How to Remove a Broken Screw With Alum

 In Tutorials, Watchmaking

Have you ever had a watch movement with a broken screw? Or an original signed crown (case parts are often harder to find than movement parts) with a piece of a broken winding stem inside?

Sure, you could use a drill to drill out the broken screw or use a screw extractor. However, not everybody has access to these tools. I don’t, for example, because I don’t often need them and they’re expensive.

But I’ve some good news. There’s a trick to remove a broken screw without using special tools.

What do you need?

  • A jar
  • Alum powder
  • Something to scratch, like a mending needle
  • Rodico or peg wood
How to remove broken screws with alum
Broken balance screw that is stuck in the base plate

How to remove a broken screw with alum

Take your jar and fill it with some water.

Mix as much alum powder with the water as it will hold. I suppose you could buy a block of alum and grind it but I bought a container of alum powder from the internet.

Before you continue, make sure to remove as many parts as possible. Alum won’t corrode stainless steel but it’ll corrode carbon steel. For example, if the broken screw is inside the baseplate, make sure to remove parts like the set lever screw.

Please note that some crowns are also made of carbon steel so the alum will ruin them. So, make sure you know which metal the crown is made of if you need to remove a broken winding stem from a crown. This might help: stainless steel has a luster and carbon steel is quite dull. Also, carbon steel is magnetic and stainless steel isn’t.

Plunge the part with the broken screw in the alum solution and leave it for about a day.

Remove the part from the solution and examine it. The alum solution should have corroded the top layer of the broken screw.

Remove a broken screw with alum
The black corroded layer of the stuck screw is very obvious

Scratch the corroded metal with the mending needle and remove it with some Rodico or peg wood.

You’ll have to rinse and repeat because each time you’ll only remove the corroded top layer of the screw. But no worries, it will be completely gone in a couple of times.

Finishing touches

Make sure to carefully dry the baseplate or part you’ve been working on because the solution is water based.

I always clean the part in the watch cleaning machine and dry the part in the heating chamber.

Carry on with the repairs like the broken screw never happened!

Do you have some tips for broken screws? Please leave them in the comments below.

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Showing 8 comments
  • David Borg

    Where can I buy alum powder, please ? On eBay alum powder seems to be a deodorant or sold in the form of stones !!

  • David Borg

    Another question please, it’s recommended that heating the alum mix makes it more effective, how can this be done ?

  • David Borg

    Hi, what about titanium, my crown is made of titanium, will the alum powder ruin the crown please ?

    • Melvin Hollenberg

      I think it’s safe because titanium has very good corrosion resistance and you only emerge it for a short(ish) period.

  • Greg

    Hello. This seems to be very useful! Is that method good for a metal screw jammed in plastic? Will alum do any harm to it? Thank you

  • Paulo Gabriel

    Parabéns pela dica, só conserto meus próprios relógios e sou principiante. Aprendo muito contigo.

    • Melvin Hollenberg

      Thanks for the kind words, Paulo. Welcome to the watch tinkering team!

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