Uwe

I can't come there an time your machine, sorry! But I can offer some tips on making it easier with not-so-perfect eyesight: 1. Use your (or somebody else's) iPhone as a magnifier loupe (https://9to5mac.com/2016/09/26/how-to-use-ios-10-magnifier-magnifying-glass-iphone/) . Android phones may have this feature, too. 2. Put a piece of paper behind the hook to see the gap between needle and hook better. Here's a video of my setup when I do very detailed adjustments: 3. Use the needle guard to keep the needle a safe distance away from the hook:

You could probably simply ignore the crack, too. If the machine sews fine, just put it in a proper, very strong table top and use it. That tiny crack at the 10" mark on your ruler should not affect functionality. Any hot welding fix attempts may just make matters worse. Properly supporting the machine may be all you need to do. Most tables only support the machine at the corners, but you can machine an edge that supports the machine along the "lip" instead of just the corners. The most likely cause of that crack is a bowing table top that only supported the machine at the very left and right edge. Many cheap industrial table tops are simply too weak to support the sewing machine without bowing in the middle. That big hole in the middle weakens the table top considerably. Especially particle board table tops are only straight for short while after you cut a big hole in them and drop 70 lbs of weight on them repeatedly. If the machine is supported only at the ends and you let it drop down often enough, a metal fatigue crack will develop in the unsupported middle on the underside. Fixing the crack without fixing the table top will only buy you a little time. Putting the machine in a proper, very sturdy table top, may keep the crack from getting worse and if the machine sews fine as it is, there may be no need to "fix" the crack. I had a machine with a similar crack in the bottom ridge that worked just fine. I built a new, straight, and sturdy table top for it (two layers of 3/4" plywood glued and screwed together). It's been performing well in a production environment going on two years now.

If, after watching that last video above, you can't figure out how to remove or install that thread tension unit, then my advice is to step away from the machine and let somebody else perform maintenance tasks.

This video shows, among other things, how to remove the tension unit and how the tension release pin works:

There's probably a set screw somewhere that holds the tension unit in place. Maybe you just need to move the tension unit out of the body a small distance to make everything work, as Wiz suggested. If the problem originates elsewhere in the tension release mechanism, replacing the tension unit itself may not fix your problem. You really need to investigate if everything moves the way it should when you raise the foot lift lever. Take off the cover plate and look at all the linkages the foot lift lever moves (or should move). One of them will press against the pin on the rear of the tension unit to separate the tension disks. You should be able to see the disk move a tiny bit when you raise the foot lift lever. Take some pictures around your tension unit so we can add an arrow or two to explain things. Don't make us beg. This video also shows nicely how this machine should be threaded:

There may be more than one thing wrong. Bottom loops have two major causes in my experience: loss of top thread tension, or top thread snagging/catching somewhere during the stitch cycle. I highly doubt the bobbin tension is causing this. It's much more likely in my mind that your bottom loops are forming because your top thread tension goes missing entirely for some reason. Check to make sure your tension release mechanism is working properly. I was working on a Pfaff 145 yesterday that occasionally had loops at the bottom. On my machine, the culprit turned out to be a thread tension release mechanism that occasionally got stuck in the "release" position after lifting the feet to make a turn. Your upper thread tension discs should definitely NOT be "wiggly" loose when you sew. Please post some pictures of your full upper thread path (spool to needle) and maybe a video snippet or two of your machine sewing (it may take two people to do that.) Here's a picture of what my bottom stitch looked like - note the loops right after a corner, with perfect stitches elsewhere.

Still for sale. Still would need to be shipped on a pallet via freight, which would probably cost nearly $300 to a residential address, and I'd spend better part of a day arranging it. Send me a PM with your address if you're interested in having this machine shipped to you. It is a very nice machine and definitely worth more than $600.

