A stupid Merlin engine question

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Hi all, I´v got one of those stupid questions that pop up in my head from time to time :rolleyes: What kind of bolts (and nuts and screws etc..) were used on the Merlin? Were they old British measurements (BS) or metric? I know of some engines from the time period (20´s - 30's) that were metric, for example the the Cirrus Minor. And, Yes, I do know why! But what is the case with the Merlin??
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Merlin - Imperial or Metric All Merlins were built using Imperial fastenings. The Packard-built Merlins were "Americanised" with (in some cases) different bolt spacings and UNF/UNC fastenings. I expect Merlin Pete will be along to give us a more elaborate answer in due course. Anon.

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The Packard threads may have been what we would now call UN, but the the unified standard was not adopted until after the war - in an attempt to solve these problems. (Parts which are labelled as Unified or have the three overlapping circles mark are deffo post war.)
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Might be worth somebody who knows their threads and bolts etc to do a little piece on this - I find it utterly bewildering! So does the three circles emblem actual denote the use of unified threads?
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They are all BSF / BA threads with some special pitches particularly on pipe fittings which are the same BSF profile but finer pitch than standard for that particular diameter. Packard used exactly the same threads but I have seen AN (UNF) hardware on modern Packard rebuilds. The only exceptions on Packards were the Bendix carburettors which they fitted and the US-built fuel pumps, these were AN. Pete
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Further to Pete's post, the Packard crankcase/lower crankcase, (and wheel case) use a larger size stud than the British versions, but once removed, the thread in the casting is exactly the same size! thus interchangeability is achieved!

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Might be worth somebody who knows their threads and bolts etc to do a little piece on this - I find it utterly bewildering! So does the three circles emblem actual denote the use of unified threads?
..Yes! The three circles sometimes on the head of the bolt, more often on one of the 'flats' as per the nut shown.
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I seem to recall from some books of the period that the 2BA spanner was a vital tool for any Merlin engine fitter. Moggy

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..Yes! The three circles sometimes on the head of the bolt, more often on one of the 'flats' as per the nut shown.
If I remember correctly, the 'three circles' on the flat is or can be a marking used by the manufacturer of the hexagonal bar ( overlapping circles run the whole length of the bar ) and not necessarily a 'unified' marking. I believe this lead to the fitment of the wrong type of bolts or nuts on an aircraft which led to an incident a few years ago although I can't remember the details ( aircraft windscreen panel ?? ) Steve

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If I remember correctly, the 'three circles' on the flat is or can be a marking used by the manufacturer of the hexagonal bar ( overlapping circles run the whole length of the bar ) and not necessarily a 'unified' marking. I believe this lead to the fitment of the wrong type of bolts or nuts on an aircraft which led to an incident a few years ago although I can't remember the details ( aircraft windscreen panel ?? ) Steve
I think the windscreen incident was the British Airways BAC 1-11, 8-32 bolts fitted in to 10-32 anchor nuts, or similar. Seemed fine on the ground, but once at altitude the cabin pressure blew the screen out...nearly taking the pilot with it!

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I seem to recall from some books of the period that the 2BA spanner was a vital tool for any Merlin engine fitter. Moggy
Don't know about Merlins but as a Rolls-Royce car mechanic for over 40 years A 2BA is THE most important spanner! I still have my Original RR one issued to me as an apprentice........................... PS. That is until BMW got there grubby 'ands on it................
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They are all BSF / BA threads with some special pitches particularly on pipe fittings which are the same BSF profile but finer pitch than standard for that particular diameter.Pete
BSP? Not only did Packard use exactly the same threads as RR, they made their own taps and dies because they couldn't find anybody in the States with the capacity to do them in time.
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Thanks for the replys guys. Looks like we have to get ourselves a 2BA spanner then :diablo::D
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BSP?
Yes, there are a certain number of BSP threads but there are more which are very fine instead, these are the non-standard type. Pete

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Yes, there are a certain number of BSP threads but there are more which are very fine instead, these are the non-standard type. Pete
Are these bsp threads the same as you find on hydraulic and air fittings?
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As a general rule - yes. BSP - British Standard Pipe.