Singer 401A Slant-o-Matic: threading, bobbin, oiling and timing

Is this your machine?

The 401A Slant-o-Matic is Singer's all-metal, gear-driven automatic zigzag machine from the late 1950s. Its needle bar and presser bar lean 9 degrees from vertical toward you, which is where the "slant" comes from, and its rotary hook turns on a vertical axis in front of the needle, two turns per stitch, under a drop-in bobbin. It has seven built-in cams plus one removable special cam. The stitch length plate on the right front of the pillar is marked MODEL 401A. Singer's instruction book for it is copyright 1958.

Slant-shank presser feet are not interchangeable with ordinary low-shank feet. Everything you buy for this machine needs to say slant shank or name the 401.

Needles, bobbins and slant-shank parts

The 401A takes 15x1 needles in sizes 9, 11, 14, 16 and 18. Singer's book: order "Class and Variety 15 x 1" and give the size. Its chart puts size 9 on filmy fabrics, 11 on sheers, 14 or 16 on lightweights, 16 on medium heavy and 18 on sailcloth, denim and ticking, with 11 or 14 on plastic. Use the same kind of thread in the needle and the bobbin.

1. Turn the hand wheel toward you to raise the needle bar to its highest point and loosen the clamp screw.
2. Push the needle up as far as it will go with the flat side to the back and the long groove toward you. Tighten the screw. The clamp takes one needle, or two for twin-needle work.

Singer's book calls the bobbin "drop-in" without naming a class. Vintage Singer Parts lists Class 66 bobbins for the 401 and 401A: metal part 172222 (which replaces original Singer 32522) and plastic 172222PS, which the same seller says is the same as 172336. Singer's current Class 66 page does not name the 401, so buy from a listing that does.

Slant-shank parts listed for the 401A by Vintage Singer Parts:

• General purpose zigzag foot 172075
• Straight stitch foot 170071
• Adjustable hinged zipper and cording foot 161166
• Special purpose foot 163483
• Zigzag needle plate 172200 (the machine came with a zigzag and a straight-stitch plate)
• Bobbin cover slide plate 163477
• Light bulb: 15 watt, 120 volt, push-in bayonet type. Singer's book: push it in and turn to release; do not try to unscrew it.

Threading the top thread

The 401A's instruction book describes threading in one line and leaves the route to its illustration and to the threading diagram printed on the inside of the face plate. Swing the face plate open and keep that diagram in view.

1. Raise the take-up lever to its highest point with the hand wheel. Singer: the top of the hand wheel always turns over toward you.
2. Put the thread on the spool pin and lead it through the threading points shown in the book and on the face plate diagram: the guide on the arm, down to the tension discs on the front of the head, the check spring, up to the take-up lever, then down through the guides to the needle.
3. Thread the needle from front to back and draw enough thread through to start sewing.

For two-needle work the book has its own threading page; follow it rather than doubling the single-needle route.

Winding and loading the bobbin

1. Hold the hand wheel and loosen the stop motion screw, the inner knob, with your right hand.
2. Raise the bobbin winder, put the bobbin on its spindle and press the winder against the hand wheel.
3. Put thread on the spool pin on the bed, lead it between the bobbin winder tension discs and through the hole in the left side of the bobbin.
4. Hold the thread end to start winding and run the machine. A full bobbin releases itself. Tighten the stop motion screw again.

Dropping it in

1. Open the slide plate and lift out the old bobbin.
2. Hold the new bobbin so the thread leads off in the direction the book's illustration shows, and drop it into the case.
3. Lead the thread into the slot and under the spring, then draw it into the notch at the end of the spring, pulling off about 3 inches across the bobbin.
4. Close the slide plate so the thread passes through its notch.

Top and bobbin tension

For straight stitching, set the needle tension dial to a lower number if the needle thread is too tight and a higher number if it is too loose. Balanced tension draws both threads equally into the fabric; the book's balanced example shows 4 at the index. Singer's service manual puts the dial's low end at 0, with no tension when the dial is turned toward the left to a point between 0 and 1.

Unlike Singer's modern machines, the 401A expects you to set the bobbin tension too, with a small screwdriver on the bobbin case screw: clockwise if the bobbin thread is too loose, counterclockwise if it is too tight. Small turns.

