tl;dr - where does the horseshoe-shaped bit at the bottom of the picture go?

Long version

I've been trying to refurbish the locking mechanism (gearbox?) for my street door after it got so stiff the key broke in the lock.

I've removed the 5 point locking strip from the door and taken the gearbox off of the strip, and I managed to get the broken key out with pliers. I've now dismantled the gearbox in the hope I could just add some grease and put it all back together again, but it kind of fell apart when I took the screws out and I've been trying to do the resulting jigsaw puzzle for a couple of hours.

I'm pretty sure got most of the parts back in the right place, but there's inevitably a small part left over that I've got no idea what to do with :-(. I'm hoping someone can point me in the right direction...

See the image below for the current state of the lock - I think it's a Millenco Lockmaster Mantis 3 - see https://www.amazon.co.uk/Millenco-Lockmaster-Backset-Spindle-Mantis/dp/B01B516QYG.

The piece I can't find a home for is the horseshoe-shaped part at the bottom of the picture. I think it goes at the bottom end of the lock somewhere, but that's not definite...

Any help appreciated with either working out where it goes, or finding a teardown guide / reference image for the lock as I've had no luck finding either of those myself.

enter image description here

  • Since these still seem to be sold, have you contacted the company to see if they've got any internal drawings that will help you? Or, send them your picture and ask them where that part goes? They'd probably be thrilled that someone cared enough to attempt to fix it (though they'd probably prefer that you chuck it in the bin and buy a new one).
    – FreeMan
    Mar 26, 2021 at 11:22
  • @FreeMan - I tried calling the number on their website but apparently the manufacturing company no longer exists, and the site (and Millenco brand) is under new ownership, ostensibly to trail off any remaining stock. Since they only bought the site they didn't get any of the maintenance documentation, and I haven't been able to find any online. I bought the one off Amazon as a fallback but I'd still like to try to fix this one if I can...
    – mclayton
    Mar 26, 2021 at 11:30
  • Bummer! I'm sure someone here will have an idea...
    – FreeMan
    Mar 26, 2021 at 11:41
  • The size does not look quite right, but that spring above the keyhole needs something to push on, I think. And perhaps the size looks wrong because I'm not considering turning it the correct way - rather than as it is, turned up on edge, I think now, and I think it's the part that interacts with the key, so it might only work right flipped one way, not the other.
    – Ecnerwal
    Mar 26, 2021 at 11:52
  • @Ecnerwal - I think that's it. I'll post it as an answer, but feel free to cut&paste the picture and post it in your own answer and I'll accept that...
    – mclayton
    Mar 26, 2021 at 12:37

1 Answer 1


Per @Ecnerwal's comment, it appears to sit astride the spring just above the keyhole, with the ends of the horseshoe slotting into holes in both faceplates when they're re-assembled...:


I'm having trouble getting the other faceplate on without knocking all the parts around, but that's a different problem althogether...

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