G14 Lightweight -Fork crown stem threads

Information relating to the Matchless G2 or AJS Model 14 250cc Lightweight
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G14 Lightweight -Fork crown stem threads

Postby David Paterson » Mon Jul 11, 2011 9:53 am

I have a G14, Lightweight and decided to repair a broken lug (mudguard bracket)on the slider and rechrome the fork slider extensions and other chrome parts while everything was in bits.

Now that it is going back together I find that the chrome nut for adjusting the fork stem will not start to thread on the fork crown stem. I have bought a new nut from the club and this likewise will not start although the domed locking nut will screw down.

Does anyone have details of the stem thread and how they can be cleaned up? I suspect this is the problem rather than the new nut.

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G14 Lightweight -Fork crown stem threads

Postby Rob Harknett » Mon Jul 11, 2011 8:35 pm

You could try cutting the old nut in half or just a quarter, then place it low down and unscrew it, this may chase up the thread, I did if once when I did not have a proper tool.

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G14 Lightweight -Fork crown stem threads

Postby David Paterson » Mon Jul 11, 2011 8:58 pm

My attempts to thread the old nut onto the stem has resulted in flattening the threads on the old nut so it would appear that the nut is softer than the stem thread - luckily - as you can buy nuts but not new stems!! Hence my interest in the original thread type and some engineering method of cleaning up the thread.


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G14 Lightweight -Fork crown stem threads

Postby wilko » Mon Jul 11, 2011 9:46 pm

Odd that the top chrome one starts but not the others?

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G14 Lightweight -Fork crown stem threads

Postby cbranni » Mon Jul 11, 2011 11:11 pm

If you want to clean up the thread on the stem, I think its 26tpi, try a thread restoring file you can find them on ebay, 26tpi files are available but very expensive £50ish, comes part of a set, you could try a 1mm metric pitch restoring file, the pitch is not perfect match but might be good enough to clean up the thread, and they cost around £5, again on ebay
26 tpi = .0384 pitch
1mm pitch =.0393 pitch
(check my maths I could be wrong)

Check the stem, has the lead/start been damaged, dropped or hammered, grind a good chamfer, if its flattening the nut thread it could be damaged.





Edited by - cbranni on 11 Jul 2011 11:30:41 PM
only dead fish go with the flow

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G14 Lightweight -Fork crown stem threads

Postby David Paterson » Mon Jul 18, 2011 1:28 pm

Many thanks for your help.

I have managed to buy a 5/16" - 26tpi thread chaser (this does appear to be the correct thread) but I must admit I'm not too sure how best to use it. I have an inside and outside thread chaser and have run the outside chaser around the stem thread holding the tool in my hand (as one does!!). But I'm not sure that this technique is going to cut it!! Nut still doesn't want to start.


Any more ideas - avoiding the obvious remarks that I'm not sure I should have encouraged.

I suspect I will have to buy a 5/16" - 26 die and try to re-cut the treads.

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G14 Lightweight -Fork crown stem threads

Postby cbranni » Mon Jul 18, 2011 2:38 pm

Where does the 5/16 come into it? the stem is 3/4 or 7/8 dia from memory. Did you buy a thread restoring file if you did clamp the stem in the vice and use it like a file. If you bought an external chaser then the stem needs to go into a lathe, using a rest and slow speed use the chaser to clean up the thread.
only dead fish go with the flow

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G14 Lightweight -Fork crown stem threads

Postby David Paterson » Mon Jul 18, 2011 6:55 pm

Sorry for the confusion. I also bought a 5/16" - 26 Thread repair (helicoil) kit for repairing other threads and mixed up the sizes.

I couldn't find a thread restoring file but bought the 26 tpi thread chaser for repairing the stem. I will now visit a friend with a lathe and try to repair the threads on the stem although I'm not quite sure how you would hold the stem in a lathe.

I am now the owner of a 5/16" - 26 tpi thread repair kit, 5/16 BSW thread repair kit and a 5/16 BSF thread repair kit so will soon be able to do most jobs.

Again, many thanks for your interest and help.

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G14 Lightweight -Fork crown stem threads

Postby cbranni » Mon Jul 18, 2011 7:05 pm

If your stem has fixed bottom yoke and the stem is hollow, turn up a mandrel, which is a good fit in the hollow bit, centre the end, fit the stem to the mandrel, support end in a live centre.
Set the rest (which is any square/ flat bar) in the toolpostset below centre so that the chaser is on centre or a tad above and as close as you can to the stem, slow speed and just nibble away at the thread, trying the nut every so often
only dead fish go with the flow

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G14 Lightweight -Fork crown stem threads

Postby David Paterson » Mon Jul 18, 2011 8:46 pm

Thanks for that - very helpful.

I'll try and coax my local farmer into having a go on his lathe and let you know how I get on.

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