Sunday, December 11, 2011

How to Re-wire a Potted Motor, Part 1: Wire, Wire Stripping, & Wire Braiding

Singer’s models 15-91 and 201-2 come with beltless, direct-drive motors colloquially called “potted” motors. Sometimes the wiring inside is shot and needs to be replaced; half a century of temperature extremes can rot the insulation away, leaving dangerous bare-metal wires inside an all-metal machine that can transmit a shock to the user.

In this series of entries I’ll teach you to re-wire a potted motor. I’m writing this for people with zero experience, so there are quite a few tools and skills we’ll need to cover. The good news is, you can practice all of these skills without touching your sewing machine, so you can be sure you’ll be able to pull this off before you open your machine up.

Also be aware that you’ll need to acquire a fair amount of tools and materials to re-wire a motor. You or your spouse may already own some. In a nutshell, what we’re ultimately going to need is:

- Wiring
- Wire Strippers/Cutters
- Soldering Iron
- Solder
- “Helping Hands” Tool
- Heat Shrink Insulation
- Heat Gun or cigarette lighter

In addition, you may also need a crimping tool and ring connectors; you'll decide for yourself after we cover how to terminate the wire in a future entry. And don’t worry if you don’t know what some of this stuff is, we’ll eventually get into all of it.

The first things we need to cover are a) the proper type of wiring to use, b) how to “strip” wires, and c) how to braid two wires together.

Types of Wiring

Electrical wiring comes in two types. The first is solid-core, also called single-strand. That’s what a coat hanger is, just one piece of wire. The other kind is called stranded wire because, well, it comes in strands. That’s what we need to use here.

Secondly, wiring comes in a variety of metals like silver, aluminum, copper, etc. We need copper wire.

Thirdly, wiring comes in different gauges, or thicknesses. Plumbing pipes come in different thicknesses too, according to the amount of water that will flow through them. Similarly, the wiring gauge or thickness is commensurate with the amount of current they need to safely convey. What we need is 18-gauge wiring, also referred to as AWG-18. (That “AWG” stands for “American Wire Gauge.” Apologies to overseas readers but I am not sure what the foreign equivalents are.)

Lastly, wiring has insulation--also called “jacketing”--that also comes in different thicknesses. The thinnest type is called SPT-1, and that’s what we need to re-wire a motor, or the sewing machine’s light fixture for that matter. SPT-1 wiring typically goes inside of devices, where there will be no abrasion. There is also a thicker jacketing called SPT-2, and that’s what we’d use to re-wire a foot pedal. SPT-2 typically goes outside of machines, where the thicker jacketing means it can stand up to abrasion. I’ll cover foot pedal re-wiring at some future date.

Left: Black 18-gauge wiring (doubled) with SPT-2 jacketing.
Right: Red 18-gauge wiring with SPT-1 jacketing.
The Wiring You Need

You need to buy “Copper-stranded 18-gauge wiring with SPT-1 jacketing.” Don’t deviate. You should be able to walk into any hardware or electrical supply store and find this stuff. It should be cheap, around 10 cents a foot. Buy two different colors. (I prefer red and black, but all that matters is that you can tell one color from another.) To re-wire your average machine you’ll only need a foot or so of each color, but since we’re going to need to practice a lot first, buy five to ten feet of each.

If they’re out of it, please resist the temptation to buy any other kind of wiring. Don’t mess around with electricity. Buy the right thing. You might be tempted to buy 18-gauge wiring with the SPT-2 jacketing, mistakenly thinking thicker is better, but that’s a mistake, as you’ll see in a future entry.

Wire Stripping

Now that you’ve got the right wire, you need to learn how to strip it. This is so you can connect new wiring to existing wiring. So you’ll need to get your hands on a wire stripper.

Wire Stripper Reviews

All wire strippers are not created equal, as I learned the hard way. Please save yourself a few bucks and have a look at this description of three different types of wire strippers. At the bottom I’ll place a video that shows me using each one.

My buddy Jeff can fix anything with practically any tool. He just has that gift, to be able to take the wrong tool and the wrong fastener and just make something work.

