Yep ! I agree, it’s a special self tapper with torx type head. Noel.

Is it one of those Taptite tri-lobal self-tappers? For example, these?

Rob

Links to https://www.jcfasteners.com/products/machine-screws/taptite-screws/

Edit – typing at the same time.

As Stihl are German, the thread could be some obscure DIN standard. It is not much to go on, but some focussed searching (DIN self tapping thread forming metal for example) might find it.

Regarding “The closest approximation of the pitch I’ve found is M5x1.25 (and it is close), but the helix angle appears to be different from a metric pitch thread”

It stands to reason that the helix angle would be different to anything you have ever seen because you never normally see a 1.25mm pitch on a 5mm nominal diameter. It will be a steeper helix than any ISO metric coarse that exists.

Drive it into a piece of wood and measure the difference in projection from the surface after one turn if a standard thread gauge is not suitable.

Bill, The EJOT is new to me as well. From your link, I think that is what it is. 33 thread angle of the bolt itself. Not enough to go on for the section, but looks to me like that is more space than standard. It also looks to be a harder material than a standard bolt , as it forms it’s own rolled thread. Maybe the reason for it being 35deg included thread angle, to lower the volume of material being pushed around and why it has a decent root radius in the thread form.

You could grind a form tool using an existing screw for the gauge.

I would not be surprised if it is actually 20 tpi pitch, instead of 1.25mm.  I know that with castings, a few manufacturers prefer the courser UNC fasteners.  So it is very possible that they have created a #10-20 tpi screw specially so that you have to use the genuine parts.

I have been lucky to not loose any screws off my Stihl saw so far.

Will keep this in mind if I do, and will buy a couple of spares , so that if it ever happens, will have some.

If you were experimenting with determining the correct pitch, clamp two pieces of wood together and drive the screw in the joint. Drive your own one into the same hole. Unclamp the pieces and look at the cross section. You would see any mismatch and might be able to determine if your own is slower or faster than the original.

Maybe if you put your camera/phone in a holder and take a super close up, square on photo against a piece of 1mm squared graph paper, you could use optical methods to give you a starting point. Cut a hole in the graph paper so the screw head drops into it and the shaft rests directly on and parallel to the surface.

Is it possible that it is total special, two possibilities come to mind

1) 5 mm dia with a 33 degree thread x 1.25 mm pitch

2) 5 mm dia with a 33 degree form but 20 tpi pitch

Until I read the 33 degree thread form, I even wondered if it was  an undersized 1/4 BSW, or a 20 tpi Whit thread cut on a 5mm shank.

The coarse thread would be good to use in Aluminium castings.

Being unlike any obvious standard it could be purely to retain aftermarket business, and prevent “amateurs” from servicing outside the dealer network. (Not unknown, The Bosch EPVE injection pump had large triangular fittings which could only be take a socket, with only one source of supply!

A Japanese injection pump supplier used fasteners with 5 sided heads, presunably with a similar object

Commerce over ruling engineering?

Howard

The tamper proofing argument doesn’t really work since specialty threads won’t prevent disassembly. Sure it would prevent people from replacing bolts but that’s not a common failure mode in my experience.