Thread pitch of screws used on Stihl equipment

[Bill PhinnParticipant @billphinn90025]

The picture below shows three types of screw thread commonly used on Stihl equipment.

The screw on the right is just a standard M5x0.8. The one on the left is a screw for plastic of the Plas-Tech variety.

The one in the middle is the one that is a mystery to me. It is the most common type of thread used on Stihl machinery, and typically it is used whenever a screw enters an aluminium casting. I would like to know what it is because I would like to be able to reproduce that thread. Most of this type that I have come across have a major diameter of approx 5mm, and they are designated by Stihl with a D, (as opposed to P for the plastic ones).

They are relatively expensive to buy.

 

 

[bernard towersParticipant @bernardtowers37738]

They would be expensive and oddball thats to keep you hooked into the brand

[Bill PhinnParticipant @billphinn90025]

I’m not so sure that’s the reason, Bernard, in this case, though Stihl certainly do like to discourage servicing and repair outside of dealerships.

 

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.

 

I think it’s actually a good choice of thread for aluminium castings as it’s harder to cross-thread a fastener, and maybe it better resists stripping generally in aluminium than an M5 coarse would.

[vintagengineerParticipant @vintagengineer]

It looks like a self tapping bolt for alloy.

[noel shelleyParticipant @noelshelley55608]

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

[Bill PhinnParticipant @billphinn90025]

Thanks for the further replies.

It’s not a 5mm wood screw self-tapping pitch, nor a DIN 7500 thread forming pitch (which is the same as metric coarse but trilobular), nor is it a DIN 7504 self-drilling screw pitch.

So what is it? And how do I source it more economically or reproduce it myself?

P.S. the Torx head isn’t important to me, only the thread type.

[Grindstone CowboyParticipant @grindstonecowboy]

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.

 

[Bill PhinnParticipant @billphinn90025]

Sorry, Rob, no. Those are DIN 7500, as mentioned. I have plenty of those at 2-3p a screw.

[DC31kParticipant @dc31k]

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 PhinnParticipant @billphinn90025]

Thanks, DC31K.

I notice some of these screws are marked RIBE

[Bill PhinnParticipant @billphinn90025]

Others are marked EJOT

[Bill Phinn]

On Bill Phinn Said:

Thanks, DC31K.

I notice some of these screws are marked RIBE

You may have answered your own question there, Bill

MichaelG.

.

Ref. https://www.ebay.co.uk/itm/276652882350

[Neil LickfoldParticipant @neillickfold44316]

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.

[Neil LickfoldParticipant @neillickfold44316]

This page shows the offset of the thread angle, Head side is 22 deg from centreline, and the bottom side is 11 deg from centreline , creating a 33 deg thread form.

registration is required to get the actual information.

They claim that standard metric screws can be used after one of these screws has been used. Not so sure of course, as I have not registered.

https://www.ejot.co.uk/Industrial-Fasteners/Applications/General-Fabrications/Castings/ALtracs%C2%AE-Plus/p/VBT_ALTRACS_PLUS

[DC31kParticipant @dc31k]

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.

[Neil Lickfold]

On Neil Lickfold Said:

They claim that standard metric screws can be used after one of these screws has been used.

That claim could rule out Bill’s screw being made to that spec.

Thinking back to his original picture, there is no standard metric screw that would substitute for what he shows (and describes as M5 x 1.25 nominal). The smallest 1.25mm pitch is on an 8mm major diameter, 60% fatter than his.

[Howard LewisParticipant @howardlewis46836]

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

[Peter TuckerParticipant @petertucker86088]

I did once read that Germans tend to use Whitworth threads in aluminum.

Peter.

[FulmenParticipant @fulmen]

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.

[Bill PhinnParticipant @billphinn90025]

Thanks a lot for the further replies.

I’ve attached pics showing a 1.25 thread gauge on the screw and an M8 screw beside it.

There’s unlikely to be any tamper-proofing intentions here by the manufacturer. They’re regular Torx heads (T27), not Tamper Torx, and the self-tapping thread type is no doubt designed for speedy assembly on the production line combined with strong holding power.

I don’t know what to make of the claim that standard metric screws might be usable in the hole after one of these screws has been in it. I’ll have to try it; I’ve got an old cylinder block in the shed I could experiment on. However, the problem is, even if this works, you may then be forced to use (and remember to use) a different screw from normal ever after in that hole.

It would be good to have a stock of these screws so that I’ve got replacements or can easily do pressure/vac tests on engine/cylinder blocks and fit the necessary blanking plates and/or adapters without the need to resort to stand-offs or cutting screws down.

It looks, though, like I may have to just shell out after all on a dozen or so screws of various lengths at £1+ a pop.

[Bill PhinnParticipant @billphinn90025]

Neil, the pic below shows a UN 20TPI gauge on the screw.

The 1.25mm is definitely closer.

[Neil LickfoldParticipant @neillickfold44316]

Thanks for the pictures with the thread gauges. Definitely 1.25mm pitch and not 20 tpi at all.

Great to learn something new as well.

The self tapping ones, will be very good for soft materials that require fastening like Syntactic foam for example that  can not be glued with any great success.  When tapping it will often tear a thread as well. It is something that crops up from time to time at work. So will recommend them and see if they will get some.

The standard screws is where they claim that standard metric screws can be used.

Clearly, these are under their catalogue as customer specific fasteners that save them time, money and a repeat sale if someone tries to use a standard screw and ruins the case. Can always replace with helicoils or recoils of course and that is another option.

I worked at a place, and used to make a magnesium model engine crankcases. They had recoil inserts from day one to take the unc screws that were going to be used. The mag would strip out otherwise. But the extra area by the recoil inserts for a 3-48 screw was quite substantial area increase.

Then use the low head option in the socket cap head screws .

I think shelling out for the spares is the easiest and cheapest time option.

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