Lycoming 0-320 Exhaust studs working loose

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
figure that if I reuse that stud/nut again I am asking for it to work
back loose because it is no longer bottomed in the hole. But I just had
another one work loose in flight that was never disturbed from new.

I always wondered why we use plain steel studs and nuts for this
application anyway. It screams "corrosion". Why not heli-coil the port
threads and use a stainless bolt and lock-washer? That way, the threads
are protected from the elements to some degree and using stainless
hardware gives you a little better resistance to rot.

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

Thanks,
Mike

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
figure that if I reuse that stud/nut again I am asking for it to work
back loose because it is no longer bottomed in the hole. But I just had
another one work loose in flight that was never disturbed from new.

I always wondered why we use plain steel studs and nuts for this
application anyway. It screams "corrosion". Why not heli-coil the port
threads and use a stainless bolt and lock-washer? That way, the threads
are protected from the elements to some degree and using stainless
hardware gives you a little better resistance to rot.

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

Thanks,
Mike

They are cheaper. Helicoil is the standard repair.

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"
http://www.JayMasino.com
http://www.OceanCityAirport.com
http://www.oc-Adolfos.com

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...


--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

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

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.

Mike Spera wrote:



I always wondered why we use plain steel studs and nuts for this
application anyway. It screams "corrosion". Why not heli-coil the port
threads and use a stainless bolt and lock-washer? That way, the threads
are protected from the elements to some degree and using stainless
hardware gives you a little better resistance to rot.

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

Thanks,
Mike

It is not true that SS is superior to other steels for all corrosion
issues. When you realize that SS has to be in contact with oxygen for
it's corrosion resistant layer to be maintained, you start to appreciate
why corrosion can be a big big problem (especially in sealed threads or
places where acids can form...)

-see http://www.corrosion-doctors.org/

Add some electrical current and/or salt and you may have a very big
problem when using dissimilar metals... Bottom line: Keep similar metals
together if at all possible and if you can't, make sure you know which
one will corrode and which can be easily replaced. A mild steel stud can
be cut, have flats filed into it or even be drilled out to effect a
perfect repair. No so easy in SS (especially when the stud eats the
metal it's screwed into)!

my 2c

Cheers MarkC

DR wrote:
Mike Spera wrote:

I always wondered why we use plain steel studs and nuts for this
application anyway. It screams "corrosion". Why not heli-coil the port
threads and use a stainless bolt and lock-washer? That way, the
threads are protected from the elements to some degree and using
stainless hardware gives you a little better resistance to rot.

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

Thanks,
Mike


It is not true that SS is superior to other steels for all corrosion
issues. When you realize that SS has to be in contact with oxygen for
it's corrosion resistant layer to be maintained, you start to appreciate
why corrosion can be a big big problem (especially in sealed threads or
places where acids can form...)

I wonder why all the stainless fasteners in my airplane last for tens of
years without corroding solid at the threads while plain steel lasts a
fraction of that time and rusts solid? Any ideas why?

-see http://www.corrosion-doctors.org/

Add some electrical current and/or salt and you may have a very big
problem when using dissimilar metals... Bottom line: Keep similar metals
together if at all possible and if you can't, make sure you know which
one will corrode and which can be easily replaced. A mild steel stud can
be cut, have flats filed into it or even be drilled out to effect a
perfect repair. No so easy in SS (especially when the stud eats the
metal it's screwed into)!


Well, you already have dissimilar metals. The head is an aluminum alloy
and the studs are steel. As for removal, stainless requires a lot slower
cutting speed. Most folks ignore this and blaze away with a HS steel
bit. The bit dulls in seconds and the person doing the drilling thinks
stainless is "harder" cuz it takes a long time to drill anything with a
dull bit.

Thanks for the feedback. I'm still wondering if this is really all about
the money and plain steel hardware is cheaper and thus desirable from
the engine maker's point of view. I don't recall ever seeing SS exhaust
studs on the market. As you pointed out, there may be good reason why
they are not used. On the other hand, if the reason is that someone is
trying to save a few pennies by using plain steel, that would be a shame.

my 2c

Cheers MarkC