I am not my buddy Jeff. I am not a natural handyman and I do not have that gift. My occasional clumsiness means it really pays off for me to use the best tool for the job, to reduce the chances I’ll screw it up. I’ve written these reviews with that in mind.

This is the most common type that you’ll find in every hardware shop. It’s also, in my opinion, a total piece of crap for stripping wires:

It's made by Gardner Bender and I bought it first because it was inexpensive (under $10) and I didn’t know any better. The way a wire stripper works is you squeeze it around the wire at the point where you want to strip off, and the sharp blade cuts into just the insulation, not the copper part.

Then you can pull the insulation off like pulling a sock off of a foot. In theory, anyway.

If you look closely at this tool, you’ll see it’s so poorly made that the holes don’t quite line up. This makes for messy stripping.

However, the Gardner Bender has a cutter at the tip for snipping wires, and also has a part for crimping, which we’ll cover later. So for those two necessary things, it remains useful.

The second one I bought was a Klein Tools Curve wire stripper ($11.52), which seemed more precisely made.

The holes line up correctly.

But I had a hard time using this tool, as you’ll see in the video below, probably because I’m a klutz. The amount of force I needed to apply to strip the wire just seemed unreasonable. I also don’t want to apply that much force to a wire connected to the motor. 

The Klein Tools wire stripper does have a cutter, so at least there’s that. But overall I do not recommend this one, and I would only buy it if it was the only thing I could get.

The third and final tool I bought, and the one that I love, is this “Katapult,” also made by Klein tools:

At $24.65 it’s the most expensive in the bunch, for a reason: It’s designed in a clever way so that when you squeeze the handle, it automatically strips the insulation and slides it off in one easy step. It places almost no stress on the wire, which I like.

One piece of advice if using the Katapult: After you squeeze the tool and it pulls the insulation off, do NOT just release the handle. If you do, the tool will close too quickly and fray the wiring. What you want to do is squeeze the tool, then gently remove the wiring by pulling it to the left before releasing the tool handles. This is really tough to describe or even show in video, you just have to play around with the tool and you’ll see what I mean. So practice on some wire scraps first.

One lame thing about the Katapult is that it does not have a cutter. If you buy only this tool, you’ll still need to buy one of the others to cut the wire. For a cutter, I’d recommend the crappy Gardner Bender because it's cheap and has a cutter and a crimper--though you may not need to crimp, as we’ll cover in a future entry. Alternatively, you can always cut wiring with a pair of tin snips, aviation shears, or EMT shears, if you have any of those handy.

Here’s the video of what it looks like to use all three of these:

You should be able to find at least some of these tools at your local hardware store. I like to support my local businesses, but for those of you who live out in the sticks, or if your stores don’t carry these things,you can order them from Amazon:

Full Disclosure: I should mention again that if you purchase these tools through these links, Amazon sends me a few cents for sending you their way, as I have signed up for their affiliate program. That does not mean I am urging you to buy things you don’t need so I can make a profit. Feel free to ignore these links! I include them in the event that you do need to mail-order the tool and would like to support this blog.

Furthermore, for those who do wish to support the blog, if you buy anything at all on Amazon (like books or whatever), if you first go to Amazon by clicking the link at the top of this page, they’ll also send me a few cents on that purchase. The price is the same for you whether you go through here or go directly to Amazon, they don’t charge you any extra. But please remember that this blog will always be free and you’re not obligated to click anything. I’m not in this for the money, but I wanted to set up a way to make it possible that a few extra bucks might roll in.

Wire Braiding

Once you’ve gotten good at stripping wires, you’ll have to practice braiding them. Braiding is how you’ll connect new wiring to old wiring in preparation for soldering. When we finally get inside the motor you’ll see you don’t have a lot of room to play around in there, so it’s imperative to practice braiding wires first by using scrap wiring.

Start off by stripping about 3/4” of insulation off the end of a wire.

Then grab it between your thumb and finger and twist, twist, twist.

It should look like this:

This reduces the chances you’re going to have frayed wire strands, which we want to avoid because it causes problems later.

Then strip another piece of wire and braid that one too.

Next, cross the two wires at the midpoints.

