Was the problem that Rolls Royce did not design the engine with mass ptoduction in mind so tolerences were 'loose' and the used skilled craftsmen to get parts to fit correctly.
Detroit got round this by redesigning the blueprints so the engine was suitable for mass production?

Is it about the intercooler sitting between the two stages of the supercharger in the Merlin?

Then is it about the difference in the carburetor setup of two engines (downdraft in the Allison, updraft in the Merlin)?

a- The NA-73X, P-51, P-51A, A-36A, P-51B/C and the RAF Mustangs I, II and III all suffered from poor rearward vision, which,

b- The RAF partially solved by modifying their Mustangs' Canopies by fitting them with Canopies which were similar to but NOT identical to the Spitfire's- These were ofttimes referred to as "Malcolm Hoods". A few USAAF Mustang types were fitted with these "Malcolm Hoods", but this did not entirely alleviate the problem, which,

c- Was partially solved by NAA engineers with the installation of the Plexi-glass (Perspex) "Bubble"-type Canopy on a P-51B/C-type modified with a "cut-down" Rear Fuselage Decking. Pilots now had a Mustang which afforded them nearly 360-degrees of "all-around vision. But, AHA! The "cut-down" Rear Fuselage Decking created some INSTABILITY, since there was now less physical area in the Rear portion of the P-51D-5s Fuselage. NAA engineers solved the problem by adding the now-famous Dorsal "Fillet" to the Vertical Stabilizer, which has become another trademark of the P-51D. Many D-5s were modified with "Field-expedient" Dorsal Fillet kits, in Theatres.

But just a second, I'm not done yet- Look at the plan-form of the P-51D/K models' Main Wing from either the TOP or the BOTTOM of the Aircraft, and compare to ALL of the EARLIER MODELS of the Mustang. Notice anything? I'll get to this directly-

I'm now going to delve into something that many of you may not be aware of as far as the Mustang is concerned. Permit me to digress to the P-38 and the P-47. How many of you are familiar with the problems which were encountered with "TAIL-BUFFET", NOT "TAIL-FLUTTER", which was what many aeronautical engineers thought the problem was when P-38s and P-47s lost their Tail Empennages in high-speed DIVES? Pilots were dying, but there were many things that the Aircraft Manufacturers were trying, and many of these things came to naught. It makes for some very interesting reading.

In Lockheed's case, the engineers did several things- First, they switched the Right Engine with the Left, so that the Propellers rotated OUTWARDS from the Center Gondola, at the TOP of their Propeller Arcs. Previously, the Props had rotated inwards. It was so simple, it almost seemed stupid (to the engineers themselves, that is), to not have thought of this before. It helped, but there was still a problem. Lockheed engineers then designed a FILLET, which was mounted at the Front of each Main Wing AT THE WING ROOTS- This helped A LOT.

OK, but young "hell-for-leather" Fighter Pilots are ALWAYS "pushing the outside of the envelope" when it comes down to breaking the rules of flying their Airplanes. The problem that they were encountering, was "COMPRESSIBILITY", which caused the P-38s and P-47s TO TUCK UNDER into flat inverted spins when in near- or absolutely vertical dives. The solution to this problem, in retrospect, was laughably simple. Dive Flaps- NOT Dive BRAKES, Dive Flaps...

(By the way, NO US, Allied or Axis Aircraft EVER "went supersonic" during World War II- That was a fiction created by the NEWS MEDIA and certain dill-wads in the USAAF "public-relations" department. Sound familiar? Colonel Cass S. Hough- pronounced "Huff"- NEVER said that he exceeded 780 Miles Per Hour in a dive, thereby "breaking the sound barrier". No WWII Fighter, US/Allied or Axis was ever capable of achieving "supersonic" speeds, because of their physical design limitations, period.)

Due to the P-51's beautifully streamlined lines and the design of her laminar flow-type airfoil Main Wings, there were similar problems with "Tail-Buffet", as was encountered with the P-38. Refer to the Main Wing plan-form of the EARLIER P-51s, from the
P-51B/C-models, P-51s, P-51As, the RAF models etc, all the way back to the NA-73X. The P-51D/K-models ALL incorporated FILLETS at the Forward Wing Root, as did the P-38. Republic Aviation solved their "Tail-Buffet problem" by beefing up the Tail Empennage of their already beefy Thunderbolt. PRESTO! No more Tail Buffet!

