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Discussion Starter · #1 ·
The one i have is a left hand thread M20 x 2.5mm pitch (measured with rule so pitch may not be accurate) does not want to screw in so most probably either i have the wrong size or the internal threads on the car have been damaged possibly by someone not realising it is a LH thread.
Can anyone confirm it should be LH M20 x 2.5mm please.
 

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It's close to LH M20 x 3.0, but actually isn't this. Closer to 3mm pitch than 2.5 on my metric thread gauge.

As near as I can see it's bang on 8 tpi on my imperial thread gauge, and the o.d is 0.775" !
So I'd say it's LH 8 AN x 0.75", but they rolled the thread 0.025" oversize.

My towing eye fits straight into the thread, so problems at all. Does sound like someone's bodged yours, sadly. Will need a suitable LH tap to clean it out properly, not a size I have/use .
 

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Discussion Starter · #3 ·
Thanks for the reply, i measured as 8 tpi and thought that be must wrong, surely it must be metric. Evidently not. A long LH 8 AN tap is not something you see everyday and will be expensive. There is a good centre on the the threaded end of towing eye, it might be possible to bore the eye shaft, cut a cross in it, grind a little from the first few threads. That used with care might be good enough to clean the thread if as i suspect it is just the first thread that is damaged. Many years ago it was quite commonly done by home self taught mechanics like me, real engineers used to faint at the thought.
 

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You may as well give it a go. I'd simply take a hacksaw to cut a few slots in the general direction of the central axis, to give the first few turns a sharp cutting edge like a tap. Flush it all well with WD40 or airline/whatever as you go, and you may well manage to re-flow the thread back into shape! All rather depends how deep the damage goes - did someone just try it with the wrong bolt & them give up, or was the wrong bolt seriously forced in & the car towed, maybe onto a low-loader, and real damage done?
 

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Discussion Starter · #5 ·
Well that was easier than expected. I used an angled pick to clean any swarf out of the first couple of turns and managed to clean up where the start of the thread had been turned over; applied some assembly dry moly and in it went with only a slight tight spot after 4 turns. The threads in the car are into an alloy which is quite soft and easily damaged, i don't understand why the part easily replaced is hard and the part permanently fixed is soft.
Thanks Andy for your input.
 

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Useless factoid #94 - only Amperas and export Mk1 Volts got the towing eye. There is a cover panel on the bumper facia on the Volt, but there was no threaded hole behind it on Nth American Volts - left or right hand!!

Also found two theories concerning the LH thread - (seems we are not alone) - take your pick:
  1. The tow rope will tend to unwind anticlockwise under tension because of the way ropes are conventionally wound (apparently)
  2. It prevents anyone forcing a wrong but close fit threaded eye in.
 

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Interestingly our friends in the USA do seem to like using imperial threads. We have a vintage Kelvin Marine engine on our boat with pre-war standard BSF threads (larger hexagons) throughout. We searched everywhere for suitable replacements and spares until eventually we found everything that we needed from a supplier in the USA.
 
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