H in t s F or H o m e b uil d e r s
Blind Fastener Tips
By Cy Galley
There are two popular brands of blind fasteners: Rivnuts
and Nutserts. They are made of aluminum and set like a
pop rivet from one side, providing threads to fasten or hold
an item like a fairing or access panel. They work fine for a
season or two, and then, instead of being able to remove
the machine screw, you'll find the body of the fastener
turns with the screw. You can't get the screw out.
When this happens, you might be able to drill the head of
the machine screw off. This can be a laborious and frustrating experience, teaching you many new unprintable
words. Even the Rivnuts with a keyway are not immune to
this problem. Many builders just swear off using them by
using steel nutplates if at all possible. But why does such
a neat little fastener give us such problems, and how can
the problem of the body spinning be reduced?
Rivnut
Rivnut with key
My SWAG is dissimilar metal corrosion between the
aluminum structure and the machine screw. This corrosion locks the machine screw to the body so that the
body turns instead of the screw. The torque of turning
out the screw is greater than the grip of the body to
the structure.
We can increase this grip of the body in several
ways. Make sure to file the notch so it seats if the
body has a keyway. Or, placing a drop of epoxy or
red Loctite on the outside of the body before setting
will help. Use a reamer or step drill so the hole is clean
and round.
To combat the corrosion, use a wax like Boeshield,
candle wax, or an anti-seize on the screw. Use one of
these every time you remove and replace the screw.
Nutsert
Finally, we can minimize the corrosion by selecting a
machine screw that doesn't react as much with aluminum. How many times do you see stainless steel screws
used with nice shiny stainless Tinnerman recessed
washers to hold on a fairing? It sure looks nice, but
there is a higher voltage potential between the aluminum and stainless steel than with cad-plated screws.
This difference is like battery acid and creates galvanic
corrosion, and the corrosion causes the screw to bind,
which creates the problem.
The choice is yours.
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