Wednesday, September 12, 2018

Weird Thread Size

My refractor has a thumbscrew to hold eyepieces in the focuser.  I doubt it was ever adequate to hold an eyepiece securely, but just to make it hard, I have put a diagonal in place of an eyepiece, and the diagonal is carrying a bit more weight than usual because it is carrying a Chromacorr apochromatic image corrector described here.  The thumbscrew is barely enough to hold everything in place so there is now a lanyard preventing the diagonal and its precious corrector from flying out. 

So I thought: why not find a thumbscrew with a cone point to really dig into the metal of the diagonal barrel?  But when I removed the thumbscrew, I was mystified by it.  I have an Imperial thread sizing tool that I made by tapping a piece of aluminum with every tap I own, from 4-40 to 1/4"-20.  It is too small for 6-32 and it will not screw into 4-40.  If it is metric, I am not finding it on the metric tap charts.  Major diameter is .109".  Measuring threads per inch is an especially hard task when the thread to thread distance is tiny, but it appears to be .075" over three threads, or .025"; it is certainly finer than 4-40.  I could I suppose go to Industrial Hardware in Garden City and use their thread sizing tool, but I am hoping one of you might lend me a clue.


  1. Looking at a thread chart, it looks like there are a couple of candidates. The major diameter sounds like a #4 fastener. It could be a 4-48. Could it also be a 5-40 or 5-44?

    Best of luck!

  2. 4-48 has been suggested. I may just take the existing screw with me, tomorrow.