Merlin engines; Rolls-Royce and Packard, thread systems
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    Default Merlin engines; Rolls-Royce and Packard, thread systems

    Packard Built Merlins

    An often asked question is; “did Packard replicate the British thread system when they built Rolls-Royce Merlins under license during World War II?” The answer is yes; all threads that were used on the Merlin were accurately replicated by Packard. This would include BSW (British Standard Whitworth), BSF (British Standard Fine), BSP (British Standard Pipe) and BA (British Association). Having said that, however, Packard Merlins> used U.S. built Bendix injection carburetors; PD-16 for single stage engines and PD-18 for two stage engines, both of which used U.S. Unified threads. British built Merlins employed S.U. carburetors using Whitworth threads. The job facing Packard when they undertook manufacture of the Merlin was daunting to say the least. It’s bad enough having to build a complex product like the Merlin but exacerbating the situation was the fact no tool maker in the U.S. made Whitworth taps or dies. Therefore, Packard were forced into making their own. Although this created a significant hurdle to overcome, the effort was well worth it, Packard and Rolls-Royce components were interchangeable.

    From: British Fasteners

    Paul

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    The Unified threadform was developed jointly by the US, Great Britain, and Canada to prevent recurrence of the logistics problems created during World War II by the difference between US Standard and British Standard screwthreads, and was adopted as those countries' new national standard threadform in 1949. I'm therefore inclined to believe that the Bendix carburetors didn't really use the Unified threadform.

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    Long before WWII, there was some controversy in the USA as to which thread form was better for certain applications. Where this seemed to come into play was with staybolt threads for boilers, mostly on steam locomotives. For some reason, certain railroad mechanical departments in the USA would specify that the boiler staybolts be made with Whitworth threads. The rest of the boiler would have National Taper Pipe threads and what was to become the Unified National Form threads in the usual standard threads for bolting.

    At the hydroelectric plant where I've worked these past 24 years, we have quite the mix of systems of measurement and screw threads. Our big pump-turbines were made in 1970-72 by Hitachi, in Japan. At the time, the bid specs called for dimensions in inches and US threads. For the most part, Hitachi complied. We have studbolts with threads as much as 8" in diameter on some of the studbolts, cut to various pitches but using the Unified National Form. On pipe threads, Hitachi seemed to favor British taper pipe and British Straight Pipe using copper crush gaskets. Here and there, on stuff made by subcontractors for Hitachi (such as valves, lube oil pumps, gauges, pressure switches, all made originally by Japanese firms), we have metric threads and all kinds of adaptors. Add to this that we can find ourselves with metric sized pipe and tubing or inch sized tubing and regular "iron pipe" sizes- all on the same units.

    The original air circuit breakers in the high voltage switchyard came from Switzerland, as did the high pressure air compressors for them. Metric fasteners, British pipe threads both straight and tapered, depending on type of fitting. The original generator step up transformers were made either in Italy, Germany, or Switzerland depending on which transformer- some by Brown Boveri, some by ASEA, some by Alstom. Same story- you go to work on a conservator tank or oil cooler on a transformer made in Europe, and one transformer will have metric fasteners, another will have Unified National Form threads and use regular "US" bolting. Pipe flanges on this stuff can drive a person up a wall, as some will be DIN (German standards), some will be ANSI, and some will be JIC (Japanese) facing/bolting patterns. Down in the plant, on the Hitachi turbines and generators, we run into flared high pressure tubing that may use JIC (Japanese standard) or SAE, no rhyme nor reason or consistency. The result is that even though we've owned and operated this plant for over 40 years, we still measure and verify fittings, bolting, tubing and pipe sizes, flanges, and on it goes. Hitachi used miles of O rings on all sorts of joints, from tiny O rings to using O ring material off of spools to seal split joints on the turbine parts. Naturally, these details on the drawings are often kind of sketchy, with nomenclature half in English, half in Japanese, and the smaller nominal dimensions are often either not there, or hard to read. So, we mike the O ring materials, measure the grooves the O ring material goes into, and I often run a separate calculation to be sure we have the right size O ring material- too small and it won't seal, too large and it will extrude at the joint line.

    We live will very nearly every imaginable type of screw thread, pipe thread, and flange or flare fitting system in our plant. Sometimes, we have to make adaptors to go from metric pipe to US iron pipe sizes, which is where having our in house shop comes in handy. Sometimes, I will detail a "compromise flange", which may be bored as a slip on flange to fit on some odd pipe size, and then faced and drilled for something else again (like DIN or JIC instead of ANSI, or faced and drilled for ANSI, but bored to fit on a metric pipe). Other times, it may be a socket weld adaptor that we machine from bar stock to couple two different systems of pipe.

