What is cross-threading? It means the threads on the male part of a rotating fastener aren't correctly lined up with the female part of the fastener. It's usually caused when one of the parts of the fastener is slightly tilted when it is tightened. Instead of the lands and grooves of the threads lining up and sliding along one another, the threads cut across one another, causing damage. The more it's forced, the more damage occurs, to the point where the fastener becomes "stripped" - the threads are so damaged, that the fastener will no longer tighten correctly.
(Stripping a fastener can also occur when it is tightened beyond design limits, causing the threads to break - this is why you should use a torque wrench!)
Spark plugs are infamous for cross-threading. The fork tube covers (wide, thin threads) are also easily cross-threaded. The larger in diameter the fastener, and the finer the threads, the more susceptible the fastener is to cross-threading. Not to say that coarse (wide, more distant) threaded fasteners can't be cross-threaded! You can even cross-thread the lid on a peanut butter jar if you try hard enough!
- Crossthreaded spark plug hole in engine block
If the bolt is torqued in even though you feel resistance, meaning it is crossthreaded, it will shave away metal on both the male and female parts of the fastener, weakening the threads:
- Crossthreaded bolt
- Crossthreaded shock bolt
So...how do you prevent this? Diligence and attention is always a good way. However, there is a very simple technique which I have used for many, many years. I don't know that anyone taught it to me, I suspect I just figured it out on my own. However, it works on every threaded fastener, and I use it every single time I screw any fastener into place, even bottle caps and jar lids! It works every time.
The secret: Screw the fastener in place BACKWARD. That's right, turn the fastener backwards instead of forward. If you normally twist the bolt clockwise to tighten it, instead twist it counter-clockwise. Do this until you hear/feel it click. This is the end of the male thread going past the end of the female thread. Once you feel this, you know the threads are perfectly aligned, and you can tighten the fastener normally.
I know it sounds a bit odd, so I made a short video demonstrating the technique:
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How to repair cross-threaded fasteners
So...it's too late, and you already cross-threaded the fastener? As long as you haven't wrenched it so tight that you've completely destroyed the threads, most of the time it can be rescued. What you need is a tap and die set, available at any hardware store:
- Tap and Die set
Normally taps and dies are used to cut threads in things that aren't already threaded - i.e. you can drill a hole in some metal, then use a tap to create threads in the hole. Or you can take a metal rod, and use a die to make it into a threaded rod. However, they are also good for repairing damaged threads.
You will need:
- Tap and die of the correct size and thread pitch (this is important! Using the wrong size or pitch will destroy your fastener even more!)
- Tapping lubricant
Use the lubricant to lubricate the tap, die, bolt, hole, everything. Not using lubricant is a good way to destroy your tap/die and quickly destroy the threads in the piece you are trying to repair.
Gently insert the tap into the hole or the nut, and make sure the threads are lined up correctly. This is not the time to cross-thread again, because the tap will very easily cut across the existing threads! Gently twist the tap in and let the threads guide it. As it encounters damaged threads, you will feel a very gentle resistance. It's ok to keep pushing through.
- Using a tap
- Using a die
A quick warning: It's entirely possible that you will cut damaged metal threads away from the fastener with the tap or die as you twist it. Particularly on spark plug holes, you don't want this metal falling into the engine! Use plenty of tapping lubricant, and every time you push through any resistance, back the tap out and clean any metal off before reapplying lubricant and starting again.
