I have a very antique light on a very antique ceiling that I do not want, by any means, to compromise (meaning have to open). A few weeks ago, the ceiling light that was installed started losing its grip on the flange. The primary structural support on which the light was mounted is a threaded rod that I measured (with a caliper) to be roughly 13/16" in diameter. It can be seen in this picture as the central protruding piece in the middle of the hole, just to the right of the white electrical tape.

enter image description here

That 13/16 is my measurement, which may be off so my first question is what standard size (by which to look for in a store or online) that resolves to as I obviously cannot take the piece with me.

Then I need to adapt this crossbar on it. The smaller size of the hole on the crossbar is why I need to reduce the threaded rod from whatever standard size it is (part of my question) to this:

enter image description here

In order to mount this flange on which the light goes

enter image description here

QUESTION 1: What standard threaded size is something that roughly measures 0.82-0.84" (roughly 13/16").

QUSTION 2: Is adapting this rod down to a modern stock size my best approach to secure this flange to the rod?

NOTE: This surrounding context of this terminal is what I have to deal with as invariable. Suggesting to change the installation because it is out of date (so that I can install a proper, modern box to which adaptation is easy) would be kind of superfluous as I am not touching the ceiling. This is a retrofitting job.

  • Do you have the nut which goes on this larger diameter rod? If not, I expect you could get one pretty easily unless the thread pitch is no longer easily available. HD has a big selection of special nuts. Also a "lamp repair" store would have special lamp nuts in various diameters. This would probably be a flat nut and you might or might not need a washer. Commented Jul 5, 2018 at 23:24
  • What is the width of the flat metal piece? If you would drill out the hole to fit over the 13/16" rod would there be enough metal left to support the fixture? A lamp store would probably have a wider piece that would have holes in the right place. What does the fixture look like? How heavy is it? Commented Jul 5, 2018 at 23:27
  • 2
    The part you’re looking for is called a “fixture hickey”— google that and click images.
    – Tyson
    Commented Jul 6, 2018 at 11:50
  • @JimStewart -- the flat metal piece is modern, the hole is 3/8" -- I need to find something that adapts the 13/16 rod to that. And I do not have the original nut
    – amphibient
    Commented Jul 6, 2018 at 14:59
  • 1
    4 kinds of hickeys , the threaded one as Tyson mentioned, a pipe hickey for bending, the one teenagers get on there neck and the doo hickey that Alaska man asks about.
    – Ed Beal
    Commented Nov 28, 2018 at 14:52

2 Answers 2


The thread sizes for these mounting fittings are specified in IPS, "Iron Pipe Straight." It's a hold over from when lights were mounted on repurposed gas lighting, which used the IPS sizing. Later IPS was replaced with NPS but the nominal sizes are not the same. Lamps still use the old IPS sizing.

I believe the size you have there is 1/2" IPS. Try a 1/2" IPS to 3/8" IPS hickey with your 3/8" IPS crossbar. I'd not expect to find it in a local store, but it won't be hard to find online.


A 3/4 inch IP nut like this might fit the threads of the existing rod. If it screws on, then you would drill out the flat metal piece you have. If drilling out the one you have leaves only very thin strips of remaining material on the sides, perhaps a washer would support the bar. Or you might have to get a wider bar.

To test the threads of the existing rod, get any 3/4" FIP adapter from Home Depot and see if it screws on.

EDIT If you want to approach this systematically, get a thread gauge set to determine the pitch of the threads (number of threads/inch) and get some calipers to accurately measure the diameter of the rod.

See video on threads.

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