    Not too much surprises me anymore. I know that at the dawn of the automotive age, metric spark plug threads became the standard. Very early spark plugs for some of the hit-n-miss engines or the early auto engines used NPT threads, but somewhere along the way, the world seemed to settle on metric spark plug threads. A car, truck, engine, motorcycle or aircraft engine built in the USA could have all the dimensions in inches and use Unified National Form threads in standard threads (UNC or UNF/SAE), but the sparkplug thread was always the exception, being metric. I never did learn the reason for this. My own guess is a lot of the early successful cars were coming out of Europe, and perhaps the early ignition systems came from Europe as well (Bosch, from Germany, perhaps Eisemann and Splitdorf for magnetos, coming from Germany & Scintilla from Switzerland). Maybe someone can shed more light on this little anomaly.

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    I have seen WII Japanese aircraft engines in museums with American spark plugs in them. Perhaps they were added later. On the other hand,when the Russians had the gall to make direct copies of an American B-29 bomber that force landed there,where did they get the tires? From American surplus sources.

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    Quote Originally Posted by paul39 View Post
    but exacerbating the situation was the fact no tool maker in the U.S. made Whitworth taps or dies. Therefore, Packard were forced into making their own.
    From: British Fasteners

    Paul
    I don't know where they got that from, I've some pre WW II Pratt and Whitney B.A. taps and dies, and (I think) a screw gauge plate.

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    As long as we are speaking of who "borrowed" whose technology or designs, the M1A1 Abrams Tank is a present day example. I watched some program (Military Channel, I think) of how the Abrams tanks are built. I was surprised to learn that the starting point is the hull of a former Soviet tank. These tank hulls are lined up in the yard where the Abrams tanks are built. The people at the plant refer to these tank hulls as "rusty's". When the order comes in for some Abrams tanks to be built, some of the ex-Russian tank hulls are hauled into the plant, shot blasted, and then modified to build into Abrams tanks.

    A lot of technology has been passed around. Chinese steam locomotives are a great example. I worked on the engineering to evaluate the boiler design on the first new Chinese steam locomotive to be brought into the USA. The boiler detail drawings were dimensioned in metric, with plenty of Chinese nomenclature and notes. Interspresed in all of this would be things like: " 1 1/2" NPT" surrounded by Chinese notations and metric dimensions. The reality is the Chinese had taken typical US steam locomotive designs from US Railroad Administration locomotives that were shipped there following WWII. The knuckle couplers are what US railroads use, same airbrakes, and most all else on the locomotives followed US practice. Most of the auxiliaries on the locomotive such as feedwater injectors, air pumps, airbrake system, all seemed to be knockoffs, or close to it, of stuff that had been made in the USA. On the heavier Chinese engines, they use mechanical stokers which are also knockoffs of mechanical stokers used on US engines. There is pretty good interchangeability on a lot of this, and some restoration of US steam locomotives has been done using Chinese after-market auxiliaries. I've heard some of this equipment literally bolts right in and the pipe connections line right up. Since the world is not breaking the door down to build steam locomotives in any kind of numbers, if the Chinese want to copy US locomotives and auxiliaries, great. Imitation is the sincerest form of flattery, I guess.

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    The Chinese space effort got off the ground by simply buying the designs of no longer used American rockets. Why not? Why go through duplicating work already done when you can just buy it and get started. Once you have developed a cadre of people with experience in fueling and launching rockets, you can start innovating.

    A little OT, I have always thought it funny that Americans were buying guns to repel a Chinese Communist invasion, largely SKSs and AKs made in China while they laughed all the way to the World Bank.

    Bill
    Last edited by 9100; 05-14-2013 at 08:59 AM. Reason: spelling

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    To the original poster (Paul39): I have always been interested in the whole story of the Packard-Merlin project. Is there a good book you could refer me to ?

    JH

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    Quote Originally Posted by James H Clark View Post
    To the original poster (Paul39): I have always been interested in the whole story of the Packard-Merlin project. Is there a good book you could refer me to ?

    JH
    Sorry no. I stumbled upon that tidbit while searching to find if the rumor of post WW2 British cars had a mix of Whitworth and war reparations German metric fasteners was true.

    Do a Google search, I am amazed at what comes up if I keep persisting.

