How To Avoid Cross Threading

You have started tightening a nut or a bolt and after just a couple of turns it won’t go any further – what is the problem? You have probably just discovered cross-threading.

How does cross-threading occur?

The likelihood is that you have cross-threaded the nut or bolt. The cause of this is usually that the nut was offered up to the bolt at an angle, rather than straight. The same goes with a bolt, if it is offered up to a threaded hole at an angle, cross-threading will occur.

CROSS THREADING

Cross-threading will also occur if non-interchangeable threads are used, i.e. the thread on the nut does not match the bolt. In this case, cross-threading will occur even if the two parts are perfectly aligned. This often happens when imperial and metric fixings are accidentally mixed.

The science bit...

The threads on nuts and bolts only work in harmony with each other if the peaks on the bolt thread are perfectly aligned with the valleys in the threaded nut, allowing them to slide across each other.

If the nut and bolt are not aligned at the correct angle, the threads can still engage but not correctly. Continued turning of a mis-aligned fastener will effectively slice the peaks off of the bolt thread – this is known as stripping the thread. The fastener will continue to turn for a short wile and then stop completely.

How to avoid cross threading

First, check that you are using matching threads. Cross-threading caused by unmatched threads can cause serious damage and sometimes it is impossible to remove the fastener without a specialist extractor.

Always ensure that the axis of the nut and bolt (or bolt and threaded hole) are perfectly aligned. If there is any tension on the fastener you should take steps to remove the tension before assembling.

Can a cross-threaded fastener be re-used?

Remove the nut or bolt carefully and inspect the threads. If they look intact you can try again, this time taking steps to ensure that the nut is properly aligned.