Tri-lobular or Taptite is what you have.

Apparently you can also get pentalobular screws. Wikipedia article which introduces the wonderful new (to me) word – lobulation.

John

Andrew beat me to it

Edited By Journeyman on 07/10/2019 15:56:34

They are a cost effective method of screwing directly into holes which are in castings of zinc or aluminium alloys and which have a draft angle, not being drilled. Generally, they are not intended to ever be unscrewed.

When the proper GKN Taptite screws are removed they leave a thread amply good enough to receive a normal screw of the same nominal size. Some of the more economically priced brands don't. Good ones work just fine in drilled holes, albeit larger than normal tapping size. Aluminium alloys or ordinary mild steel. I have a box of M5 ones that call out 4.5 mm for drilled holes. I ought to have the full list of drill sizes for original GKN Taptite screws somewhere. But I've not seen it for 25 years or so!

If you simply screw a taptite in the resulting thread is a bit distorted and tight on the screw. Effective inherent self locking. Especially in plastic. Removing it cleans up the thread so the self locking is lost if you re-fit it. The thread is looser too and maximum torque reduced. Using a normal screw brings the torque / strength thing back up but the self locking effect is largely lost. Best to add a drop of screw lock.

Clive

Edited By Clive Foster on 07/10/2019 22:27:31

Hi Neil,

Diecast box screws, including Taptites are normally M3.5. This odd size appears to be for compatability with 4BA which was used on the original Eddystone boxes (and electical "back" boxes)

This rather than the tri-lobular form is likely why ir seemed small for a M4.

Robert G8RPI.

If the box is Hammond or of USA origin its most likely a UNC thread

Ian P

While most Hammod boxes are 6-32 UNC and tapped holes, not all are

https://www.hammfg.com/electronics/small-case/diecast/eddystone

6-32 is 3.5mm major diameter but coarser pitch than metric but a taptite will fit…….

Robert G8RPI

Taptites were typically used for cost reduction ,to save the tapping and subsequent inspection of tapped holes,best results are when used in ductile materials,used on very high volume commercial work ie aluminium pressure die castings with minimal draft on the cored holes,saves drillng and tapping. roll thread taps have similar profile with minimal swarf produced and a stronger thread form again works better on wrought than cast material.