Tuesday, February 8, 2022

All For Want of a 1/4"-20 1.25" long Socket Head Shoulder Screw

 I am machining an adapter to mount a Vixen dovetail on an EQ1 equatorial mount:

The adapter holds to the mount with 1/4"-20 screws, but I need 1.25" long.  What makes this so tragic is that when I owned ScopeRoller, I mostly used 1/4"-20 screws.  In my collection of surviving fasteners I have 1/4"-20 screws with flat blade, Philips head, socket head cap, and round head cap heads from lengths 1/5" to 3", but none 1.25" long.  Yes, I could go to Ace Hardware and I may do so after lunch, it just upsets me to not have such a common size in my dozens of drawers.  I tried to use 1 1/2" long screws with a nut as a spacer, but nuts are only .15" long.  1" is too short; I want as many threads as I can holding this in place.  At least I can hope they have something shiny, so it goes with the finish of the dovetail.

Okay, I bought two 1 1/4" long stainless steel screws and they shrunk to 1" on the way home.  (Why aren't the drawers properly labeled.)  I will try again after exercise.  I put it together with 1 1/2" screws, nuts and lock washers.

This is the saddle of the mount:

This is the Vixen dovetail shoe:

This is adapter plate:

I started with a 4" long x 2" wide by .4375" piece of 6061, then edge milled the ends, and face milled the sides to get a parallelism that it did not really need.  It was now 3.97" x 1.988" x .4375".
  I drilled, countersunk, and tapped holes for the shoe (at y=.994" and x=1.3005" and 2.6695") and the bolts to hold the plate to the mount saddle (y=0.994". x=.534" and 3.435").  Because I would be attacking the end holes from the bottom, I suddenly realized that I needed the counterbore these holes as well.  Here is where I screwed up.  The bolts that held the scope rings to the saddle I had tapped as 1/4"-28.  I have no idea why.  Almost everything I do uses 1/4"-20 threads.  So instead of tapping only the bolt holes for the mount saddle (the two at the ends) 1/4"-28, I did all of them 1/4"-28.

Ace had 1/4"-28 flathead machine screws that held the dovetail shoe in place.  The bolts for the mount saddle did not need to be 1/4"-28 and finding 1/4"-28 socket head cap machine screws (SHCS) at Ace was hopeless.  So I retapped this holes 1/4"-20.  Ordinarily this is not wise, but the major diameter of 1/4"-20 and 1/4"-28 are not that much different.  No screw is going to pull out over a .002" difference.

I needed 1.25" long machine screws for this, and as previously mentioned, I could not find any in my shop.  I bought 1" long ones by mistake.  I ended up using 1.5" long fully threaded SHCS from my collection with a nut and a lockwasher to get the right length.

I really did not need to sand the faces to 1000 grit, but why not make it pretty?  The two ends that I edge milled were awful; I had used a coarse end mill and they were never going to be pretty.


The saddle is not flat against the adapter.  I should have started with a 1" thick piece of material and milled it to fit into the slot.   But it is not going anywhere.  This was fun, but doing it again, even in acetal, would not be enough fun to do this again.


  1. If you have longer screws, simply put a nut on them and then cut them down to whatever length you need then thread the nut back of to straighten the threads. . Seconds with a hacksaw, a bit of trimming and cleaning up with a file, and you have the custom screw you need.

  2. Just wondering why you didn't cut down a 1.5" screw to 1.25"

  3. Buy a hacksaw. Problem solved.

  4. Several suggested hacksaw. I have one, but threading the newly circumsized screw would be a hassle. Turning the nut back off is a good alternative, but unless the nut is harder than the screw...

  5. Nah, it's not a problem, I do it all the time. It's all in HOW you do it.