AOPA Pilot Magazine - January 2020 - 94

P&E

SAVVY MAINTENANCE

to stretch. You can see this clearly in the
photo of Lycoming connecting rod bolts
and a crankcase through bolt on the preceding page. By making the bolts more
"stretchable" (i.e., less stiff ), the joint
becomes harder and the bolts feel less
stress when the joint is under load.
TIGHTENING THE BOLT

By far the most popular method of obtaining the necessary bolt preload is using
a calibrated torque wrench to tighten
the bolt to a specified torque value. This
method is simple and well understood by
every mechanic. Unfortunately, it's also a
very inaccurate method, a fact that is not
well understood by most mechanics.
The reason the torque method is so
inaccurate is that roughly 85 percent of
the torque applied by the torque wrench
is used to overcome friction, leaving only
about 15 percent to actually stretch the
bolt. One study showed that 47 percent of
the torque is dissipated overcoming friction between the heads of the bolt and nut
and the items being joined, and 38 percent
is dissipated overcoming friction between
the bolt and nut threads.

TIGHTENING TO TORQUE is the most common

USING THE TORQUE-ANGLE method in an

a preload of anywhere between 1,500 and
2,000 pounds. Clearly not a good situation.
The most accurate way to obtain a predictable preload is to measure the stretch
of the bolt. This is precisely how many
Lycoming connecting rod bolts are tightened. The bolt length is measured with a
special micrometer, then the nut is tightened until the micrometer shows that the
bolt has stretched by a specified amount.

an accuracy that's nearly as good as the
stretch method. Known variously as the
torque-turn or torque-angle method, it's
widely used for tightening critical fasteners on everything from race cars to diesel
locomotives. Inexplicably, however, it's
almost unheard of in GA maintenance, and
that's a shame.
Tightening a fastener using the torqueangle method involves two steps. First, a
torque wrench is used to tighten the fastener to a specified "snug torque" to ensure
that the joint is in metal-to-metal contact.
Second, the fastener is further tightened by
rotating it by a specified number of degrees
to obtain the final preload, something that's
easy to do by using an inexpensive torqueangle gauge. This step results in a precise
amount of stretch that is completely independent of friction, and therefore vastly
more accurate than using torque alone.
Catastrophic engine failures following
cylinder work are depressingly common
and are almost always attributed to inadequate fastener preload during cylinder
installation in the field. If Continental
and Lycoming encouraged mechanics to
use the torque-angle method to tighten
through bolts and hold-down studs instead
of the far less-accurate torque method, I'm
convinced that many of these failures could
be avoided. Why don't they?
AOPA

(but inaccurate) way to ensure proper bolt
preload.

The most popular method is using a calibrated
torque wrench to tighten the bolt to a specified
torque value. This method is simple and well
understood by every mechanic. Unfortunately,
it's also very inaccurate.
What's worse, these frictional losses
tend to be highly variable and difficult
to predict. They're profoundly affected
by the presence or absence of lubricant,
the presence or absence of plating, the
condition of the threads (new, worn,
damaged, or corroded, for instance),
the use of washers or flange-headed nuts
and bolts, and how many times the fasteners have been reused.
Studies have shown that when fasteners are tightened using the torque method,
the resulting preload may vary by as much
as 25 percent. Thus, tightening a bolt that
should have a preload of 2,000 pounds
using the torque method might result in
94 | AOPA PILOT January 2020

This stretch method is extremely accurate
and virtually guarantees that the resulting
preload is spot-on.
Unfortunately, the stretch method
is impractical for many kinds of tension
joints. It's impossible to use when tightening a screw or stud because one end
of the fastener is inaccessible, so the fastener length can't be measured accurately.
It's also infeasible when tightening engine
through bolts because a micrometer big
enough to bridge the entire width of the
crankcase doesn't exist. That's why the
less-accurate torque method is used.
However, a third method combines
the simplicity of the torque method with

automotive application. Why don't we do
this on our Continentals and Lycomings?

MIKE BUSCH is an A&P/IA.
EMAIL mike.busch@savvyaviator.com

savvyaviation.com
http://www.savvyaviation.com
AOPA Pilot Magazine - January 2020

Table of Contents for the Digital Edition of AOPA Pilot Magazine - January 2020