Satin stitching needs a lighter needle tension than straight stitching, and the wider the stitch the lighter the tension. Test on a scrap with the special purpose foot; puckering means the needle tension is too high. Presser foot pressure is the thumb screw on top: turn it upward to decrease and downward to increase. Too little pressure gives irregular feeding.

Cleaning, oiling and gear lubricant

Singer's book asks you to clean the 401A periodically and to oil it after each cleaning, with SINGER Oil on the oil points and SINGER Lubricant, a gear grease, on the gears. Never mix them up: the book says in capitals never to apply oil to the gears, and that the motor requires no lubrication.

Cleaning

1. Clean between the tension discs, and around the feed and throat plate area.
2. Remove the throat plate and open the slide plate. Lift the bobbin case holder and pull it to the right to unlock it, then lift the bobbin case out.
3. Clean the lint and thread ends out of the hook area.
4. Refit the case by seating its groove under the tension spring on the hook raceway, with its fork straddling the positioning stud. Push the holder left until it snaps down and locks. Close the slide and refit the throat plate.

Oiling and greasing, in the book's order

1. Top. A drop of oil at each marked point along the top of the arm and on the bed by the bobbin area.
2. Upper gears. Remove the screws and lift off the arm top cover. Apply a small amount of lubricant to the gears shown. Not oil.
3. Face plate. Swing the face plate to the left and oil the marked points on the needle bar and presser bar mechanism.
4. Hook. Open the slide plate and put a drop of oil on the hook raceway.
5. Underside. Tilt the machine back, remove the thumb nut (keep the felt washer) and take off the bottom cover. Oil each point marked by arrows on the shafts and linkages, and occasionally put a small quantity of lubricant on the teeth of the lower gears. Refit the cover.

For storage, Singer suggests a thorough brushing and then swabbing the exposed parts with a lint-free brush dipped in SINGER Oil to keep rust off.

Troubleshooting by symptom

The 401A's instruction book has no troubleshooting table, so this one is built from its needle, tension, pressure and cleaning pages.

Checking hook timing and needle bar height

These are the checks in Singer's service manual for swing-needle machines (Form 20732), Class 401 section. They use the two timing marks on the needle bar. Set the red lever at 3 and the stitch selector knobs at A and K first, so the needle is not swinging.

Needle bar height

1. Insert two needles of the same size.
2. Turn the hand wheel toward you until the needle bar is at its lowest point. The upper timing mark should be level with the lower end of the needle bar ball bushing.
3. Turn on until the hook point is behind the needles. Both needles should be the same distance in front of it.
4. If not, with the bar at its lowest, loosen screw W, raise or lower the bar to the mark, keep it correctly turned, and tighten W.

Hook timing

1. With needle bar height set, fit a straight size 18 needle. Remove the throat plate, bed slide, bobbin case and bottom cover; the feed dog can stay.
2. Turn the hand wheel toward you until, on the needle bar's upward stroke, the lower timing mark is level with the lower edge of the bushing.
3. The hook point should now be at the center of the needle.
4. If not, loosen the two set screws in the gear on the hook drive. Hold the needle bar at the mark and turn the hook shaft gear until the hook point is at the needle's center. Tighten both screws, checking the gears mesh without binding or backlash.

Singer's service manual also sets the hook's distance from the needle (.018 inch between the needle and the position finger, with a feeler gauge, by moving the hook bracket) and times the needle swing before the hook. Those are adjuster's jobs; if the marks check out and it still skips, take it to a technician.

Leave these to a technician

The enclosed motor, its carbon brushes and wiring, the power cord, the terminal blocks and the foot or knee controller. The 401A's book itself refers adjustment and repair to a Singer Center. Pull the plug before oiling, cleaning or opening any cover.

Manuals and parts lookup

Singer has no current support page for the 401A. ISMACS hosts scans of the Singer documents.

• Singer 401A instruction book, 1958 (PDF scan, ISMACS)
• Singer service manual for swing-needle machines, Form 20732, with the Class 401 section (PDF scan, ISMACS)
• 401A parts at Vintage Singer Parts