Diving the P-51 wasn't anywhere near as harrowing as it could be in the P-38 or the P-47. Even though the '51 was a bit heavier than the Spitfire, once you "pushed-over" in a Mustang, you had to "fly her down", not like in a Lightning or in a Jug, which when "loaded for bear" with certain types of combat loads, could weigh two or more times as much as the P-51. The P-38 and the
P-47 dropped like rocks. Yet, BOTH of these heavy Fighters could out-maneuver the P-51 in certain situations. Argue all you want, but those are the facts, quoted by the guys who flew all three types in COMBAT...

OK, let me try another tack- The P-51 was always vulnerable, (actually this was her "Achilles' Heel", because she was liquid-cooled, and the Radiator was located at the bottom of the Fuselage, just slightly aft of the Cockpit. More P-51s were lost by enemy ground-fire than by being shot down by enemy Pilots. ONE bullet or a piece of shrapnel in the P-51's coolant Radiator, and YOU weren't going to make it back to your base. P-51 ground strafing missions were somewhat curtailed; this was part of the "cure" for the P-51's "ground-pounder problem". The second part of the cure was to TRANSFER most 8th Air Force P-38s and P-47s in the ETO, (EXCEPT for the 56th Fighter Group), to the 9th Air Force, which was a "tactical" organization. The P-47, with her "ironclad" ruggedness and dependability, made for an EXCELLENT "ground-pounder"; lifting capability, survive-ability and her ability to absorb tremendous amounts of battle-damage made the Thunderbolt an attack-platform par-excellence. Not for nothing is our A-10 called the "Thunderbolt II".

The P-38s-

General Kenney, Commander of the 5th Air Force in the Pacific Theatre was screaming for P-38s, so most of the P-38H, J, and L models found themselves being shipped to the Pacific, where they destroyed MORE Japanese aircraft than all of the other Allied air-assets, combined...

Our two Top Aces, Dick Bong and Tommy McGuire, destroyed the greatest proportion of their "victories" while flying P-38s...

I know, I know... The four-fold question was built around the
P-51. I'll laugh out loud if the answers turn out to be something silly like a simple old Landing Light!!!

I'll try once again: to counteract the heavier weight of the Merlin, the copper radiator was replaced with a lighter aluminum housing, causing corrosion in the cooling system that led to engine-overheat. The solution was to cover the the radiator with the same fluid used in beer-kegs to prevent corrosion.

ok, here goes:
What was the most produced seaplane prior to the build-up to World War II; what plane was it based on; and what “first” is attributed to it?

Not to split hairs, but "Sea Planes" are a wide group of aircraft and are grouped into two categories-- Floatplanes and Flying Boats. Float Planes have external pontoons. In Flying boats, the hull and fuselage are synonymous. So there is a difference.

If it's a Flying Boat, it might be the Consolidated PBY Catalina. Produced in large numbers pre-war, it also made the first non-stop transcontinental flight by a flying boat from California to New York in 1937, breaking the speed record for the flying boat class along the way. It was developed from the earlier Consolidated P2Y Ranger. But the Ranger was considered a "sesquiplane" having a shortened span lower wing. The P2Y was also commercially successful as the Consolidated "Commodore".

But it could also be the Savoia Marchetti S55 series, of which 243 of the class (S55-S64) were produced before WWII. The "first" for the SM 55 would have to be the 1933 Transatlantic formation flight of 24 S55Xs flown in formation from Rome to Chicago (with several steps along the way) for the Century of Progress, coining a new term-- a "Balbo" which is any large formation of identical aircraft-- Named for the leader and organizer of the flight-- Italo Balbo. The S55 series was a mix of both Floatplane and Flying Boat, since there was no fuselage, it was a flying wing with twin hull pontoons which carried the passengers, although still grouped as a flying boat.

By the way, the Dornier "Wal" series (Wal, Wal J and Wal II) and the Dornier DO "X" are entirely different aircraft, the Dornier X being one of the largest aircraft ever built, powered by 12 engines (there were only three built), and it was not considered a successful design, able to only gain about 1600ft of altitude. About the only thing The "X" and the "Wal" series share is the planing surface design and the stabilizers as mentioned above.
VR, Russ