Threaded 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 — especially 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 mounting

I normally use socket head bolts and blind nuts for mounting plate type landing gear (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 either type.

Wood screws are used as their name suggest — to thread into wood.  They can also be threaded into non-brittle plastics, such as nylon.

Appropriate uses of wood screws:

• Attaching landing gear straps for music wire landing gear

• Attaching cowls

• 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 first place.

Head style

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 tool or the fastener are cheap.  Good drivers have hardened tips and are worth the investment.

There are five types of heads that I am aware of:

• Allen head
• Hex
• Japanese Industrial Standard
• Phillips
• Slot
• Torx

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 things happens.

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 having torx fasteners so I don't have anything to say about them other than that they exist.

Nuts

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 the engine).

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 compartment.

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 airplane.

Nylon hardware

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 about it.

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. Johnson