M 11.5 bolts?...

Weird. Why not use 7/16 inch bolts?

I was going to suggest this could be an imperial thread. For comparison with TPI and thread pitch this might be useful:

Have these to screw into captive threaded ends, or can one simply fit smaller bolts and nuts on the other end. I'm Sure strumech used to use odd sized bolts back in the day, but no idea where they got them from, never could afford such a luxury!

Brian

Sure it isn't an imperial tread? BSF/Whitworth. Some sizes do sort of fit the nearest metric.

UNF and UNC wasn't that common on UK made stuff other than cars.

Best option I can think of if retapping the holes is very awkward is single point tooling to turn the new bolts down to the size you want them. I reckon a cut of about 10 thou off ought to do it.

PITA and you would need to galvanise them again (spray on?).

The size you specify of 11.5mm is suspiciously close to one of the US UNC thread sizes that has a 11.51mm clearance hole but TPI would be 14 rather than 14.5 - that should be just enough to spot a difference:

If US based (even if made elsewhere) it likely has American Fine or American Coarse. And those are not the same in every size as UNF and UNC.

Actually you may be right there that is very close indeed anyway I'm off to Glenwood bolts tomorrow the bloke there claims to have anything and everything thats been made in bolts!

The odd thing is that this equipment was made its believed by an outfit called Rasmussen that later changed to AS Steels and we did have a drawing that said these bolts were M12!!!

They were in either Sweden or Denmark so what are UNC or UNF bolts types doing there;!?>...

Maybe after the plates or whatever had been drilled and galvanised, there was no longer clearance for M12's in some locations. Rather than faff around drilling or reaming, and worrying whether the galvanizing had been damaged, bunging in a few 7/16ths seems like a pragmatic way around it. I doubt if there was very much formal design for an amateur radio mast, and the amount of conservatism and redundancy means there is little risk in having a few undersized bolts. Most of the structural strength comes friction between the plates rather than the shear strength of the bolts in any case. I'd bet they were all torqued up to the same setting.

Well all rather odd as these weren't for the Amateur market, far from it!

Its not a friction on plates more if you can imagine a 3 inch diameter tube in that sits a 20 foot steel scaffold pole the M12's go thru the outer round section then hold the 2 inch scaffold pole as such, they can be varied a bit to make sure the pole is upright there is quite a hefty windload these towers are 30 meters high!..

OK, I realise you said "bolts" originally but I'd assumed you meant a fabricated structure with nuts and bolts. Are you saying the 3 inch tube is tapped to take these radial bolts? Or perhaps has captive nuts, or nuts welded on the inside. In that case, the important parameter is the thread pitch.

How many M12's? I agree with your point about wind-loading, this does not seem to be a particularly secure way to support a round pole. Presumably the weight is taken by a socket at the bottom, which also provides radial restraint.

Found the problem!, matey at the bolts shop said the problem is you got a Galvanised bolt there, thats thicker than a normal M12!

Ah! So showed him one of the original Galvanised looking bolts he said I bet they had theses custom made!

Standard bright Zinc plated bolts fit the sample lock nut fine so some of these and we'll coat them with Galvafroid and that will be that thanks to all you helped much appreciated!

In case anyone is interested measured the Galvanised bolt its 12 mm exactly, the Zinc plated one is 11.7 mm. Holding that next to the original there does seem to be a very slight difference between the threads, same per inch 14, but perhaps the angles are also ever so slightly different which may add substance to a custom made unit?!?.

"As they say. Every day is a school-day"

Does that really make sense to you?

UNF and UNC are common on marine items though, as most originate from US.

I have a boat with a new Volvo Penta engine - you would assume Swedish means metric..... half the screws are metric nut half are UNF particularly on what are shelf components s- water pump, alternator etc.

In article snipped-for-privacy@davenoise.co.uk>, Dave Plowman (News) snipped-for-privacy@davenoise.co.uk> scribeth thus

No! But we've now got some stainless M12 ones that fit fine and won't rust so;!

It would have been interesting to have compared the original with a 7/16 UNC thread but the requirement is met now:)

Not so.

The US sizes are American Fine and American Course. UNF and UNC were an attempt to have a common thread during WW2. And some sizes and can mix and match, but not ideal for high loads.

UNF and UNC were common on some UK car makes between WW2 and the mid 70s, when new designs started using metric.