Then, braid those two wires together. It should look something like this:

It’s trickier than it looks, at least for me, as my fingers always feel too big. But just keep practicing, practicing, practicing, and eventually you’ll get good at it.

Your first goal should be to braid two wires together tightly enough that when you pull on them gently, they do not come apart. I recommend starting with two 3/4” lengths of exposed wire, or even 1” if you’re clumsy, to get the hang of it.

Then, once you’ve mastered that, start using shorter lengths of stripped wire. Your end goal is to get it down to about 1/2” of exposed wire that you can cleanly braid together.

Once you can consistently braid 1/2” of wiring together nice and tightly, you’ll be ready to move on to soldering, which we’ll get to next.

As you practice braiding each piece, don’t throw those pieces away! Even the crappy ones, save them. Cut each side off so that you’ve got your braid in the middle, and about two inches of insulation on either side. We’re going to use those pieces to practice soldering on.

Go on to Part 2: Tools & Materials for Soldering


  1. What a fantastic tutorial so far! I'm looking forward to the next part and I don't even have a potted motor to rewire :-) (I do have some belt-motors with issues though). I love your blog, it makes me want to get out my own vintage sewing machines and get tinkering, although I haven't done so yet. I love the details you give about the different tools too (the screwdriver posts and now the wire strippers), very helpful. So thank you!
    Hilde (in the Netherlands)

  2. Rain, thanks for the step by step guides. You are my guru.

  3. If I could bow to you online, I would do it. Words cannot express how much I love your tutorial, and I am waiting for the rest of it!

  4. Thank you for doing this. Looking forward to part II

  5. Thanks so much for these fantastic tutorials, especially the close-up photos that show exactly what you're describing. Looking forward to parts 2 and 3 and ... since we have at least two potted motors on 15-91's that need rewiring.

  6. Great work so far, excellent pictures! I just rewired a motor for a 15-91 over the weekend and am interested in seeing how you proceeded. BTW, tool #1 will work much easier for wire stripping if you turn it around so that the flat side (not the bevel) of the cutter faces the section of wire that you are stripping. You are correct about tool #2 being junk, and tool #3 is great for doing a lot of wiring - I've rewired dozens of motorcycles from scratch and that type of stripper (#3) saves a lot of time while delivering excellent, consistent results. Keep up the great blog!

    Marc Lamb, White River Junction, VT

  7. OOOO LALA I am going to get me that katapult TODAY. I hate stripping wires. And I have three, count them, three 201 motors to rewire. I also have 750 feet of 18AWG SPT-1 lamp cord. So I will color one red and leave the other black. Or not. Maybe if I keep repeating "Smooth=red, smooth = red" I can remember. Thanks so much for this wonderful tutorial. Are you going to video the soldering???? Can't wait

  8. This comment has been removed by a blog administrator.

  9. I am so grateful/excited that you are covering how to do this. I can't find any repair shops willing to rewire my 201. I probably won't take it on for a while but it is great to know that I will be able to refer to your detailed instructions when I'm ready. Thank you for so generously sharing your time and knowledge!

  10. Thanks so much for sharing your knowledge. This is just the info I have been looking for!

  11. I'm hoping this isn't a dumb question, but how do you know if your motor needs rewiring? Can I just look at the motor to tell or does it have to come off? I have cleaned the entire machine so far but have not seen anything to help with the motor. Now that you have a tutorial, I may dig around in there with a little more confidence!

  12. I love your site and have all the tools you have recommended, although finding the 18 awg stranded copper wire in a spt-1 jacket has been a lot harder to find. I have tried all the big home improvement stores, they only go as high as 16 gauge. Two lighting stores which have the two cord or wires. I already have some of that. Maybe I can strip one wire's worth off if needed. I even tried two major electrical supply stores and almost bought a roll off 500 ft and would of but they couldn't tell me if it was spt-1 or 2. For something that is suposed to be easy to get, I am not finding this. Any advise? You are clear as day yet still having trouble. Also No one has non insulated terminals. Maybe they went sincerely Jim

  13. found 18 gauge at the automotive store. whew!

  14. This is a wonderful tutorial! Thank you for taking the time to post it. I want to rewire my old (1910) National Electric Machine, but first I'm going to buy some wire and practice until I get it right.