A Bolt with a Hexagon head is measured by the thread diameter and the length from under the head to the end.

The tensile strength of a bolt is indicated with a Class or a Grade.

  • A Metric bolt has a Class tensile strength
  • An Imperial bolt has a Grade tensile strength

The table below shows the approximate equivalents between a Class and a Grade

Metric Imperial
Class 4.6 Mildsteel  
Class 8.8 High Tensile Grade 5 High Tensile
Class 10.9 High Tensile Grade 8 High Tensile
Class 12.9 High Tensile Grade 9 High Tensile

No, there can be various Metric Fine thread pitches available. We recommend the thread to always be measured with a thread gauge or checked with a nut to ensure the correct thread pitch is ordered. We can cut any thread on a bolt in our specialised Workshop. Just advise us of the thread pitch and the length of the thread required and we can make it for you.


A Set Screw is a fully threaded bolt. It has a Hexagon head and has no plain shank. (Not to be confused with a Socket Set screw).

Most commonly available in Metric and in Class 4.6 Mildsteel & High Tensile Class 8.8, although also available in some sizes in Classes 10.9 & 12.9.

Set Screws are sold without the nuts because they are mainly used for in tapped holes with the longer Set screws being used for adjusting situations.


This can vary depending on the Length of the Bolt, and the Standard it is manufactured to.  

The table below can be used as a general guide...

Bolt Length Thread Length Calculation
up to 125mm long 2 X Diameter + 6mm
up to 200mm long 2 X Diameter + 12mm
Over 200mm long 2 X Diameter + 25mm


If the measurement of the thread or the Parallel Plain shank of the bolt is crucial, we highly recommend that you contact us and we can give you an accurate measurement.

We can also manufacture Bolts with specific thread lengths in our Workshop - contact us for a Quote.

Need a fitted bolt ? We can also make these for you.

You can use the Part Turn Tightening Method...

  • On assembly all bolts and nuts in the joint are first tightened to a snug tight condition.
  • Snug tight is defined as the tightness attained by the full effort of a man using a standard podger spanner or by a few impacts of an impact wrench.
  • Location marks are then established to mark the relative position of the bolt and nut.
  • The bolts are then finally tightened by the amount shown in the Table below...

Nut Rotation from the Snug Tight condition.

Nominal Length of Bolt Length of Bolt
Bolt Nut rotation Nut rotation
Diameterrotation 1/2 turnrotation 3/4 turnrotation
M16 Up to 120mm Over 120 up to 240mm
M20 Up to 120mm rotation Over 120 up to 240mm rotation
M22 Up to 120mm rotation Over 120 up to 240mm rotation
M24 Up to 160mm rotation Over 160 up to 350mm rotation
M30 Up to 160mm rotation Over 160 up to 350mm rotation
M36 Up to 160mm rotation Over 160 up to 350mm rotation

Please note:

  1. Tolerances on rotation: one-sixth of a turn (60° rotation) over; no under tolerance,
  2. The bolt tension achieved with the amount of nut rotation (specified in the Table above) will be at least equal to the minimum bolt tension.
  3. Nut rotation is rotation relative to the bolt, whichever component is turned.

The Length of all Countersunk bolts and screws are measured overall - including the head.

This is because when installing, the head sinks into the material, therefore becoming part of the length.

A Socket Screw is a screw that has an Inner Hex drive and can be fastened with an Allen key or a hex Drive bit.

Available in the following head types...

  • Socket Head Cap Screws
  • Countersunk Socket Screws - length is measured overall, including the head
  • Button Head Socket Screws
  • Socket Set Screws - also known as Grub Screws
  • Socket Shoulder Bolts - length is measured from under the head to the end of the shoulder

The diameter of a Tension pin is measured by it's Hole size.  It is not measured by the actual size of the pin.

For example:

M6 Tension pin                Nominal Hole Size = 6.0mm             Actual Diameter of pin = 6.7mm




Metric Taper pins are measured differently to Imperial Taper pins.


When measuring Metric Taper pins , the diameter is measured at the smallest end of the pin.

Metric Taper pins have a taper of 1:50

Click here for measuring example



When measuring Imperial Taper pins, the diameter is measured at the largest end of the pin.

Imperial Taper pins have a taper of 1:48

The Length of a Clevis Pin is only the Effective Length

Click here to view example

The length is measured from under the eye to the end of the shorter leg.

The diameters of Self tapping & Self drilling Screws are measured in Gauges rather than millimeters, below is a conversion table.

Screw Gauge Millimetres
4 Gauge 2.9mm
6 Gauge 3.6mm
8 Gauge 4.2mm
9 Gauge 4.5mm
10 Gauge 4.9mm
12 Gauge 5.6mm
14 Gauge 6.2mm

The thickness of a Washer is often measured in Gauges, here is a conversion table into millimetres.

Washer Gauge Millimetres
22 Gauge 0.75 mm
20 Gauge 0.95 mm
18 Gauge 1.2 mm
16 Gauge 1.6 mm
14 Gauge 2.0 mm
12 Gauge 2.5 mm
10 Gauge 3.0 mm

When ordering U Bolts, we need to know the following measurements and information...

  1. Material                ie: Mildsteel, High Tensile & what grade, Stainless & what grade etc.. 

  2. Material thickness ie: M10, M12 etc..

  3. Centre                  ie: the measurement from the Centre of one leg to the centre of the other leg.

  4. Overall length    ie: from the outside top of the U bend, to the end of the legs.

  5. Thread length    ie: the length of thread on each leg.

  6. Thread pitch      for example:  Metric Coarse, UNC, UNF etc...

We also need to know...

  • Do you need Round or Square ?
  • What finish ? Black, Zinc or Galvanized ?
  • Are nuts and washers required ?

The Nominal Bore is the internal diameter of the pipe the U Bolt will fit.

There are four NZ Courier pick-ups from The Boltholder every day (Monday to Friday). Most destinations within the North Island can be reached overnight, and many within the same day.

We deliver three times a day to Hamilton, Rotorua and Tauranga, twice a day to Taupo and once a day to Auckland, Napier.

If your order is going to a rural address, expect to add one additional day to delivery times.

For larger orders, we use other Transport Companies.


If we don't stock the product you're looking for, we may very well be able to source it for you or fabricate it for you in our workshop.  

We have an excellent network of suppliers, both within NZ and across the world from which we can source bolts and other products. Contact our Sales team and let us know what you're looking for.

No problem, you can either open an account by downloading our Credit Account Application form, or just let us know what you are after and we can email you an invoice and our bank account details for payment.

We accept Credit cards for any Email or Phone orders, subject to a 3% Credit card processing fee.

If you would like to pay on collection of goods, we only accept EFTPOS payments at our premises.