@Frank: Many thanks, found Essex Fasteners at Orbital HS – apparently they've merged. There is no on-line product list (or shopping) but there is a PDF which at least shows the ranges they carry. You were indeed right to say "they carry a lot of weird stuff" – they seem to have almost every type of fastener under the sun! I shall contact them on Monday and see what they say.

@Roger: That's a very kind offer, thank you! It sounds like the bolt you found is very nearly a perfect match to what I'm looking for, it's only the shoulder that sounds like it could be a problem (see also below); on the one I have the shoulder is a tiny fraction over 1/2". I confess I don't understand the Imperial vernier on my calipers (I was born metric!) but an exact measurement is 12.8mm (or 0.504 inches) – it's likely that 0.495" would leave enough of a gap that the blade will wobble, doubly so since I'm measuring a worn bolt! I'll certainly keep your offer in mind though!

@John: Ah yes, I should have made my uncertainty about the threading clearer; I at first counted 13 threads / inch and assumed it was a UNC thread, but checking again it's actually only 12, which matches BSW. This also makes sense as, hidden between two screwed together parts, I found a "Made in England" stamp I should get a set of gauges really; this isn't the first time I've been confused over a thread! Thanks for your suggestions, and for pointing me in the right direction, thread wise.

Edited By Bent Spanner on 15/11/2014 23:27:42

Edited By Bent Spanner on 15/11/2014 23:28:03

Thanks to everyone for their input – I have now heard back from Essex Fasteners and they are unable to supply this bolt. I've also looked at that auction site, but a search for "1/2" (whichworth,bsw) bolt turns up nothing suitable. So unless anyone can suggest another possible source it looks like I'm going to have to find some way of reusing the existing bolt – or take up Roger's offer of a second-hand one. Today I received 1kg Oxalic Acid which will be used (not all of it!) to clean the surface rust from the blade, and the other parts have been cleaned and sanded, ready for spraying. That bolt will soon be the only thing standing in the way of completing the restoration!

Golly, thanks a bunch for all the interesting comments & suggestions – I knew this would be a good forum! I'll try to respond to each of the points made, but first I have to share these "before and after" photos of the guillotine blade:

Before:

After:

Pretty amazing, don't you think? That's after degreasing and soaking in oxalic acid for two hours (~400g oxalic acid ~5L warm water). It actually looks even better in real life; the bits of oxide left are not "rust" but black oxide, which oxalic acid won't remove (I seem to have a lot of colour balance problems with my photos!). I may try to buff these out, but I'm not too fussed. The other side of the blade looks even better, almost like new, and it still has a good edge despite being 50-odd years old. That said, if I wanted to sharpen the blade, does anyone have any suggestions how to go about it? Might be best to take it to a company that specialises in sharpening tools?

@Marcus: Michael is correct, it's just the colour balance that is off (I took the photo under my desklamp, which has a tungsten bulb). The bolt is most definitely steel.

@Neil & @mechman48: You are both right, the beefy leaf spring (350mm long, 35mm wide, 3mm thick) is there to hold the blade firmly against the shearing edge on the platform. Again, the angle of the blade is the critical paramater; if it doesn't pass the edge at exactly 90&#deg; the paper will be sqeezed and folded instead of neatly cut (and angle it the other way and it will hit the platform!). So the bolt has three jobs: 1) to give the blade a point to swivel around 2) to keep the blade laterally in position 3) to maintain the correct blade angle. Radial position is not critical; a small amount of radial movement won't affect the cutting action.

Now the main problem with the worn bolt is that the head has been thinned on one side from friction against the leaf spring – this means it is unable to force the end of the spring flat, which severly reduces the amount of pressure the spring exerts on the other end. The base of the spring needs to be forced flat against the bearing sleeve on the blade to do its job properly. If I was to reuse the bolt I would have to find some way to make the underside of the head flat, but I only have a small bench grinder and an angle grinder – I doubt I would be able to get a satisfactory result with either!

@Bodgit: Thanks for the suggestion, but I don't have oxy/acetylene gear, just a small propane torch, so I don't think I can braze steel. Even if I could, I don't have a lathe either, so wouldn't be able to turn the bolt afterwards.

@Chris: Many thanks for looking – I suspect you may be right about 1/2" bolts not being 1/2" at the shoulder. A brass bushing would be ideal, but I would have to enlarge the hole in the blade's mounting sleeve, which I don't think I can do. A very thin shimming tube that is slit open might be the best option, but again I do not know where I might find such a thing!

@Keith: Hehe, yeah, that's a typo, and I did search for "Whitworth". You and many others have suggested contacting "a professional bolt supplier" – indeed that was exactly why I posted my question here; to get some suggestions on "professional bolt suppliers". You make it sound easy to find one, but it hasn't proved all that easy to me! The 1/2" bolts from Cromwell tools look great, but unfortunately they have BSF fine thread which won't fit. They are also a tad pricey @ £12 each, but that would have been acceptable if only it would fit!

Thanks again to everyone for your kind help!