This article was first published in 2005.
These days, very few people rebuild engines at home. In fact, of the last few
hundred feature cars that we’ve covered in AutoSpeed, off-hand I can’t think of
one which had an owner-built engine. So when we talk about using measuring tools
in car modification, there’s not a lot of point in covering dial indicators and
internal and external micrometers – almost no-one DIY modifying their car uses
However, there’s still a multitude of important uses for measuring tools.
From as simple a task as making a bracket (but bracket making is actually seldom
simple!) through to mounting an amplifier, drilling holes, measuring coil spring
wire thickness, moving the battery from under the bonnet to the boot – to do a
good job, all will require the use of measuring tools.
So what are they and how do you use them?
For non-precise, relatively long measurements, use a tape measure.
Retractable metal designs give fine accuracy (don’t use cloth tape measures!)
and are available very cheaply.
The tape should have a moveable end tip – when the tape is pushed up against
a surface, the tape measures the distance from the surface...
...but when the tape is hooked over an edge, it measures from the inside of the
edge. In this way the moveable end tip cancels out its own thickness.
About the only other point to make is that you should always buy a tape
that’s brightly coloured – it makes it so much easier to find...
The next level up of accuracy in general purpose measuring instruments is
provided by a steel rule. Again, it’s a very cheap instrument. A typical steel
rule is 300mm long with markings each single millimetre. However, better quality
rules have markings over the first 10 or 20mm that are in half-millimetres.
There are two important aspects to keep in mind when using a steel rule.
The first is that the rule should be placed so that the graduations are as
close to the work as possible. For example, where possible, the rule should be
placed at 90 degrees to the work so that the graduations actually touch the work
Secondly, a habit to avoid is measuring a work piece from the beginning of
the rule – the very end markings of a steel rule may have been damaged (this
occurs much more easily than to markings along the inner length of the rule) and
it’s also much easier to accurately line up two markings, rather than one
marking and the end of the rule. However, when you use the rule in this way,
remember to subtract the initial numbered amount from the total reading.
A steel rule can also be used to assess the flatness of a surface. Place the
rule on edge across the surface then hold the work and the rule up to the light.
If the surface is flat, no light will be able to penetrate between the edge of
the rule and the work piece.
In any typical home workshop, steel rules wear out – normally because of
inadvertent abuse like being trodden on, cut with power tools and so on. Expect
to replace a steel rule every few years.
One metre long steel rules are also extremely useful – as a marking edge, to
check for flatness and of course, to measure lengths. However, long steel rules
tend to be much more expensive than short ‘uns.
Digital calipers are nearly as useful as the humble tape measure and steel
rule. They are also now so cheap that no do-it-yourselfer should be without one.
Digital calipers are most commonly available in 150 and 200mm measuring
sizes. Both sizes are useful – the smaller calipers will fit into tighter spaces
and the larger ones will – duh! – measure larger work-pieces.
Calipers are fitted with surfaces to make external, internal and depth
measurements. The external ones fit across the work (eg to measure the diameter
of a rod), the internal ones measure the width of openings (eg of a cut-out
being made in a panel to mount a head-unit) and the depth provision can be used
to measure the length of a drilled hole.
The primary use of a digital caliper is the measurement of the thickness of
an item. This measurement requires the use of the external calipers. The
instrument is fully closed, zero’d, then the external calipers placed so that
they are a just-snug fit on the item. The reading is then made before the
calipers are removed. Used in this way, digital calipers are especially useful
in measuring screws and bolts to make accurate sizing of the drill-bit easy,
measuring coil spring wire thickness (something that it’s important to do very
accurately when comparing spring rates) and measuring sheet metal gauge.
And literally hundreds of other uses!
Internal measurements with digital calipers should be made only when the
opening has parallel sides. If you try to use the internal measuring calipers to
measure the diameter of a hole, the thickness of the internal measuring jaws
will give an incorrect reading. Instead, in this situation you need to use
thinner manual calipers, as described below.
With the low price of stainless steel digital calipers, it’s not worth buying
plastic imitations. Arguably, it’s also not worth hanging on to old-style
vernier calipers – the digital readout is much quicker and easier to read, with
less likelihood of mistakes being made.
There are many situations where a digital caliper cannot be conveniently used
– perhaps because the jaws are not long enough to reach the work-piece. In this
situation, manual calipers perform well.
The calipers are opened (either by being pulled open against a friction screw
or having an adjustment knob unwound) until the jaws of the caliper are a gentle
push-fit over the work. The calipers are then transferred to either
the digital calipers or steel rule and a reading made of the distance between
the manual caliper’s jaws. With care, this process can be highly accurate.
Some manual calipers can have their offset jaws completely rotated to allow
them to measure both internally and externally, while others are limited to just
the one function or the other. With good care, calipers last for many decades
and so these are good tools to pick up secondhand. (Note that many older
calipers are made from mild steel and so should be oiled to prevent
It’s not world-shattering stuff, but I for one wish I’d bought a pair of
digital calipers and a pair of manual calipers years before I actually did.
Their use would have saved plenty of too small and too large holes from being
drilled and would have made lots of other examples of my workmanship much more