The genesis of the unified thread form.

[qantasqf1]

In the dark days of June 1940 the British government told Rolls-Royce to send a complete set of Merlin drawings to the US government in case the unthinkable happened. Thinking Britain was bound to be defeated by the Germans Henry Ford refused to have anything to do with manufacturing the engine so the car manufacturer Packard agreed to manufacture the Merlin instead. This turned out to be an incredibly difficult task for Packard and it is to their eternal credit that they actually managed it, although it would have been made slightly easier if Packard management had agreed to send some of their engineers to Derby to study the Merlin. One of the major obstacles Packard encountered was the use of the British screw threads BA, BSP, BSF and Whitworth because these thread forms were a completely unknown quantity to US engineers since the two governments and industry representatives had never thought about agreeing to a common standard after WW1 this issue was clearly a problem to be addressed after the war. In the meantime Packard had to find a US company that could manufacture taps, dies etc. to UK specifications and since the US was starting to go on a war footing in those days that caused some delays, but finally Packard overcame the problem and their Merlins went into production, first manufacturing them in 1941 for the rest of the war. To understand the challenge of duplicating British threads (including oversize threads) there were about 140 different types used on the engine, many of which were not documented so Packard engineers had to resort to physically measuring them. Components for the Packard engines such as magnetos, generators, carburettors, etc. were all US-made and thus employed American threads.
After the war the US, UK, Canadian governments and engineering representatives got together to agree on common thread forms. Not surprisingly the new thread form was called Unified and it encompassed coarse, fine, extremely fine, pipe, J-thread and so on, hence today we have UNF, UNC etc.
It would be remiss of me here not to mention the outstanding work by Packard to manufacture an engine of about 13,000 separate parts that was never designed to be assembled on a production line basis: Rolls-Royce engineers assembled them by selective fit, whereas the US production line method meant the dimensions of Packard (and, for that matter, Ford UK) Merlins had to be revised to allow interchangeability. Redrawing the affected parts to accomodate the assembly line method must have been the job from hell. The fallout of the production line method is reflected by the Packard Merlin Overhaul Manual which is more or less the same as a conventional workshop manual (except vaster) so semi-skilled workers could follow the assembly sequence without resorting to hand fitting as per the Rolls-Royce method whereas the Rolls-Royce manual (they called it a ‘handbook’) is nothing like as detailed and about a tenth of the size.
Finally, since Dan Snow’s 3-part TV series The Dam Busters is being shown here in Australia, it may be of interest - if you’ve got this far - that the Lancasters deployed on the dams raid were all fitted with Merlin 28 engines which were made by Packard, USA (complete with British threads)!

[Brian-H]

That was a very interesting read, thanks.

I've seen the 3-part TV series about the Dam Busters, which IIRC also includes the Tall Boy usage on other targets. I don't think they mentioned that the engines were Packard built - or did they ?

[qantasqf1]

Brian, I’ve only seen the first two parts. There’s nothing said about the engines, but I can’t speak for the entire series as part 3 isn’t on until next Sunday afternoon and well after Norwich lose. Again. However I would guess there’s no mention of the engine manufacturer because after all it’s not ‘on topic’ so to speak.
Steve

[Brian-H]

Well Norwich were playing at Chelsea's home ground on Saturday, and losing there was obvious, even Liverpool would struggle to draw there. But next Sunday Norwich are at home against Leeds, so the result is not easy to predict.

I think I might be confusing the Dan Snow Dambusters with another documentary on 617 squadron in which they went on to use Tallboys against various targets including the Tirpitz .

[A.N.Other]

I am following this thread with interest .
Come on Leeds

[Brian-H]

If I were doing the football pools, I'd put Norwich v Leeds as a no-score draw.

[tpickering]

That would put the cat amongst the Canaries

[A.N.Other]

I like a thread with a bit of a twist.

[tpickering]

Yep you can't beat a good yarn!

[Brian-H]

Looks like the Cat got the Canary

Bizarre yesterday with Brighton getting a draw at Liverpool after being 2-0 down
And Crystal Palace playing away at Man City and beating them

Maybe admin could move most of this to a Footie Thread