Reassembling the turret - and buggered castle nut

During my absence on Thursday / Friday, the o-ring for the turret arrived. It's very slightly larger in diameter than the one that came off. However, I followed the recommended procedure for measuring and specifying an o-ring (measure the ID ie the groove inner diameter) and the actual diameter of the o-ring section itself. Here's the new (top) and old (bottom). Let's fit the thing and move on. Having said that, obvs I bought 2 of the buggers, as it doesn't look easy to refit them without nipping the edge off, as seen in the old one:

That went fairly well, although I've no means of telling until I pressure test it. Apart from oiling the parts beforehand, there's not a lot you can do to ensure the assembly goes well. I'd checked the entry to the bore was clean and smooth previously.

This is the time to refit the external dust seal, as the screws are not accessible once the turret is fitted. That also means the turret will need to be inserted past the seal over a large circumference simultaneously, which could be tricky.

The next operation is to insert the turret, then.

Time for a good slathering of oil.

Not forgetting the needle thrust bearing and its 2 washers.

But wait. Nothing's as simple as that and sure enough, the castle nut isn't looking too clever. Nor does it want to simply spin onto the end of the shaft. I smell a rat here.

Sure enough, the thread is well and truly buggered, with bits of swarf hanging off internally. Once those have been pried away with various scrapers, it's clear there's almost no thread to speak of. Or just enough to jam the nut when you try to thread it on. Standing back somewhat, it's pretty clear that this is actually a replacement, home made affair.

Time to measure up what we have here. Is it metric or imperial? So far, all the fixings on this turret have been metric, so I'll expect that first. I'm guessing this turret wasn't actually made by Tree, otherwise we'd be seeing UNC / UNF threads.

But firstly, I need to reassemble the thread gauge. This was rusted TF, so I tried out my Evaporust on it. This worked OK but now I'll need to sort them all out into metric and imperial, then order then in ascending order....

...like this...

...and then fit them to the frame. Phew. Wasn't as bad as I'd feared.

Here we go - 1.5mm pitch:

...and 18mm  diameter - LEFT HAND THREAD.

So it seems that it's a standard thread - MF18 ie metric fine, 18mm diameter by 1.5mm pitch. When I dismantled the turret, there was no split pin evident in the castle nut until the thing had come off, at which point I was able to see the remains of a split pin in the hole here. So some clot had simply forced the nut off, shearing the pin and buggering the thread of the castle nut in the process. NO, it wasn't me - or even The Stupid Fat Bloke.

So, what to do here? The nut's clearly fucked, so needs to be replaced. This is a problem. Although my Bantam would love to thread this, I can't easily remove the male threaded (mating) part. So even if I manage to make the required nut, I'd not be able to try it on the shaft unless I was able to remove the entire nut and chuck. I can do this with a D1-3 spindle nose, as we find on the Bantam but it would be far from easy to try out the fit with a bloody great collet chuck hanging off the back of it. 

So I have a choice:

1. Make up a blank nut and thread it with an off the shelf tap.
2. Make and thread a nut myself, removing the whole nut / chuck to try the fit.
3. Make a dummy male thread of the correct size.
4. Somehow procure an MF18 go / no go thread gauge.

I like option 1, if I can find something at a sensible price. 2-4 don't appeal unless I'm really in a corner. I would normally hope to find something at Drill Service, Cutwel or possibly even The Tap and Die Company But no obvious cigar appears...

However, all is not yet lost. It seems I can buy a set of LH MF18 taps for £75 plus VAT and postage from MSC UK. At that price, we can happily forget that as an option. Alternatively, we have Eurafco(?) who sell individual LH MF18 taps for ~£25, like this one. That works out at £31, which would save a lot of faffing about. Deal done. You never know, I may actually receive it by next weekend. However, I'll need to make myself up a blank nut anyway, so I couldn't use one even if I got it tomorrow.

Currently, the castle nut requires a rather special driver to tighten it, due to the awkward shroud around the end of the shaft. If I'm going to make this myself, I'll go for a different, simpler solution, like a good old fashioned hex section. I could even turn down the shroud and make life much simpler....