Drilled Bolts for Safety-Wire

["John Stricker" <jstricke@domain.elided>]

A couple of digests ago, Joe took issue with drilled bolts for safety wire.
This is commonly done in aviation and if done properly will NOT weaken the
bolt whatsoever.  Following is a copy of some information I sent to Joe
earlier to explain how, and why it's done.

If a bolt head is properly drilled, the change in strength of the bolt is
insignificant.  In addition to this, it is a REQUIREMENT on any aircraft
bolt deemed critical to safety of flight to be safety-wired.  (which, BTW,
is most all of them)  How do you properly drill it?  With a fixture and a
very small hole (a number 60 drill bit works well for .032 stainless).  The
most common size is .032 dia, but you can get it as small as .020 and as
large as .041.  If I recall, the MilSpec number is MS20995C for stainless,
with a dash number of the diameter.

It's not uncommon to have AN spec bolts drilled three ways, once through
each flat.  The strength of these bolts is roughly equivalent to grade 5
bolts, with comparable torque (but not identical) limits for the bolts,
drilled or undrilled.

While I'm on the subject, there is a right and wrong way to tighten and
torque and install bolts.  Bolt heads should always be on the top or facing
the prevailing airflow or direction of travel.  This is to keep them from
falling out for as long as possible if the nut should back off.  If you
torque a nut/bolt combination, always hold the bolt stationary and torque
the nut.  Unless specified, threads should be clean and dry.  You also have
to account for friction drag in a/c on the torque, but for use in
automobiles and truck this is accounted for in the torque specifications.
Never, ever, reuse cotter pins or safety wire.

All of this is off the top of my head, I'm sure I've forgotten some things
that I take for granted.  If you REALLY want to learn about bolts and what
is and is not acceptable practice, order a copy of AC 43.13 from the FAA
(that's the Advisory circular of Acceptable Methods, Techniques, and
Practices of Aircraft Inspection, Repair, & Alterations)

When you look at the high dollar race cars and see all the nifty little
fasteners, safety wire, etc. and wonder where they came from, they came
straight from this book.  (At least the methods did)  Why??  Because over
the last 80 years or so of aviation these are the methods that keep things
from falling apart when you really don't want them to.

BTW, this book will also tell you how to test for the hardness of metals,
how to stress relieve and normalize welds, basic electrical information
(particularly in wire mounting, support, and connections), and a host of
other items.

I know there are a couple of pilots on the list that can verify the bulk of
this and I can assure you that they wouldn't fly in an aircraft that wasn't
properly safety-wired on all the critical fasteners.

John Stricker

jstricke@domain.elided

"I didn't spend all these years getting to the top of the food chain
just to become a vegetarian"