Source for 4-40 acorn nuts (flexible staybolt caps)?

[amadlinger]

Hi all,

I am in need of low-crown 4-40 acorn nuts to use as flexible staybolt caps for backhead/throat sheet/side sheet wrappers, as Jack and others have done. The ones from McMaster, and several other suppliers, are all too tall...0.250". Several suppliers *say* they offer low-crown versions that are only 0.200" according to their spec sheets (which would be perfect), but when they double check the actual nuts they are 0.250"+. I have gone through this with at least 6 vendors so far. Very frustrating.

Does anyone have a confirmed source for 4-40 low crown acorn nuts?

P.S. This is for my 1.5" scale, 7.25" gauge B&M K8b 2-8-0.

Sincerely,
Adam

[NP317]

Nope. Don't know of a source.
I used the .25" tall ones for my fake staybolt caps.
No one has complained yet...
RussN

[Dick_Morris]

McMaster-Carr says depth of thread on brass is 3/32" and it's 5/32" for steel 4-40 nuts. There might be enough material to take .050 off of the top. Use a form tool and a threaded stud in the lathe. A chemical black coloring would make the brass look right.

You could make them from 1/4" hex. The short thread depth into a blind hole might be a challenge. Maybe you could use a bottoming tap with any point on the end ground off in a guided tap holder that you feed by hand. Hold the tap against rotation with a knurled shaft that you can release so that it spins freely just before the tap breaks. A single form tool could put the dome on and do the cut off.

[Fender]

When comparing the full-size staybolt caps (shown in Flannery 1938 literature) to the 4-40 nuts, the most significant difference is with the height of the hex, not the height of the dome. So, you could probably trim off the bottom of the hex with a cutoff tool in the lathe (using an arbor with 4-40 threads on the end) to get a better proportion. See screenshots below.

[Fender]

The diagrams for cap nuts at Bolt Depot look promising, but they don't list any actual dimensions for their nuts.

[amadlinger]

Hi all,

I sincerely appreciate all the thoughts and suggestions! I have actually tried turning down commercial acorn nuts to a more appropriate size, but I found that the thread depth spec of 3/32" is a bit overstated (threads are shallower in actuality) but the tap drill hole is deeper than expected. The result is that you can only take off maybe 0.020" on either the hex or crown ends without running into trouble. Since I need qty (264) of these stinkers, it just doesn't seem worth the trouble. If only I had Trainman's Hardinge DSMA I'd be all set!

Dan, Bolt Depot was actually one of the six vendors I tried, but they confirmed the 0.250" height on their stock.

At the moment, I am inclined to just use the 0.250" nuts as-is, as Russ suggested.

Many thanks again; any and all thoughts/suggestions are most welcome.

Sincerely,
Adam

[Mike Walsh]

If only I had Trainman's Hardinge DSMA I'd be all set!

Might be worth looking into seeing what it'd cost for a custom run. I would think that if these came out well, you might have an opportunity to make a few bucks...

[Bill Shields]

If you want shallow with threads to the bottom find someone who knows how to thread mill in a swiss type of lathe. This way you need zero chip clearance for the tap that is not being used..and give a full thread all the way to the bottom.

This is generally not done on mass production items such as you need...but will get the job done.

Setup will cost you $ but if you make them from leaded hex bar the items will cost a few cents on a production run of maybe 1000.

[Harold_V]

No reason they can't be made on a manual machine, using a free machining material. The thread could be generated with a thread forming tap instead of cut. A tapping head would make that easy. The crown would be machined with a simple form tool, readily ground from a HSS blank. In reality, the hardest part of doing it is finding the desired material.

Making parts like this is an excellent way to become familiar with simple production techniques, where one works to the dial settings, or to DRO settings. Nice part is, this can be done on one's own equipment....no need to find any particular machinery.

Just sayin'

H

[Mike McCotter]

This is how I made them years ago(1993). By measuring the actual staybolt, I machined the correct size hex stock to the scale dimensions, drilled a 1/8" hole and by using a I/8" steel rivet, tig-tacked the rivet in the firebox sheet.

[Fender]

Mike, that is a really nifty solution!

[Berkman]

Mike that is some true dedication to literal rivet counting right there! I'd probably have a panic attack half way through making the 300 hundred needed and go buy a GP9.

Always enjoy seeing pics of your mikado though! Incredible detail!

What does Jack use ? I'd start there.