AOPA Pilot Magazine - January 2020 - 91

P&E

SAVVY MAINTENANCE
Savvy Maintenance coverage
sponsored by AIRCRAFT SPRUCE

TIGHTENING A CYLINDER hold-down nut is

an extraordinarily critical task. Failure to do
it exactly right can result in catastrophic
engine failure.

OPINION

|

Tense bolts
When using threaded fasteners in tension,
it's all about the preload
BY MIKE BUSCH
THREADED FASTENERS are ubiquitous in
aviation. Look at any GA aircraft and you'll
find hundreds of them, if not thousands.
They attach wings to the fuselage, cylinders to the crankcase, connecting rods to
the crankshaft, and instruments and avionics to the panel. They hold on cowlings,
fairings, inspection plates, floorboards, and
just about anything else that might need
to be removed to gain maintenance access.
They're so numerous and so familiar that
we tend to take them for granted.
But, when used in safety-critical highstress applications-such as holding on
wings, cylinders, and connecting rods-
there's complexity to threaded fasteners that

often is not well understood or fully appreciated by the maintenance personnel who
are responsible for ensuring that they're
safe and secure. Mechanics often don't treat
these critical fasteners with the respect they
deserve. The result can be scary.
Threaded fasteners go by a variety of
names. As a general rule, they are called
bolts if they are designed to mate with one
or two threaded nuts, screws if they are
designed to mate with a threaded hole in
one of the items to be joined, and studs if
they are designed to mate with a nut on one
end and a threaded hole on the other end.
The cylinders on most Continental
and Lycoming engines are secured to the

crankcase by eight threaded fasteners: six
studs that thread into the crankcase casting
on one end and mate with threaded nuts on
the other end, plus two long through bolts
that pass all the way through the crankcase
and are secured by two threaded nuts.
This naming convention is often violated. For example, most machine screws
are designed to mate with nuts and so they
really should be called bolts. In this article,
I'll use the term bolted joint to refer to any
joint that is held together by any kind of
threaded fastener.
Bolted joints come in two basic flavors:
those where the bolt is loaded in tension
(i.e., along its axis) and those where the
bolt is loaded in shear (i.e., at right angles
to its axis). The wing bolts of Beechcraft
Bonanzas and Barons are loaded in tension, while the wing bolts of Cessnas and
Pipers are loaded in shear. The cylinder
hold-down studs and through bolts and
connecting rod bolts of Continental and
Lycoming engines are all loaded in tension.
This article will focus on tension joints;
we'll look at shear joints next month.
HOW TENSION JOINTS WORK

SCREW

BOLT

STUD
GENERALLY, thread-

ed fasteners are
called bolts if they
mate with a nut,
screws if they mate
with a threaded
hole, and studs if
they do both.

A bolted tension joint is one where loads
are trying to pull the joint apart with forces
parallel to the bolt's axis. The bolt is tightened in order to prevent those forces
from separating the joint. Think about the
bolted joint that attaches a cylinder to the
crankcase of a Continental or Lycoming
engine-combustion forces in the cylinder
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AOPA Pilot Magazine - January 2020

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