403 breaking thread
 

My boss at the LQS brought in a Singer 403 for us to use at the shop (we did have a Stylist Touch and Sew, but the bobbin winder was giving us fits). She said it was working fine at home but now it keeps breaking the top thread. We've tried everything and just can't seem to figure it out. There was a thread stuck under the bobbin race, thought that was the problem but it didn't change anything when she pulled it out with tweezers. Any ideas?

For starters, take the bobbin race out and clean it up. Clean the tension area - check for burrs etc everywhere.
Check the tension adjustment - both top and bottom. Change out the spool of thread, needle and bobbin. I have an industrial machine that had just a wee bit of dried up gunky oil on the tension and it was breaking thread like crazy - took a while to figure it out.

I have a 403A. If it is the top thread, then it may either be a tension problem, or a needle problem. I would first look at the needle to make sure that is in new condition and put in correctly. I believe that the flat side of the needle faces the back. If it's a tension problem, I would check out the tension knob to make sure that there is nothing stuck between the discs and that the knob with the tension numbers on it is put on there correctly.
~Cindy

Make sure there's a little space between the bobbin case & the positioning finger...enough for the thread to pass thru easily. If it's too close it will break the top thread.

Where is it breaking? At the tension assembly? At the spool? At the needleplate?

Thank you everyone for the suggestions. I will forward this to my boss.

Tammi, the thread is breaking in the bobbin area, under the needleplate.

I agree, it's likely tension or needle related then, but my guess is tension, and I want to say upper... I know you've already checked this, but the spool isn't catching the thread in that little "slit" that you can use to make sure the spool doesn't unravel when it's off the machine, right? I think that usually breaks at the top, hence my question.

How does the bobbin tension feel? If you pull on the thread how much resistance would you say it has? There should be "some" but only a little more than barely there.
Do the same for the top tension. If you thread the machine up to the needle (but not through it), put the presser foot down, turn the dial to 0, when you pull on the thread it should have "barely perceptible" tension. If you turn the tension dial up, the tension should increase. If it's really tight at 0, or it tightens extremely fast compared to the numbers on the tension assembly you might need to adjust the assembly to be more "Accurate".

There's no burr on the needle plate is there? A needle strike can cause that on the hook or the needle plate.

Sometimes when I get a mess like that I change out the thread, the needle and the bobbin - totally re-thread the machine - make sure you look in the manual - yeah check the tensions, check for burrs. Those pesky slits on the spools can give you grief - also make sure the spool isn't spinning too fast and looping thread around the spool pin. You can have burrs on the bobbin hook, too. Even dried up oil can cause problems.

The 401A that I was rehabbing this weekend had this problem. Sometimes it would break the thread outright, sometimes it would just shred it and leave part intact. I changed out the throat plate and the problem disappeared. Looking closer at the plate, someone had hit it hard enough that it looked like it had been drilled. The needle had to have been buried up to the shaft!

You can use some emery string type stuff - sew-classic sells it - and polish up the hole.