[Ducati] Which 900 engines had the bad studs

Tue May 2 12:02:46 EDT 2006

Good info. I was not so familiar with fasteners designed to be used once, well everyone knows about locknuts being designed in many cases for single use but not bolts. In automobiles one would guess weight savings is not so much an issue. Do you think waisted bolts make for better torque to yield fasteners, meaning by design they can be made to work effectively easier than non waisted?  You would think stress and strain would be different in waisted. In motorcycles where gram conscious engineers brag about every gram saved and the aggregate of all parts can be pounds...maybe waisted is critical. I look at Honda fasteners for example and trimmed down fasteners are commonplace, contrasted to say Suzuki which seems to focus less on fastenrs and more elsewhere.   Clearly Honda looks at fasteners differently than the competition and has for some time. they are generally higher quality and have been for decades. I thikn they clearly waist in some cases to save weight and in
 others for torque.

Scott Cloninger <scloninger at astecinc.com> wrote:  Actually, many automotive companies routinely use "torque-to-yield" fasteners to secure heads and other critical connections as far back as the 1980s (the first ones I saw were Ford Escort head bolts). They don't have to be "waisted". 

I believe the use of such fasteners is strictly done to save money (smaller fasteners = less cost), but that doesn't diminish the fact that they work. I could imagine some geometric arrangements where smaller fasteners would be advantageous, but I don't believe that's why they're used in most situations. Also in aircraft or racing engines they are lighter than their non torque-to-yield brothers.

Without going too much into detail, the stress-strain curve of steel materials (that is applied stress versus deformation) is relatively linear to a point called the yield point. Once the material is stressed beyond the yield point permanent deformation in the material occurs, if the stress is removed the deformation beyond yield remains.

Generally, engineers design to remain well below this yield point. However, by designing to take fullest advantage of the material's loading properties (by approaching or even slightly exceeding the yield point), a fastener can be depended upon to carry a greater load. Smaller fasteners can take the place of larger ones and money can be saved.

Now for the bad news. Torque-to-yield fasteners are NEVER to be reused. Once disassembled, they must be replaced. Often when these fasteners are reused they will break upon tightening or, worse yet, will fail in service.

More bad news....the tightening process is critical when using these fasteners. The threads should be lightly oiled and a CALIBRATED torque wrench should be used. I don't believe the type of oil is critical, but I generally use a light air tool oil in my service efforts. Also, these assemblies should use a hearty flat washer between the nut (or the head of the bolt if it's not a stud) and the fastened member to avoid frictional losses at that interface. Tightening order is also critical. Most manuals will give a stepped tightening procedure with multiple torque wrench settings and an explicit tightening pattern.

Hope this helps clarify the situation.

Cloner
User of a CALIBRATED torque wrench.

-----Original Message-----
From: Rich Roberts [mailto:bigredxrunner at yahoo.com]
Sent: Monday, May 01, 2006 7:08 PM
To: Ducati Owners Group
Subject: RE: [Ducati] Which 900 engines had the bad studs

Uh okay...what does that mean, fastener is designed to stretch to give you the torque? Interesting, since threads are a big part of that torque as well as the amount at head, the thing that always scares me about such things is exactly how do you standardize the procedure of torquing, how much lube on threads, what kind of lube etc etc. As we all know lubrication changes things in a big way. Use too much or the wrong kind and torquing becomes an exercise in futility until that waisted section breaks. Do you work in the aviation mechanics field? Where do you see such fasteners routinely?

frugalyankee-cycles at yahoo.com wrote: Some of the 'waisted' fasteners I've worked with over the years are the 'stretch' type (torque to yield).....

Brad
CT.
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