Fasteners and fittings

There are two terrific references that can serve as a starting point for discussions of fasteners, both by Carroll Smith: Nuts, Bolts, Fasteners and Plumbing Handbook’ and Engineer to Win.  My purpose here is to discuss some of the specific fasteners I used and my experiences with them.

Fasteners photo album link

Rivets:

The rivets used in the construction of the Mk IV are ‘blind rivets’, so-called because access to the back side of the material being clamped is not accessible.  The rivet is composed of a tube with a mandrel through the center of the tube.  The rivet and mandrel can be made of different or similar materials, e.g., aluminum/aluminum or aluminum/steel.  Broadly speaking there are two types of pop rivets: structural and non-structural (i.e., trim).  In structural rivets, the mandrel extends through the shear plane for maximum strength: see drawing on right.

In non-structural rivets, the mandrel breaks off part way through the rivet body.  Thus, for structural rivets, the selection of the grip dimension is particularly important.  Structural rivets are substantially stronger in tension and shear than are trim rivets: 75 - 80% greater than trim rivets according to Smith.  Because the 6061 aluminum sub-chassis is intended to add strength to the overall chassis, I used Cherry-Q structural rivets for sub-chassis attachment:  BSPQ-42, BSPQ-46 and AAPQ-42.  

In the figure above, Cherry-Q structural rivets are compared to the Factory Five supplied rivets for clamping two sheets of 0.040 6061 Al together; below, right, for attaching the Al sheets to the square steel tubing.  (See also the fasteners photo album).  Cherry-Q rivets are structural rivets and the mandrel penetrates through the shear plane.

Rivnuts (also known as rivet nuts, nutserts or blind nut):

These are internally threaded tubular rivets that can be inserted to produce a threaded space anchored from one side.  Depending on the  Rivnut, it can also serve a function similar to the rivet: clamping two pieces of metal.

The third photo down in the right column shows the Rivnut placing tool that I used for 10-32 bolts (Pegasus Racing #659-3).  To the right of the tool is a Rivnut (e.g., Pegasus # 650-3-080).  

IMG_3093

A Rivnut is attached to the placement tool by threading the Rivnut on the 'bolt' portion of the tool (left) .  The nut spins freely on the placing tool’s hex-headed bolt.  When the nut is held from turning and the hex-headed bolt is rotated, the Rivnut ‘tube’ is pulled up against the nut, pinching any material in between.  The Rivnut is released by unscrewing the hex-headed bolt.

Note:  It’s relatively easy to over-tighten the Rivnut upon placement, stripping the Rivnut threads.  I found that one rotation of the hex-headed bolt after the tool was snug against the uppermost surface was sufficient for my needs.

Clecos:

Clecos are fasteners used in the place of rivets to temporarily hold parts in place.  Clecos are manufactured to fit specific hole sizes (e.g., 3/16”) and are put into place and removed using a Cleco pliers.  I purchased 40 1/8” and 20 3/16” Clecos (Summit: G1852 and G1853, respectively) and I’ve found that this number is about what is needed.  Cleco fasteners are tremendously useful and, I would say, ‘a must’.

Bolts:

A long and conplicated subject.  Smith's two books, referenced above, contain lots of science and opinion on bolts.  Two points are made here: 1) every bolt / nut combination has a correct torque spec; 2) regarding the orientation of bolts, from 'Nuts, bolts, fasteners and plumbing handbook' by Carroll Smith:

Myth: “Bolts must always be installed with the bolt head up and facing forward so that, if the nut should fall off, gravity and the force of the airstream will tend to keep the bolt in place.” There is nothing wrong with installing bolts in this manner.  In fact, in the interest of standardization, I usually do.  But to hope that gravity or air pressure will keep a bolt in place is unrealistic.  There is a place in the world for dreamers- but that place is not in engineering.  Neither is it anywhere near an airfield or a racetrack.  There is absolutely nothing wrong with installing bolts wrong end up or backwards, when it is more convenient to do so.’


A constant concern is the loss of a bolt’s clamping force due to vibration and other factors.  An interesting video on bolts loosening is from Nord-Lock, a maker of a type of washer meant to hinder nuts from unfastening.

Bolt descriptions:

Bolts (an externally-threaded, headed fastener used in conjunction with a nut) are described using the following nomenclature: major diameter-threadcount x length.  In Unified threads (measured in inches), bolt sizes (major diameters) are: #0 through #10, then sizes (diameters) in fractional inches (e.g., 1/4 inch).  Bolt sizes are even-numbered.  The Bolt Depot (boltdepot.com; supplemental technical information) has a number of informational pdfs on bolts, screws and other fasteners.

The bolt diameter (the major diameter) is almost always the 'thread diameter' (the figure below is from the Bolt Depot website).  Note that the head size (the distance across the hex head, for example) is not the correct size to specify when ordering bolts.

In the case of the numbered sizes, the major diameter is given by 0.060" + [(0.013") x (the bolt number)].  For example, consider the following bolt description: #4-40 x 0.5

The major diameter = 0.060" + (0.013" x 4) = 0.112"

The thread count (threads per inch) = 40

The bolt length (from the bottom of the head to the end of the bolt) = 0.5"

Note: In the case of screws that are countersunk (e.g., flat-headed screws), the screw length is measured form the top of the screw (the flat head) to the end of the screw.