Quick question... Are there any circumstances where it is unsafe to replace OEM bolts with A4 grade stainless? TIA
Yes, as I understand it, brake caliper bolts, studs and nuts for the exhaust ports and anywhere else where a bolt is subject to a shearing force as opposed to a tensional force.
You should probably get familiar with the principles of galvanic corrosion to better understand where and why Ducati uses zinc plated steel fasteners (and purposely not stainless) to sacrifice the plating to corrosion instead of the expensive aluminum parts that they thread into. One of the design considerations for a motorcycle is corrosion resistance. So when you disassemble it, make changes or replace components, and reassemble it, you need to be aware of a few things. Galvanic Corrosion The manufacturer uses a number of different metal alloys, plastics, and coatings - each selected for its cost, weight, strength, appearance, and corrosion resistance among other things. What also needed to be considered, is that when any two different metals touch each other, electricity flows between them (which is how a battery works), and the surface of the metal lower on the list (below) corrodes. For example, when aluminum or magnesium are in contact with carbon or stainless steel, this galvanic action will corrode the aluminum or magnesium. So the approach is to use steel fasteners to fasten steel parts together, whenever possible. One problem is that aluminum fasteners aren’t very strong, so aluminum parts are held with steel fasteners, but in special ways to reduce corrosion. Carbon steel bolts threaded directly into aluminum is generally avoided, for example. Here’s a list of some commonly used metals. The farther apart (top to bottom) on the list the two materials are, the more corrosion that will occur to the material lower on the list when they are held in contact. Gold Graphite Silver Stainless steel, type 316 Titanium Nickel (passive) Silver solder Bronze Copper Brass Tin Lead Cast iron Mild steel Aluminum Cadmium Galvanized steel Zinc (often used as a sacrificial anode in marine environments) Magnesium One way to control this galvanic corrosion is to use metals closer to each other in the above list, or by electrically isolating metals from each other. Zinc plating of steel fasteners for example, is used to reduce the metal dissimilarities with aluminum and magnesium. Paint and coatings are used to prevent metals from touching. Keeping the two dissimilar metals dry will also slow the corrosion process but just the moisture in the air on a humid day is enough to cause a problem. Anti-Seize Products If a fastener won’t get disassembled for long periods of time, it’s a candidate for using an anti-seize compound during assembly. There are three formulations widely available based on copper, aluminum or nickel. The way anti-seize compounds work is by placing a third dissimilar metal between the two base metals. So the corrosion of a thread in a magnesium part caused by a titanium bolt is reduced by an intermediate copper-rich or nickel-rich thread coating grease. The aluminum anti-seize compound is for use between (say) stainless steel and magnesium. Torque Values If the same materials are being fastened together, then they are assembled dry to the manufacturer’s torque values - unless otherwise specified. In critical fasteners such as the axle nut that holds the rear wheel on superbikes, the spec calls for lubricating the threads prior to assembly. The torque spec assumes a lubricated thread. Read your manual. In general, a thread treated with either an anti-seize or regular grease requires a lower torque value (than a higher-friction dry thread) to create the same tension in the fastener. So, if you make a modification that changes a component material, such that anti-seize is now needed, you’ll need to torque the fastener to an approximately 10% lower value to avoid over-tensioning the fastener (according to Machinery's Handbook, 25th ed.). A new torque wrench is usually accurate to ± 3%. Vibration If a bolt is torqued to the specified value there’s no need for thread locking adhesives. When the manufacturer is designing a critical connection that will be subjected to vibration, a lock washer is incorporated to prevent loosening. So to sum-up, if you use titanium hardware to replace the zinc-plated steel hardware, you can develop worse corrosion problems. The zinc is sacrificial in the sense that it corrodes preferentially, thereby protecting aluminum and magnesium components assembled by/to it. It depends on how the titanium (in fact, any material) fastener is used. The above picture shows the galvanic corrosion of an aluminum plate (after just six months) caused by using a stainless steel screw. The stainless itself doesn’t corrode, it causes the aluminum to corrode. The corrosion using a titanium fastener will be worse than for stainless. In the above situation if you used a zinc-plated steel screw (like Ducati stock hardware) the zinc plating would corrode first, and in doing so, protect the aluminum part instead of the other way around. In fact, that’s what you’re seeing when you’re looking to replace that scruffy-looking hardware ... the inexpensive fastener’s plating protecting the expensive aluminum and magnesium parts. So, what I’m warning here is that if you replace your corroded fasteners with a corrosion-resistant (but more-dissimilar) material such as titanium or stainless steel, you can shift the corrosion to the aluminum or magnesium if the two dissimilar metals are touching. Anti-seize materials will help by inserting a third material that itself will corrode but only slow down the galvanic action.
In general you should not use stainless fixings where torque values are important. When stressed, stainless steel flows, to the point you cannot undo the nut from the bolt. At work we only use grade 8.8 plated fixings, except on one or two high torque applications where even higher grades are required.
If you choose to use ss or ti bolts you can get them DLC coated which does help considerably with galvanic corrosion issues.also helps prevent galling issues associated with ss
@Shazaam! wow, great post. I understoond the principle, but you managed to summarise the detail really well in your post. I hadn't considered the scale difference to be so critical in these applications. Even though these are really common issues on Watercooling loops e.g. in Computer kit, which I'm familiar with. Great post.
Well, that got some interesting replies, thanks all - especially @Shazaam! I didn't realise that the bolts were designed to corrode, which they certainly do. Long story short, I've gone off the idea of using stainless bolts and will stick to whatever Ducati made them of. I don't know if Ducati sell OEM spec bolts? If not where to purchase. I've got a lot of stuff from Westfield in the past. As well as stainless bolts they list "BZP 12.9" and "ZNFLK 12.9". Are either of these what I'm looking for? Thanks again.
For galvanic corrosion to be a issue a electrolyte has to be present. Salty water will do it, hence the need for sacrificial anodes in marine environments but on your motorbike that's living in a garage and never sees rain it's not a thing. I use lots of titanium bolts and carbon on my bikes and have never seen the aluminium corrode away as it's been suggested. It's bollox to be frank.
Correct. If you live in a no-humidity desert environment nothing corrodes. But seaside salt air, a damp garage and repeated rain exposure can be a problem to varying degrees.
My personal experience is that if the fasteners appear on the official parts listing, they can be ordered from Ducati dealers, and are usually supplied in Ducati branded packages. However, I have not found them to be particularly cheap, but in some cases size, head type, thread form and LH or RH thread type makes paying more for the original items easier than searching for alternatives. Availability can be checked according to the bike model, using official parts listings such as the examples in this link: https://www.ducatiomaha.com/pages/ducati-oem-parts . Good luck, Tom.
Check on the tensile strength required, although harder than carbon steel. Stainless can be prone to stretch under load, leading to the fastener coming loose or worse breaking. If you are looking for shiny fasteners, go for plated in the same grade or titanium (expensive) In non structural applications ie casing covers SS will be fine.
I bought a stainless steel bolt kit for my 749 from these guys: https://a2stainless.co.uk/product-category/motorcycle-kits/european/ducati/ I've only used them decoratively, for easy visible spots—my timing belt covers have nice stainless bolts now!
@Shazaam! Just one thing to add, there is also ceramic based anti-seize. (according to the marketing) it pretty much stops galvanic corrosion as well as working at extreme temperatures, as even if the grease is cooked off, the ceramic stays behind and continues working. As for where to use stainless etc, the metallurgy is extremely important. If it's safety critical, I use whatever the manufacturer went with.
