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.
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:
In order to mount this flange on which the light goes
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 Suggesting to change the installation because it is out of date (so that I can install a proper, modern box to which adaptation is easy) would be kind of superfluous as I am not touching the ceiling.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.
Best Answer
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.