    See: https://www.google.com/#hl=en&gs_rn=...iw=853&bih=484

    Paul

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    They will be laughing even harder when we have to give them Alaska and South Carolina to settle our debt.

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    To get back to the original thread: I heard stories from a couple of oldtimers about the Merlin engines in WWII. As is well known the first Merlins to retrofitted into US fighter aircraft were made in England. When these engines were received in the USA, there was some disbelief as to the lack of consistency and quality in them. Supposedly, these first Merlins were taken apart and gone through, some modifications made, and a process similar to "blue printing" the engines was done. Then, those Merlins ran and performed a lot better. When Packard was awarded a contract to build the Merlins, the oldtimers told me it was a much better engine with tighter tolerances maintained. Whether this is simply some wishful thinking, a myth, or true remains to be found out. When I remarked about Rolls Royce building the Merlins, and my having a hard time swallowing the story that RR would turn out anything like sloppy work, I was reminded about British cars and motorcycles of the 50's and 60's. I never rode in a Rolls Royce in my life, and I never got closer than seeing one parked at the curb or in museums, so have no firsthand knowledge of anything RR built. I tend to go with the legends surrounding Rolls Royce, and have a hard time believing that the RR Merlins were not up to the standard Packard built them to. No disrespect intended to anyone.

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    In the related and earlier thread (Piston Aero Engines) I mentioned this book which has a number of pages devoted to the subject. Packard - A History Of The Motorcar And The Company

    The other thread:

    http://www.practicalmachinist.com/vb...ng-213940-new/




    Quote Originally Posted by James H Clark View Post
    To the original poster (Paul39): I have always been interested in the whole story of the Packard-Merlin project. Is there a good book you could refer me to ?

    JH

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    Joe,Chuck Yeager,the first man to fly supersonic was a WWII pilot. For some reason he hated all things British. No idea why. He remarked that the Merlin was not much of an engine till Packard got hold of it. I have always wondered why he would say that.

    I have heard that many parts made in England for things like tanks had to be hand filed and tinkered with to be made to fit. They liked American tanks because they were easier to work on. No hand fitting was needed.

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    I was hoping to sit this one out, but it was inevitable that hearsay and the Packard mythology would emerge, about how they redesigned it, how they improved its performance, made it more reliable, how Rolls-Royce had old time craftsman building the engines by hand in penny numbers, while Packard churned them out, to higher standards, too.

    If anyone can offer any facts to support the mythology, let’s hear them.

    People are free to believe what they want regarding rival claims about the relative merits of different engine producers, but I hate to see the efforts and sacrifices diminished of Rolls-Royce and its designers, and of the brave test pilots, given the debt that people in Britain owe to the Merlin, not least for its vital role in the Battle of Britain, in 1940.

    The story of the Merlin is a fascinating one, and luckily it has been well told by people who were closely involved. I have read a number of books published by the Rolls-Royce Heritage Trust.

    The Merlin was being continuously developed to squeeze out more and more power, and the books give an excellent insight into the problems that arose, not just with increased power output, but with different applications, and the solutions found to overcome the problems.

    The need for modification, to provide increased output and to cater for different engine applications and to adapt for local manufacture placed a great strain on the design team. In fact some of R-R's leading engineers went to Packard to act as liaison engineers. These included T B Barrington, the Rolls-Royce Chief Designer, and J Ellor, Chief Experimental Engineer.

    Sir Stanley Hooker, in his fascinating autobiography ‘Not Much of an Engineer’, recounts that these two 'bore such a crushing load that Barrington died in the USA and Ellor soon after his return’.

    In addition to their responsibility for design changes which could be of critical importance for men - including Yeager - flying deep into the heart of Germany, the engineers were presumably having to work long hours suited to liaison with their colleagues in the UK.

    One of the RRHT books is Rolls-Royce Piston Engines - a Designer Remembers by A A Rubbra. Rubbra took over as Chief Designer when Barrington was seconded to Packard. The book gives a first hand account of how problems were overcome, and also contains superb drawings of engines and components. The Merlin seemed to consist mainly of gears! Lots of nightmarish lathe work, too, with thin, hollow shafts with lots of diameters needing to be made concentric with each other. Another thing that comes across is how ‘close to the edge’ military piston aero-engines had to be designed, with regard to stressing, output, power-to-weight, and overall dimensions. The Merlin was quite a compact engine compared with its German rivals.

    Having quickly looked through the books, the information relating to Packard is quite limited. However, I will summarise some of the points made. Where problems arose, they are described in a sympathetic way, not out of criticism.

