Special Screw - not "ooh err", more "oh rats"

[Cutter]

The so-called (by Ducati) "special screw" locates the rod-end bearing on the end of the suspension arch into the bracket on the swinging arm (this is on first-gen monsters). It's a pin with a hex cap on one end and a threaded portion on the other that takes a nut. Haynes gives torque as 22-24nm. The right hand one tightened up no problem. The left hand one tightened then failed (capped hex end snapped off).

I ordered another and tested my torque wrench which is a Draper I've had about a year. Second try and well before the click the head of the special screw starts to deform.

This is getting depressing - anyone got any ideas? Am I naive to trust Haynes torque figures?

[utopia]

I've always found Haynes manuals to be reliable, and that torque sounds about right. It certainly shouldn't be enough to shear the bolt anyway.
Exactly where was the head starting to deform the second time? If it was the hex socket itself, then I'd suspect a worn or badly fitting allen key.
If, however, the bolt itself was deforming, ie starting to shear, then I can only think that it must be overstressed by too much torque. How did you test the wrench. And are you certain that its set on the correct scale, ie Nm/ft.lb. etc ?
I think its an 8mm bolt...it should be nowhere near shearing at 22/24 Nm. Thats only "comfortably tight" (no comment) with a std spanner.

[Cutter]

Thanks for commenting, I appreciate it. Testing wrench was seat of the pants on something I didn't mind overtightening, but I trust my pants if you know what I mean. Also deffo on Nm scale. Inch/Pound is on other side of wrench and figures in different format.
These pins seem to be made of cheese. I threw away the first one when the hex cap end sheared off, but if memory serves me right it was hollow - need to get it out of the chickenshack bin and check.
The second one just started to bend at the cap as it tried to pull itself through the bracket.That's what popped it off on the first one. It wasn't the hex aperture but the cap that surrounds it, if you see what I mean. To explain further the force was applied to the nut with the torque wrench while I prevented the hex end from turning with a small T-bar allen key. Bit worrying when you think these bolts/pins/special screws secure the swinging arm to the rear suspension. But then they are not put under shearing strain doing that job.
I guess I know the answer. Buy another one and do it up using my internal torque wrench, the nut is a self-locker so I don't think it's critical as long as it's reasonably tight. Bit of a pain though...

[crust]

I had one shear and replaced it with a stainless bolt as a temporary measure.

A few years later I spotted its replacement still in the bag in the garage, ho hum.

[Cutter]

Well now you mention it maybe something suitable along those lines is not such a bad idea...:devil:

[utopia]

Hmmm. ..now I'm curious enough to want to take one of mine out and have a closer look.....but then again, it might shear when I replace it.............:cry:
I'll have a closer look tomorrow, but now I'm thinking that the bolt head is maybe quite shallow, such that the depth of the hex socket goes beyond the head itself and into the shank, leaving not a lot of metal left to connect the two...?
If that is the weak point, then a smaller hex size would help, but I think its quite small anyway. A philips would be too arkward, and an external hex probably too big cos there's not much room. I wonder whether a torx head would be stronger, or available in a smaller socket size.
Yep, I'm gonna have to have a closer look now.
I might nip down the bolt shop first though, and see what they've got.

Internal torque wrench sounds like the best plan.

[Dookbob]

I just took a look at mine, I have removed and replaced quite a lot of these without the problem that you are having. An 8mm bolt requires between 22 and 25 nm of torque, so that is not the problem. Utopia has a good point about the head depth in relation to the internal depth of the hex. I wonder if the bolt would lock up enough to torque the nut if the suspension was loaded by someone sitting on the bike while you tightened it. Failing that, and unless someone has the answer, I would turn down the head of a high tensile bolt, reducing the depth of the hex. head as shallow as possible to still get a spanner on it.

[Dookbob]

Just thought of one more thing, if the two rose joints are not screwed into the arc brace ends exactly the same amount, (even half a turn is too much) then you would require more torque to pull the bolt into place before the torque is applied. Clutching at straws here , but the wrong answer often leads someone to the right one eventually.

[utopia]

... Clutching at straws here , but the wrong answer often leads someone to the right one eventually.

Top marks for that point of view.

It lead off at a tangent, as usual......
1) You wouldn't want to fit any old replacement bolt. I still haven't looked (lovely day, went fishing), but I imagine the std bolt has a plain shoulder as a journal for the rose joint. A replacement bolt threaded all the way to the head would probably create some undesireable sloppiness in the suspension linkage, and...
2) without the plain shoulder, the thread would act as a stress-raiser on the already flimsy bolt head, whereas the shoulder shifts this further along the bolt.

So, maybe its...high-grade, stainless, shouldered bolt with thinned-down, hex head.

Looks good for the river again tomorrow, though.
(two fat chub on homemade spicy cheese-paste, btw).

[Cutter]

Thanks to all for replies. Just to say that as far as I can tell the distortion is not radial around the hex aperture. It's more as though the cap/shoulder (?) starts to fold up as it can't resist the force trying to pull the bolt through the bracket towards the nut. Then it twists off. However, as I mentioned before I think I will order another one and do it up firm, but not too firm, according to my biological torque wrench. Once bike up and running will keep an eye on it.

Now I am fed up because powdercoating started peeling off cycle cat top yoke last night!! :banghead: Will talk to powder coaters re free re-coat and maybe key surface first this time.

Still project finally gathering speed (in a snail like way) - see http://chickenshackducatiproject.blogspot.com if interested.
Thanks again all.

[Dookbob]

I remember now why I havn,t had this problem before. I dont use the torque wrench on them, its obvious when looking at it that you wouldn,t need to stretch this bolt by torqueing, the retaining nut is a lock nut so it isn,t going anywhere once it is tightened with a good tug on a shortish spanner, I use a 1/4" drive ratchet on them to avoid over tightening. The plain shoulder on the new bolt that you tried would only have to be very slightly short to cause the torque to pull on the thin bolt head instead of pulling on the threaded portion only.

[Cutter]

Makes sense:thumbsup: