Fasteners used in Model Aircraft
Steel screws, nuts and bolts are used throughout the model. Many
kits come with soft steel fasteners that should be replaced
those used for mounting the engine to the mount or the mount to the
firewall. They are usually slot-head or Phillips-head and may be machine
screws or sheet metal screws.
The hardness of the steel varies considerably from low quality, soft
steel to stainless steel and titanium. I have never purchased a kit
that came with either of the latter choices though. I generally use
stainless steel hardware in the engine compartment.
There are four significant problems with soft steel bolts and screws:
- Heads strip easily making them very difficult to remove.
- Threads strip easily.
- Bolt can twist in half leaving the body of the bolt threaded in.
In some cases the body can be removed but in other cases the part the bolt
is threaded into must be replaced.
- Can shear apart due to vibration or impact shock.
Machine screws (bolts)
Often times imported kits and ARF's come with metric hardware.
Thanks to the proliferation of R/C cars and Helicopters, finding
replacement hardware is not as difficult as it once was. There was a
time when I would replace all metric hardware with imperial hardware so
that if I lost or damaged a fastener, I wouldn't have to make calls all
over the U.S. trying to find a replacement.
Appropriate uses of machine screws:
Mounting control horns using the nylon plate that is included with
the horn (wood screws are also appropriate).
Mounting bellcranks in an inaccessible area
using a lock nut. This is not a place to use a wood screw because
the constant force on the bellcrank will most likely cause it to rock
until the screw is ripped from the mount.
Bolting an engine mount to a firewall. Never use wood screws
here unless using a 1/2A engine (.049).
Mounting an engine to an engine mount of any
type. When using a plastic or metal mount, the mount can be
tapped. When using wood beam mounts, lock nuts should be used on
the opposite side.
More about engine
I normally use socket head bolts and blind nuts for mounting plate type
(dural, carbon fiber, etc.). That is just a personal preference. It is heavier, but
I do not have to worry about the threads in the wood stripping over
time due to oil making it is way into the holes (which it will).
I like to use stainless steel hardware around the engine compartment .
Again, I always use socket (Allen) head bolts because they allow the driver
to engage positively. One thing you will find is that you can not always
see what you are doing, so it is even easier for the driver to slip from a
slot-head fastener which is annoying and can potentially damage things.
Wood or sheet metal screws
For all practical purposes, wood screws and sheet metal screws are
interchangeable. In other words, when I say wood screw, I mean
Wood screws are used as their name suggest — to thread into wood.
They can also be threaded into non-brittle plastics, such as
Appropriate uses of wood screws:
Attaching landing gear straps for music wire landing gear
Attaching control horns to plywood or hardwood blocks in control
surfaces or to a nylon plate on the opposite side of a control surface
Mounting servos to a plastic or plywood tray or to hardwood rails
Mounting plate type landing gear (dural aluminum, fiberglass or
carbon fiber) to a plywood plate (I prefer
socket bolts with blind nuts)
Wood screws are sometimes included with nylon engine mounts. I
throw them away and replace them with machine bolts. The screws that
are included with nylon mounts are always soft steel and I have seen more
than my share with stripped heads and I have seen some shear off due to
engine vibration. It is a lot easier to use quality hardware in the
Always use a driver that fits the head properly. The main reason
fastener heads or tools get chewed up are because too small of a driver is used
particularly Phillips or Allen-head types. The second most common
reason is that the
the fastener are cheap.
drivers have hardened tips and are worth the investment.
There are five types of heads that I am aware of:
- Hex (Allen head)
- Japanese Industrial Standard
I avoid slot-head fasteners whenever possible. Drivers slip out
of these type heads much too easily which can cause damage to you, the
model, the fastener or the tool. Usually more than one of these
Phillips head and Japanese industrial standard (JIS) fasteners look the
same, but they aren't. I can't tell the difference by looking at
them. When I'm working on a model that was made in Japan, such as my
helicopter, then I try a JIS driver first. If it doesn't fit
properly then I try a Phillips driver. Ultimately I use whichever
driver engages the head most positively. Generally speaking you
should use the largest driver that will fit.
Allen head fasteners are my first choice because they tend to have the
most positive driver/head fit. That isn't so true in smaller sizes
though. For example, the set screws in wheel collars tend to round
off the tip of the driver because of the size and the fit. Again,
hardened drivers last much longer than cheaper tool steel types.
I have never had a model that torx fasteners so I don't have anything
to say about them other than that they exist.
There are three types of nuts that are most commonly used. Common
hex nuts, nylon insert lock nuts and blind nuts (also called T-nuts).
Hex nuts can be used almost anywhere that the nut is accessible.
However, lock nuts are a better choice in high vibration environments such
as the engine compartment (if using a bolt and nut arrangement to mount
I also like to use lock nuts in places that I do not normally inspect,
such as when mounting a servo in a hatch in the wing. Hex nuts could
be used here, but to be safe, a thread-locking compound should be used to
ensure the nut will not come loose.
Blind nuts are used for three reasons: When the nut is not
accessible, such as behind the firewall, when it is desirable to not have
protrusions or just because you can. For example, I often use blind nuts
with fuselage-mounted landing gear because using regular nuts would
require me to remove some of the radio to get at the nuts. Plus I
can cut the bolts flush with the nuts so there are no protrusions into the
You should always use a nut on a threaded rod when using a metal
clevis. The nut is used as a jam nut to prevent vibration from
causing the clevis to chew up the threads of the soft metal threaded rod.
This is not another of my obsessive compulsions. It is a real problem
and one that is easily solved. If the threads are damaged enough,
the clevis can slide off the rod and that will be the end of your
Bolted on wings almost always use nylon bolts. In addition to
being lighter and adequately strong, they are supposed to shear in a crash
and save some damage. Unfortunately, most planes are not built
properly to take advantage of this. First, the bolts used are almost
always larger than necessary. Second, the block in the fuselage that
the bolt threads into must be flush with the wing.
If there is a gap
between the wing and the block, the bolt will flex, but not break.
Generally it is a moot point, because even the times that the bolts do
break, there is still significant damage in a crash.
Some people also use nylon bolts to attach the landing gear.
Personally, I believe that their thinking is flawed. They claim that in a
hard landing the gear will break loose instead of ripping out the
underside of the fuselage. I use steel bolts and have plenty of hard
landings. I have yet to rip out the gear in a hard landing or break
the bolts. I have landed hard enough to flatten a gear, but that is
The guys I have seen who use nylon bolts have their gear pop loose as
planned and then the gear does damage to the plane after it is loose.
I do not see how that is an advantage, but to each their own.
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Copyright © 2004 Paul K.