    In The Merlin in Perspective - the combat years, the author (Alec Harvey-Bailey) says 'As one would expect, Packard built engines to very high standards of quality. Technical problems were not dissimilar from those experienced on British engines and when comparing like with like modification standards there was nothing to choose between engine sources.

    'At squadron level there were times when there were fortuitous variations in reliability either way but when dealing with large numbers of engines at Group or Command level there was good consistency between engine sources
    .'

    Incidentally, this book has comprehensive tables showing the numerous Marks produced by R-R and Packard and summarising the changes embodied.

    Rolls-Royce had the advantage over Packard of having close and immediate involvement in resolving any problems that arose, and also they’d learned by experience of subtle factors that can cause problems. There were cases where Packard (or North American Aviation with the Mustang) went their own way on apparently minor matters, and problems arose (for example with engine coolant formulation, and pipework routing and materials). Ref: Rolls-Royce and the Mustang by David Birch.

    I’ll just mention one specific problem that arose at an early stage, not because I'm trying to dimininish Packard's work, but because I think it's interesting. The Merlin in Perspective mentions a problem with occasional fatigue failure on connecting rods, which R-R had designed out. 'Because Packard were less flexible in embodiment of modification, engines from this source used the Mod 399 rod for much longer and ran into a number of characteristic failures. When examined the Packard rods appeared to be well finished with heavy polishing of the rods and associated areas. British experience was that rods free from finishing scores were far less prone to failure and this apparent contradiction was surprising. However, when the rods were etched it was found that the polishing operation had swaged material into deep score marks, masking them from visual inspection, and failures could therefore be expected. It was a lesson in the effect of heavy over-polishing.'

    There are a number of interesting examples of problems solved which I’d like to air, but they’re for another day, or not.

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    One issue to not ignore: aircraft engines of the time were constantly being pushed for higher power outputs. So engines made early in the
    war in britain probably had a lower max rating than those made in britain, later in the war.

    The problem would be when you compare early british merlins, with late-war ones by packard.

    My understanding is that some of those engines had peak hp ratings practically doubled over the
    course of development during the war.

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    A great example, though neither vee type nor liquid cooled was the Wright Aeronautical 1820 radial, a nine cylinder design 6.125 X 6.875". The first 1820 in about 1930 was the 1820E at about 575 HP and 900 lbs. Less than twenty years later it weighed about 1600 and made 1525 HP as installed in the T-28 - from identical displacement.

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    My reply was not intended to be hostile to the English. Merely to mention,with no information on my part,what Yeager said. Two of my best friends are off the boat Englishmen: The Master bookbinder,and the harness maker. The old harness maker is the real deal. He was raised in the trade. During the filming of the movie "The Battle of Britain",he made or repaired many of the helmets,boots,and other things used in the film. He told me that some of the spitfires in the movie were equipped with Jaguar engines,and could barely get off the ground. He came here to the museum about 25 years ago. He was and is a very helpful guy,always making little gifts for others in the museum,including myself. I recently gave him a vintage Webley mark 1 air pistol(I collect Webley and other fine old air guns). I also like the british fellow in charge of coach and livestock. His brother,I believe,is or was in charge of the Royal Artillery (I can't recall their correct name). They dress in period costumes and do parades and fire pre WWI cannons(which I'd love to make a working model of,if I could get accurate plans). They were the type used in "Lawrence of Arabia". Short barrels with recoil cylinders on top of them and spoked wheels.
    Two others were absolutely huge PITA's,like anyone else,some good,some not.

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    Hello, regarding all the original confusion about the different threads, this begs the question, that if UNC was similar to BSW, and UNF again similar to BSF, why if the British threads had worked OK for a hundred years or so, did the US not carry on using them ?. Surely, if the thread system had worked in Britain for that long, why change it.? As 9100 said, when the Chinese started their space program, they used well proven American rockets. Mr Whit worth is probably looking down and thinking, bloody hell, I tried to sort all of these different thread pitches out in in the 1800's, and its just the same in 2013. !!!. Only worse.
    Just my two cents worth.

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    Good question, Roger...

    The Whitworth / BSF thread form's stronger, as it has a radius at the root instead of a flat, so much less of a stress-raiser - it may be that the Unified thread is more consistent with the rest of the world, Metric etc. also being 60* threads with flats at root and crest? I imagine it's a lot easier to make tooling for flats than exact radii too...
    There was a much larger manufacturing capacity in the USA post-war, too, as the Luftwaffe didn't manage to bomb many American factories... So strength of numbers would have swayed the decision!


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