thread end and press the winder down until its latch drops and holds the winder pulley against the hand wheel. Run the machine and hold the end until it breaks off.
4. The winder stops by itself when the bobbin is full. Remove the bobbin and tighten the stop motion screw.

If the bobbin winds high on the right, loosen the tension bracket screw and move the bracket left; if high on the left, move it right. Singer notes you can wind a bobbin while sewing.

Loading the bobbin case

1. Raise the take-up lever to its highest point. Draw the bed slide plate on the left out, open the bobbin case latch and lift the case out.
2. Put the bobbin into the case the way Singer's Figure 13 shows the thread unwinding. Pull the thread into the slot, under the tension spring and into the slot at the end of the spring, leaving about 3 inches hanging.
3. Hold the case by its latch and put it on the stud of the shuttle body with the position finger opposite the notch at the top of the shuttle race. Release the latch and press until it enters the groove in the stud. Close the slide plate. Keep the slide closed while sewing.
4. Hold the needle thread and turn the hand wheel toward you until the needle goes down and comes up and the take-up lever is at its highest. Pull the needle thread up to bring the bobbin thread with it, and lay both back under the foot.

Top and bobbin tension

The needle tension dial is numbered 0 to 9; Singer says the numbers mean neither thread size nor ounces. Turn the thumb screw right (clockwise) to increase and left to decrease, only with the presser foot down. Note the number that suits your usual sewing. The bobbin tension screw is on the tension spring outside the bobbin case: right to increase, left to decrease, and seldom needs changing once set.

The presser foot pressure is the thumb screw on top of the machine: up (left) to lighten it for filmy fabric, down (right) for heavy or napped fabric.

Resetting the tension from scratch

1. Use size 50 mercerized thread on top and in the bobbin.
2. Set the needle tension thumb nut to 4.
3. Sew through two layers of thin material and adjust the bobbin tension until the stitch locks in the middle.

Oiling and cleaning the 15-91

Singer's book: oil daily if the machine is used continuously, occasionally if used moderately, one drop at each point the figures mark with arrows. In words:

1. Front and top. The oil holes shown on the front and top of the machine.
2. Shuttle race. Draw the bed slide to the left, clear out lint and dust, put a drop on the shuttle race and close the slide.
3. Back. Loosen the thumb screw in the round cover plate at the back and turn the plate up. Turn the hand wheel until the connecting rod is at its highest, and put a few drops through the hole in the top of the machine onto the wick in the connecting rod cap. Oil the other moving parts inside and close the cover.
4. Face plate end. Loosen the screw near the top of the face plate, raise the plate and slip it off, oil each marked point, and replace it.
5. Underneath. Tip the machine back on its hinges and oil each marked point in the base.

The motor gets grease, never oil. This printing of the book says the two motor grease cups are filled at the factory for about a year and should be refilled with SINGER Motor Lubricant at least once a year: tip the machine back, remove the two thumb screws from the cups, clean the cups out, fill them, and refit the screws. Earlier 15-91s may have grease wicks and tubes instead, which Singer's 1930s adjusters manual mentions; follow the booklet for your machine's era, and if you cannot tell, leave the motor to a technician.

Cleaning the shuttle

1. Draw the slide left. Turn the hand wheel until the needle is at its highest and the shuttle point is where Singer's Figure 36 shows it.
2. Take out the bobbin case. Remove the thumb screw, the spring and the shuttle race back, and lift out the shuttle.
3. Clean the lint out of the shuttle and race.
4. Refit with the needle at its highest and the shuttle point in the same position: shuttle, race back, spring, thumb screw, then the bobbin case. Close the slide.

If the machine runs heavily after standing unused, Singer's book suggests a little kerosene in the oil holes, running it fast, wiping clean and then oiling.

Troubleshooting by symptom

Checking shuttle timing and needle bar height

The 15-91's shuttle timing is set at the factory by a shuttle driver pinned to its shaft. Singer's adjusters manual (Form 18847) calls that setting permanent unless the machine has been damaged badly enough to move the driver, so on an undamaged machine a skip is almost always the needle, the threading or lint. Replacement drivers come with two screws instead of a pin and must be timed.

Needle bar height

1. Turn the hand wheel toward you until the needle bar is at its lowest point.
2. The upper timing mark should be just visible at the lower end of the needle bar bushing.
3. If not, turn the wheel until the needle bar set screw shows through its access hole, loosen it, set the bar so the upper mark is just visible, and tighten the screw.

Shuttle timing (after a driver has been replaced)

1. With the needle bar on its upward stroke, turn the hand wheel until the lower timing mark is just visible at the lower end of the bushing.
2. Turn the shuttle driver on its shaft until the shuttle point is at the center of the needle. Make sure the shuttle's heel is not compressing the end of the driver's cushion spring.
3. Tighten the two screws in the driver hub with the hub held against the shaft bushing.

If the needle bar bushing has been moved, Singer's reference is the shuttle point at the center of the needle and a scant 1/16 inch above the top of the eye. Fitting a new driver means removing the feed dog, position plate and shuttle race; if that is further than you want to go, a technician can do it.

Leave these to a technician

The potted motor's wiring, brushes and commutator, the power cord, the light cord, and the foot or knee controller. Singer's book asks you always to pull the plug when you finish sewing; pull it before you oil or open the machine as well.

Manuals and parts lookup

• Singer 15-91 instruction book, 1950s printing (Internet Archive)
• Singer adjusters manual for the 15-91, Form 18847 (Internet Archive)
• ISMACS page on the Singer 15-91
• Class 15 machine parts at Vintage Singer Parts
• Singer 15-91 parts at Sewing Machines Plus