HINTS FOR HOMEBUILDERS
BY FRANK HOFMANN
IF YOU ARE GOING TO INSTALL a 1/4-inch bolt somewhere, the last
thing you should do is grab the nearest 1/4-inch drill bit. There
is much more to the process.
The strength and durability of a bolted joint is partially dependent on the closeness of fit between bolt and hole; the closer the
match, the stronger the joint. A loose joint will allow relative
motion (wear and fatigue), moisture (corrosion), cause hardening
of the bearing surface (eventual cracking), transmit loads unevenly
to the material it is supposed to hold (stress cracks), and allow the
bolt to cant sideways in a single shear application (cracking under
the head of the bolt or thinning a softer material, weakening it).
The craftsman has two problems: AN bolts are nominally
undersize by about 0.003 inch; the other problem is that a drill
bit creates a slightly triangular hole. Measure a typical AN bolt
and then insert it into a hole drilled into a scrap of sheet aluminum. Careful inspection will show light coming through on
three sides around the bolt.
How can this poor fit be avoided and a stronger joint be created? Check the different kinds of drill indexes you can buy.
Standard is the fractional set—usually from 1/32 inch to 1/2 inch
in 1/32-inch increments. Then there are numbered sets—from
No. 1 to 60 (0.228 inch to 0.040 inch). Numbered bits are 0.003-
inch oversize relative to their nominal fractional equivalent.
For example, a No. 30 bit is 0.128 inch, and if you use it, the
hole will be 0.003 inch larger than the 1/8-inch (0.125-inch) it is
meant to replace. This bit is used when you use 1/8-inch rivets
for example, where you want a bit of clearance so the rivet can
be installed easily and where the rivet will expand to take up the
slack. This clearance is not a good idea for bolted joints. Lastly
there are the lettered sets, from A to Z (0.234 inch to 0.413 inch).
Lettered bits are 0.003-inch undersize relative to their nearest
Thus a “D” bit is 0.247 inch, or 0.003 inch smaller than the
1/4-inch (0.250-inch) fractional bit. You could not get a true 1/4-
inch diameter bolt into a “D” hole, but an undersize bolt will fit
With this bit (excuse the pun) of background, here are
some guidelines to follow:
• Good sense dictates that the drill should be sharp, in good
condition, held perpendicular to the material and the work,
and held as rigidly as the setup will allow.
• Step drill. If you want a 1/4-inch hole, drill through with a
3/32-inch bit, then a 7/32-inch, and finally the 1/4-inch bit.
Better drill indexes have bits in 1/64-inch increments, but
there will be many bits in a 1/2-inch set that you might
never use. The idea is to come up to the final dimension in
• Expansion or taper ream. A tapered reamer is used much like a
tap, but its gradual taper allows you to open up a hole gradually
enough to fit your bolt exactly. For example, a 1/4-inch taper
reamer goes from 7/32 inch to ¼ inch over a length of about 3
inches, giving it a very gradual change in diameter. With a bit of
practice you will be able to judge just how far you need to work
the reamer into the hole to get a tight fit.
A fancier version of a taper reamer is an expansion reamer.
The cutting edges of this tool can be adjusted so that you can
create a clean, parallel-faced circular hole to fit a particular bolt
exactly. You might use this type of reamer if you need to have an
exact hole (not tapered) through some thicker material, such as
a wing spar or landing gear leg, where tight fits are mandatory.
• For a good tight fit of an AN bolt you should step drill, but
instead of that final cut using a 1/4-inch bit, use a letter “D”
bit. It is 0.003 undersize, so instead of drilling a 0.250-inch
hole for that nominal 0.250-inch AN bolt (which is actually
only about 0.247-inch), you will end up creating a 0.247-inch
hole for a 0.247-inch bolt. That is a zero clearance hole, and
the bolt will have to be driven in. It is unlikely ever to work its
way out. More importantly, however, the bolt will bear evenly
against the material it is meant to hold and therefore create a
joint capable of giving the design strength of the joint.
Frank Hofmann, EAA 21791, is an EAA technical counselor. You can visit his
website at www.Hoftec.com.