Lycoming 0-320 Exhaust studs working loose

Orval Fairbairn wrote:
In article et,
Mike Spera wrote:

I have had intermittent problems with 0-320 exhaust studs working loose,
usually with overhauled cylinders. I understand it if I have a stubborn
nut to remove for a gasket change and the stud comes out with it. I
SNIP

Any engineers know why studs are used on exhaust systems in general?


They are cheaper. Helicoil is the standard repair.

In addition, there are 0.010 oversized studs available to tighten up the
loose fit.

It would be interesting to hear from someone with metalurgic experience.
I wonder if stainless studs may not be as able to withstand the very
high heat that these studs experience?


--- Jay


--

Jay Masino "Home is where My critters are"
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From a mechanical engineering standpoint (and not necessarily
aircraft!)

Studs are generally used so that normal disassembly doesn't wear on a
non-replaceable soft part (i.e. the cyl head). Studs are stronger for
that reason. My personal preference with mechanical machine design is
to not use lockwashers as they perform quite poorly (such as breaking)
in a severe fatigue environment. The design of a threaded fastener
joint really is best if there is no flex in the joint, and the length
of the fastener is maximized and tightened to a preload sufficient to
keep the assembly locked together - such as in a metal propeller-
crankshaft interface. A marginal joint would be a cylinder base
(where the flange might flex some) or of course an exhaust flange.

Have you tried using high temerature antisieze paste? The stuff I've
used for general purpose is called "Neva-Sieze" as I recall. It is
good at keeping out corrosion even underwater and at high
temperatures. Be careful tightening up a nut with this stuff under it
as it is really slippery. I also use it on spark plugs but again be
sure to torque it only to the low limit.

Stainless has a bad tendency to gall with other metals and
particularily with itself. In addition. Anti-seize should be always
used with stainless for general work - although I don't know anything
about how the turbine stuff is maintained.

Helicoils are effective for getting good strong threads in soft
materials, but once you wreck it, there is no additional fix without
removing an unacceptable amount of parent material. I suspect that
(and $) is the reason they are not used in exhaust attachments.
Fortunately they do use them effectively in sparkplug holes as you
know.

nrp wrote:

From a mechanical engineering standpoint (and not necessarily
aircraft!)

Studs are generally used so that normal disassembly doesn't wear on a
non-replaceable soft part (i.e. the cyl head). Studs are stronger for
that reason.
I figured that was the reason. However, these studs corrode instantly
and make future removal difficult or impossible. So it would seem this
method is not accomplishing that. Many use various penetrating oils to
attempt to break the corroded nuts free. Although once you get the nut
off, you may see that the stud has corroded sufficiently that it is
unusable anyway. I have used a cutoff wheel on a Dremel to cut the
siezed nut off a couple of times.

..stuff snipped
Have you tried using high temerature antisieze paste?
. more stuff snipped
Never tried anti-sieze because I worry that the nuts would work loose.
Never talked to an aircraft wrench who used it and/or reported success.

.still more stuff snipped

Helicoils are effective for getting good strong threads in soft
materials, but once you wreck it, there is no additional fix without
removing an unacceptable amount of parent material. I suspect that
(and $) is the reason they are not used in exhaust attachments.
Fortunately they do use them effectively in sparkplug holes as you
know.
It is common for cylinder overhaulers to weld all the exhaust holes shut
and then redrill and tap them as part of the overhaul process.

Thanks for the feedback.
Mike

Mike Spera writes:

It is common for cylinder overhaulers to weld all the exhaust holes shut
and then redrill and tap them as part of the overhaul process.

I'm wondering how you fill up a stud hole with a welder. It strikes
me as a non-trivial task...


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Mike - I'm not an A&P but always used antiseize on my O-320E2D
Lycoming exhaust studs and small muffler bolts for 30 years without
any trouble with them loosening or backing out. Never even gave it a
thought.

I also used it on spark plugs (torquing to the low limit), Once I had
a little bit dribble over the end that required the plug to be removed
& washed to get it to fire.

The Lycoming studs have a slightly larger thread on the
head end and are an interference fit. They must be double-nutted and
driven all the way. If your studs are backing out they're likely the
wrong studs or the cylinders have been overhauled too many times.
We run six Lycs and have no trouble with studs coming out.
Of course, these are all factory overhauls and Lycoming won't reuse
cylinders.
The proper nuts are plated with somethng better than the
usual stuff and do not easily corrode. You might have plain AN coarse-
thread nuts instead of the Lycoming nuts.

Dan