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.
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 one of the metals 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 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.
18-8-3 Stainless steel, type 316 (passive)
18-8 Stainless steel, type 304 (passive)
Stainless steel, type 316 (active)
Stainless steel, type 304 (active)
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. Stainless steel and cadmium plating are used to reduce the metal dissimilarities with aluminum and magnesium. Paint and coatings are used to prevent metals from touching.
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. The aluminum anti-seize compound is for use between (say) stainless steel and magnesium.
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 a lubricant 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 a approximately 10% lower value to avoid over-tensioning the fastener. A new torque wrench is probably accurate to ± 3%.
As always Sire an excellent and timely post.
We use products called Copper-slip and Nickel-ease in the chemical plant I now work at. Nickel-ease especially in petrochemical applications.
Copperslip works well for automotive use the only draw back being if it gets on clothing its very, very difficult to get the stain out - but worth the effort.
I'll show the article to my wife as proof that I need to continue buying all the stainless fasteners going for my Duke :lol:
Cheers - Frank
[Edited on 20-3-2004 by Iconic944ss]
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