Constabulary

machine seems to be a Singer 17 or Seiko TE-6 and they have supper small bobbin. Surprises me that they sew with fairly think thread this fairly thick leather with a machine like this > not much thread on the bobbin. When I look at the stitch length lever the it seems to me just a screw w/o reverse - or not? However - IIRC there was a reverse device of the Singer Class 7 that did not come with reverse ex factory... Maybe they tinkered something similar. Well.... lots of guessing here. No matter what they do a great job there - I´m quite impressed. EDIT: weeeeell - I´m not sure - new TE-6 have reverse AFAIK but not sure what I see there. EDIT II: Looks like this one - which looks like a non reverse machine

thread is coming of the spool a bit the odd way. Maybe one of these thingies may be helpful. However if it works for you then don´t mind. 😉

okay, may depend on the machine 😉

I´m updating here cause I have an Astor M5 on my bench for restoration - the inner thread is metric M10x1.5 (at least on mine). So the information MikeRock got from the distributor is correct. So if you need a cheap tool adapter for M6 or M8 tools / dies then this could be an option: https://www.ebay.de/itm/402466257921 Hope this helps

not sure what you mean... Is it that your last stitch is probably to long and not hitting the 1st needle hole where the seam began? If so then you can shorten the stitch with the reverse lever and hand crank the last stitch - if you know what I mean.

not sure where how you figured that but I have different experience (I´m living in a metric country). Maybe someone overcut the original threads on your machines. Nothing really metric I´d say: https://www.singersewinginfo.co.uk/technical/parts/screw_threads/

Or NTB in the City where the Adlers came from: https://naehtechnik.eu/

Check with Liersch: https://liersch.com/online-shop/ersatzteile/ or Zieger: https://www.np-z.com/ or Busche: https://busche-online.de/ If they don´t have it in the Shop send them an Email - Answers may take longer due to Christmas season.

I never did that drop test, the manuals show how to set the tension right. That works for decades, just follow it. But as always - many roads lead to Rome - or each to his own. 😉

BTW - I put one off these in a plastic box and wired it to the motor. https://www.ebay.de/itm/396873227544 But I´m using it for my electric bobbin winder which run with domestic sewing machine motors. Not sure if this is a solution for you. They are available everywhere and I think even cheaper. This was my project back in 2017

servo motor and speed reducer come to my mind but depends on your sewing machine table and if the machines have a hand wheel with pulley and no built in motor.

these original manuals pop up every now and then. i just recently bought two from a guy in France he still has some: https://www.ebay.de/sch/i.html?ssPageName=&_ssn=zougoulouba

Guessing - most likely a motor problem. I guess the machine has a servo motor so I would guess the lever of the accelerator unit or the hall sensor is loose / has moved and is probably stuck in the "run" position. Please post some pictures of your motor and the accelerator unit and lever where the it man rod is attached and maybe a video showing the issue (please not just 5 second video or so" upload it to YT and post a link here.

yeah - keep that old cast iron alive - I seldom went wrong with that! 😉 I love my 51w but the 2nd type as yours would be even more preferred but I love mine too. Singer crazy me is. 😁

often Ebay is a bit cheaper than Amazon - even from the same seller. Just saying...

make sure the whole machine incl rod and coarse threading is well oiled and the stitch length mechanism parts are not gummed with old oil.

basically that's it but I would check the needle hook timing too - that never can be wrong.

are all the parts as shown in your diagram present (See text / parts list)? The tip on your adjustment rod is damaged as you already noticed, tip should be very sharp (see picture below). Not sure how much this affects the stitch length regulation. Is the small stud inside the adjustment knurled knob present? Is the coarse threading at end of the top shaft in good order? Please post some pictures. edit: Quick picture of the stud from my 107w1 rod and the top shaft threading. Stud slides inside the coarse threading.

Check with Fuchs Elektronik in Germany they have the Quick Rotan Service: http://www.fuchselektronik.de/index.php?vid=302&mid=1&lid=2 However - not sure if they are still in business - looks like an early 2000´s website

7x23 are slightly longer IIRC. Minor needle bar height adjustment could be necessary but they may work even w/o adjustment.

sure: https://www.ebay.com/itm/234079435236 this is with what the 31K48 came ex factory but roller size may vary. however we have not seen your machine and how it is set up. Could be this type as well https://www.ebay.com/itm/143603133032

happened to me quite often. Waiting for a better price... for money to come in... lack of time... just being toooo lazy... looking for too long for information... When you see one you want grab it before someone else takes the 1st shot.

I think I mentioned somewhere that I may have one.... Will check later.... heading to work now...

I have a complete singer shuttle race for the class 7 not sure if it will fit I have some other bit but have to look for them.

Fxck AI - don´t trust it too much. Not sure what needle this subclass is using but you can use different needle systems with the smae shaft diameter but have to alter the needle bar height. That usually it not big deal. Singer 16 (more common is 287 or 1738 as needle system) have a 1.63mm diameter IIRC and 135x17 have 2mm diameter needle shaft. It may work but you have to try it. Or you try to find a needle bar that ex factory is made for 2mm shaft needles. I did that with my old Singer 51w but it using a different needle bar that does not fit the Singer 17. EDIT: oh- BTW I once owned a 34K that ex factory is using a 2mm shaft needle and I had to replace the NB and I took one from a Singer 17 (!!!) and the machine after the NB replacement worked with the 2mm shaft needles but ex factory the NB was made for Singer 16 needles (as yours) so I would just try the needles and adjusts the the NB in height. Worth trying - never say no - just try it. But hand crank the machine before you are using it with a motor. This was my hunt for a Singer 34K NB 10 yrs ago 😅 https://leatherworker.net/forum/topic/66729-singer-needle-bar-i-could-use-some-help/