That's probably a 1/8"-44 or #5-44 screw written in weird metric/imperial mishmash dimensions. 1/8"=3.175mm That missing hook screw might be tough to find individually. Your best bet may be to buy a cheap hook that has a good chance of using the same screw. I doubt Hirose invented a new screw for each of their hundreds of hook variations. One candidate is the Hirose HSH-7.94B hook, which is used in dozens of machines and therefore cheap. Available for around $15 on Ebay and I guess some of our trusty banner vendor have them, too. At this prices, the clone manufacturers have little incentive to make their own. Here's the list of machines this hook fits, according to the Hirose Catalog (Hirose Catalog Low Resolution OCR.pdf) :

You can still buy Adler 30-10 and Adler 30-70 patcher sewing machines new, made in Germany, and with a price tag to match. The production, development, and support of these machines was taken over by a company called, take a breath, "SL-Spezialnaehmaschinenbau Limbach GmbH & Co. KG", exhale. You can find them online at http://www.sl-spezial.de/sl/index.php/company.html Our resident patcher experts on LW are @Constabulary, who had some beautiful patchers for sale just recently, and @shoepatcher, who apparently owns every patcher ever made, or something close to that. In the clone world, I had a Techsew 2900 (a copy of a late Singer 29 style patcher) some time ago that appeared to be very nice quality:

It's probably a Singer 45K-SV99 like the one described on this page: https://www.jomimachines.co.uk/shop/uncategorized/singer-45k-sv99-industrial-sewing-machine/ It's a Special Version of the base Singer 45K. What exactly it is that makes that particular version special is hard to tell. It's also often hard or impossible to find documentation about particular SV models.

If you're sewing Kevlar thread, you'll definitely need a machine that you can still get parts for. That Singer 51W55 is not one of those machines. Kevlar thread is hard on machinery and will wear through metal parts over time. In the vintage post bed category, I'd recommend looking into machines like Durkopp Adler 168 or 268, or Pfaff 595 or 1295. In the cylinder arm (beam) category, the Durkopp Adler 269, and Juki LS-341 class machines are candidates, as are the heavy duty Adler 205 and Juki TSC-441 class of machines. Some of these machines have modern aftermarket clones. I have some used and new candidates in that category, just in case you're interested. I built a binding workstation around an Adler 205 cylinder arm machine once to sew laptop fire containment bags (for airplanes) with Kevlar thread. It's holding up nicely in a demanding production environment.

Please share the information you already found and upload documents and a picture or two here so that others can benefit from it in the future.

On my Juki LS-341 that bobbin case opening finger is rock solid, has no noticeable play and makes no clicking sound when pushed. Perhaps something didn't go back together quite the right way after you removed the hook. I'm also a little surprised that we have totally different hooks in our supposedly identical machines.

Page 4 and 5 of the Juki LS341N Engineer Manual (Juki_LS-341N_Engineers_Manual.pdf) describe how to adjust the feed dog position sideways to center the feed dog hole with the needle.The feed dog touching the side of the throat plate opening is probably not a super big deal (unless the feed dog dips below and catches coming up.) The feed dog touching the front or back of the throat plate opening, on the other hand, IS a big deal.

Well, the LW let-me-google-that-for-you service (me) says it's a drop feed post bed with roller foot and edge trimmer for sewing leather and gloves. Apparently made in the late 30's and 40's, perhaps longer. I'm sure it's great if you want to sew gloves, not so great for saddles and holsters. Relatively small stitches at 8 per inch. No reverse. It's not a very common machine, since today is the first time I've ever heard of it. There's one for sale for $950:

If it's complete and works then it's probably worth way more than 120€. I doubt you can get many (or any) parts for it other than generic needles and maybe a hook, if you're lucky. This Singer 18 is a totally different design and super rare compared to the common Singer 29K. The Singer 18 probably weighs three times as much as a Singer 29K, too.

