What grade of bolts and nuts should I use?

Grade 8.8 high tensile nuts and bolts are commonly used in tow bars. However, grade 10.9 nuts and bolts may be used occasionally. The correct grade of nuts and bolts is specified in the installation instructions.

How can I tell the grade of a nut or bolt?

The grade of a nut or bolt will be marked as 8.8 or 10.9 on the bolt’s head. This figure is located on the top face of the bolts head.

The head of the bolt has other markings, what are they?

Bolt manufacturers stamp an identification mark and grade mark on bolt heads. Tow bar bolts all previously carried a ‘3F’ marking before transitioning to a ‘DF’ marking.  You may have to source bolts from alternative suppliers on occasion. The identification mark would be different in this instance.

What are the correct torque settings?

The recommended torque settings for all fasteners are clearly stated on installation instructions. This has been standard practice for the last ten years. It is wise to consult the installation manual if you have any doubts. Torque settings can be found on the fitting instructions for each tow bar. 

What is ‘torque’?

When fasteners are tightened, a pre-load is created in the bolt. The amount of force used to rotate (or tighten) the nut/bolt is measured as torque. Prior to the introduction of metric measurements torque was measured in ‘ft. lbs’ or foot pounds but the metric equivalent is ‘N.m’ or Newton meters.

Many prefer to measure torque using ‘ft. lbs’. This is the value of forces in lbs applied at a distance of 1 foot from the fastener’s centre of axis. A majority of torque wrenches measure torque in N.m and ft.lbs.

Do I need to tighten all bolts with a torque wrench?

You should always tighten bolts with a torque wrench. This is the only tool that will reliably and consistently gauge the applied forces. 

Should I lubricate nuts or bolts?

You must never lubricate nuts or bolts! If a nut or bolt is lubricated in any way this has the effect of doubling the tensile load that can be applied. This results in the potential applied load for a bolt being doubled at the same tightening torque.

Now here is how it works. Take an un-lubricated, clean and dry 1.5mm pitch M10 bolt with a few added washers to keep the nut on the threaded portion of the bolt. Now carefully tighten that bolt. Once you reach double the recommended torque setting of 100 Nm the result will be shearing.
However, if this test is repeated using a good quality lubricant applied under the bolt’s head and to the threads, you will see why using lubricant is a bad idea! It will only take around the recommended torque level setting to strip the threads off the bolt, because the lubrication allows for a higher force to be applied for the same torque.

What sort of finish is on the nuts and bolts?

Witter tow bar fasteners come with an 8 micron zinc electroplate coating as standard. The yellow or golden finish indicates that they have been ‘chromium passivated’, thus rendering them more resistant to corrosion.

Is chromium eco friendly?

EC and EU requirements dictate that eco friendly ‘tri-valent’ chromium must replace ‘hexavalent’ chromium in our products in order to meet new recycling standards. We are in the process of changing over, but this will not affect the performance or any other characteristics of our fasteners.

Can air driven wrenches be used to tighten nuts and bolts?

It can, but only if it has a torque control to ensure that nuts and bolts are not over tightened. This is especially important with the smaller M8 and M10 fasteners. We always recommend that you finish tightening by hand using a hand torque wrench.

How often should I check the nuts and bolts for tightness?

There is a ‘bedding in’ period for your tow bar’s components and this is perfectly normal. During this time small irregularities on surfaces can flatten out. We advise that the fasteners are checked after the first 500 miles. After this you might want to refer to your warranty statement, but generally we recommend that you check the fastener’s tightness once a year.

Why is the diameter of the bolt much smaller than the holes in the tow bar’s components?

Holes in the tow bar’s components are designed to accommodate normal vehicle-to-tow bar variations and to assist with alignment. Witter bolted joints rely on the application of the pre-load to the bolt, using a ‘friction grip’ concept. The forces of the components hold the parts together using friction.

What does ‘thread pitch’ mean?

Thread pitch refers to the thread’s spacing and this is generally measured using mm. You can measure this yourself with a thread gauge or good quality ruler. To measure thread pitch with a ruler you need to measure a section of the bolt and count how many threads are within that space. If you divide this by the distance you have the thread pitch. 
The standard thread pitch is 1.5mm on 10M nuts and bolts and 1.75mm on M12 nuts and bolts. However, nuts and bolts of different thread pitches must never been mixed otherwise the fastener will be damaged.

Why do you need different thread pitches?

Thread pitches are often vehicle specific for tow bar fixtures, depending on the vehicle manufacturer. Not only do we supply fasteners to match various fixtures, but we also supply standard pitch fasteners for tow bar-to-tow bar connections alongside these.

Fasteners with a finer pitch (with less space between the threads) are much more resilient to vibrations and are less likely to loosen. The connection between the nut and bolt is stronger and far more secure. Fasteners with a finer pitch are, however, more vulnerable to binding and damage from particles such as dust and grit.

Are there any requirements for how much thread protrudes past a nut?

It is generally accepted that a minimum of one full thread should protrude past the nut’s surface. There is not, however, any specified value for this.

Can nuts and bolts be reused?

Should you need to replace, move or adjust any components, it is best practice to use new fasteners each time. It is important to note that Nyloc nuts must never be reused.

What if the fixed fastener threads on a tow bar or car are clogged with paint or other debris that prevents the insertion of a bolt?
It is imperative to remove any debris from the threads, a top with the correct thread pitch and diameter can be used. If you were to continue tightening a bolt you would risk damaging or even stripping the threads off the fasteners. The clamping force versus the applied torque would be greatly reduced.

Is a thread locking compound a good idea?

Applying a thread locking compound is definitely beneficial. This helps prevent a loss of clamping force and loosening from loading and vibration. However, the instructions must be followed.