Screws for LittleArm 2C Arduino

I’m chasing down the extra screws needed to make the LittleArm 2C but I’m having issues finding the exact ones.

Here’s a list that I’ve found

  • Arduino Screws - Tapped, #4 x 1/4 screws
  • Horn Mounting Screws - Tapped, #0 x 5/16 screws
  • Gripper Screws - 2-56 x 3/4 machine screws
  • Gripper Nuts - 2-56, 3/16 WD, 1/16 HT
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Hi Aaron,

Sorry for the delay in getting back to you. I’ve had a look at the videos and it seems they’re just not using the correct English terms for these parts.

You’re actually after:

Screw sizes are a bit complex! There’s an awful lot of engineering that goes into the humble screw!

The Arduino Screw size is read as a “Number four by one-quarter inch long, self-tapping screw”.
The Gripper Screws are read as “Two dash fifty-six by three-quarter inch long machine screws”, while the Gripper nut size is read as “two dash fifty-six nut, three-sixteenths of an inch across the flats, by one-sixteenth of an inch thick”.

These are US (Imperial) sizes, whereas most other countries in the world tend to use Metric sized fasteners (although Imperial fasteners are still quite widely available.

Breaking it down:
#4” and “2-” are the nominal diameter of the screw ie. the name of that size of screw. It’s roughly the same as the real physical diameter, but when you get down to the nitty-gritty you’ll find there are quite a few different diameters of different parts of a screw and its nut.

Number sizes are very small imperial sizes. There are mathematical formulae used to determine all the dimensions of a Number size screw, and the name Number comes from the number used in the formulae.
Confusingly, these are also known and sold as Gauge sizes (G) - but this is actually incorrect as gauge is something else, related to wire drawing, and the sizing works in the opposite direction (where a higher number means the wire was drawn through more gauges and is therefore a smaller size).

Dash sizes are the next smallest imperial size of screw. Dash sizes are the diameter of the screw (or other part) in 16ths of an inch, so a 2 dash screw is 2/16" = 1/8" (note that " means inches).

Thread Count/Pitch
The “56” is the thread count, and refers to how many threads per inch (TPI) there are on the screw. Metric standards usually use the actual distance (or pitch) between two threads instead (eg. 1.5mm).

The length of a screw is measured from the fastening suface:

Other features
Tapping is the act of cutting a female thread (Bolts have male threads, nuts have female threads). A self-tapping screw is one that cuts a thread into the workpiece as you install it.

A machine screw generally refers to a bolt like threaded fastener, which conforms to either the UN, BSW, or ISO thread standards used on bolts, but is entirely threaded, unlike a bolt which usually has an unthreaded portion of the shank (particularly the case for longer bolts).

Finally, AF (or Across the Flats) refers to the distance between two opposing flat surfaces:

Thick refers to the dimension m in the diagram above. A stanard 2-56 UNC Hex Nut is 1/16" thick, so it’s not usually specified unless it’s important to the application (critical applications may require more back-up threads, or other specialty nuts such as Nyloc anti-vibration nuts may have different dimensions).

I’m glad my degree was at least a little bit useful!

Here’s a really good resource:


Thank you very much Oliver. I will look into getting these.


2-56 is an American UNC screw. In the electronics/communications industry UNC seemed to be used for a short time during the interim period between BA (British Association) threads and metric.
Disregarding thread pitches I think the diameters approximated as follows. (I think, long time ago)
8BA = 2-56 = M2
6BA = 4-40 = M3
4BA = 6-32 = M4
2BA = 8-32 (UNF) = M5
0BA = 10-32 = M6
3BA and 5BA were not quite as common but used fairly extensively in switch assemblies (Oak etc)
It was a bit of a pain actually as the UNC nut sizes were physically larger than the BA counterpart so if a part was designed to accommodate a BA nut it caused no end of strife trying to use a UNC nut. Screw heads were the same but not as pronounced as the nuts and were usually able to get away with the size difference.
If the designated fastener for this project is a 2-56 screw and nut you could probably use M2.
Cheers Bob
PS: I think the retaining screws and nuts for a lot of “D” connectors are still 4-40 thread. Can be M3 (I think Sony were the first M3 I came across) Some of the Male/Female screw/nut parts of these retainers are both, 4-40 male and M3 female and vice versa.

PPS: From memory I think the countersink angle was different too. 82 Deg as against the usual 90. May be wrong here but there was some difference.


With the Horn Mounting Screws - #0 x 5/16" Self Tapping screws, I just contacted that company and it looks like they don’t have them. I’ll have to see if I can find them somewhere else.

don’t they come with the kit???
Cheers Bob

Hi Bob,

You buy the 3d printing files from them and source the rest of the parts yourself.

#0 x 5/16 screw. looks like they all come from overseas. Found these dimensions. Surely must be an equivalent locally.
Sizes will be in inches. You will have to convert.
Cheers Bob

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Please excuse my Paint-Fu :sweat_smile: All to the nearest 0.01mm



Hi Aaron
Sorry about the pic quality, it was a screen capture.
The dimensions are probably too hard to read so will list them:
Head Dia. 0.114" = 2.89mm
Head height 0.044" = 1.12mm
Thread length 0.313" = 7.95mm
Thread Dia. 0.060" = 1.52mm
Hope this helps.
Cheers Bob
PS: Sorry, didn’t see Bryce’s post above. He has already converted.


Interestingly 1" = 25.4mm is not just a conversion, it’s the actual definition of an inch. Pounds too are defined in terms of the kg. Freedom units are actually metric!

Quite correct. Little sense in having heaps of standards so internationally the metre is the standard and imperial measurements are derived from that.
The history of the standard metre is a read in itself. But we are digressing from the original post. Anyone interested look up the Wikipediaq version of events.
Cheers Bob

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Hi all
A quick question. Are these screws going into plastic or similar material.
From past experience self tapping screws designed for sheet or thinner metal do not perform too well in plastic type materials. You might get away with it initially but the thread will strip out if removed and replaced. There are special screws which used to have a name but damned if I can think of it now. They are described at this site (but not named):

There may be an alternative, UNC thread forms pretty well in plastic materials if you have the correct size hole so if you can get some 2-56 screws you may be able to use these. Try in a piece of plastic first.
I fairly recently needed to replace some chrome (going rusty) ST screws with stainless. Problem they were #7. Almost impossible to get odd gauges in “normal” hardware store. Very close to 6-32 UNC so was able to successfully get the thread to form in the plastic and replaced with these screws. I don’t know how many times I could remove and replace before failure but good up to now.
Cheers Bob

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Thanks for the information all.

@Robert93820 They would be going into plastic.

Modification to above. Just checked diameters, 2-56 UNC is a bit larger than #0 screw. Nearest I can see would be 0-80 UNF which may or may not form the thread OK in your plastic. All you can do is try.
Available at:
They also have self tappers but only seem to go down to #2.
All the #0 x 5/16 I can find seem to come from USA but you would think somebody here would have them.
Core have a range of 2-56 screws of varying lengths, I suppose they have the nuts to match. search 2-56 screw.
Cheers Bob

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Another suggestion for #0 x 5/16 screws, you could try the hobby shops. They probably have lots of little screws going into plastic.
Cheers Bob

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Try this site:

They have 3 sizes of “Kaydee” screw, 0-48 x 1/8, 0-48 x 1/4 and ?? -?? x 3/8. They look like the type of screw that goes into plastic. The 3rd one does not have the size listed but if it is 0-48 that is what you are looking for. It is 1/16’’ too long but that may not matter. Only you will know this.
Hope this may solve your problem.
Cheers Bob