Lights-on picture of a Singer 18-2:

Ask the seller for a picture where we can actually see something. It looks very much like a black cat in a dark alley on a moonless night to me. Or just go there, check it out in person and take some decent pictures yourself to share here. It may be a very cool machine, actually. You'll never know for sure unless you go see it in person.

I have a Durkopp Adler 272 in very nice condition and ready to sew. Full function with all-electric actuators - no compressed air required. Back tack, reverse, thread wipe, thread cutting, needle positioning, stitch count mode, bar tack mode. It won't sew holsters or saddles, but it'll do plenty of other useful things with lighter weight leathers, vinyl, or canvas, and of course garment fabrics. This machine runs on 220V 3-phase. A basic rotary phase converter will allow you to run it in your basement. Priced to sell at $600. No shipping, sorry. It's worth a day trip to the Detroit area for sure. Pictures: I made a sewing demo video, but Youtube is currently overwhelmed by people uploading cat videos - it may be a moment or nine before this link works . . .

If you installed the thread guide where it belongs, I really don't see how it could cause the trouble you're describing. There may be something else going on.

This binding trouble may have to do with the bobbin case opener in two totally different ways. 1. When the throat plate is removed like in your first video, the bobbin case rotates more than it normally does and the bobbin case opener finger pushes against the triangular bobbin case tab and causes a bind. You can see the bobbin case popping backwards when the bind releases. 2. When the throat plate is installed, your bobbin case may simply be pulled back too much, causing the tab inside the throat plate cutout to touch and cause a bind. Neither of the videos shows that detail to verify or disprove. I made a video that hopefully will illustrate what I was trying to say in words above:

Ha, I'm having a déjà vu experience here. My Pfaff 545 also had mangled screw heads in the clamps on the foot lift linkage bar. I had ordered new screws based on the parts manual numbers (11-108 225-15) but they don't fit my machine either. Based on my measurements, I'm voting for the screws in my Pfaff 545 being size 7/32"-40, which is a weird size even in the sewing machine world. I was expecting it to be a 15/64" or 13/64" , which are somewhat common in sewing machines. I had some Juki 1/4"-40 L-11 screws which looked promising, but only for a brief moment. Who knows, maybe Pfaff just found a big box of weird screws on their shelves and decided to just use them somewhere (I really hope I'm wrong here.) I found taps for size 7/32"-40 on Amazon so that weird screw size does exist. Your best bet for finding screws of that 7/32"-40 size is to look at part lists by manufacturers like Juki, Mitsubishi, etc. that list actual screws sizes. Then order some of those screws and hope for the best. I've not been able to locate any OEM screws with that size. Regarding that mystery pin on top of the stitch length mechanism, it's for attaching a return spring. It's only used on the N10 subclass (like on my Pfaff 545), where the stitch length mechanism operates in the opposite direction (there's also no reverse with N10).

After 32 replies I was really hoping to see a roller foot on that machine.

The tiny pin inside the tension disc post pushes the tension discs apart when you raise the presser feet via the manual foot lift lever. If that pin is missing (likely) or worn down (less likely) then your thread tension won't release when you lift the feet. It's not a super big deal since it doesn't really affect the sewing itself, just when you try to pull out the material after lifting the feet. You can make a pin yourself from a suitable nail that you cut off at the right length and file the ends smooth. Simple trial and error will get you there. Once you file the pin too short (oops) and it no longer pushes the tension discs apart, you start over. I must have made a dozen of these little pins by now. I even managed to lose a few original pins myself, knowing full well here they are hiding and that they go AWOL in the blink of an eye. Please upload your pictures directly here to your LW posts (drag your resized image files on top of the editor area where it says "Drag your files here to attach".) Those Google drive links or other workarounds won't be around a few years from now when people browse the LW archives, and they're cumbersome to click on just to see the picture in a new window.

While you're fine tuning your machine, I'd suggest installing a proper lower thread guide. The one on your machine looks like a homemade replacement for the original, which looks like this: