The P-51 Mustang header tank (P/N – 106-46002), is fitted immediately behind the propeller spinner and is connected to each cylinder head coolant outlet and arranged around the reduction gear housing. The tank is designed with two outlets, one on each side of the gear housing; the lines from the outlets join in a “Y” just forward of the firewall and continue as a single line extending to the radiator.
This is a critical component that can be described as performing several key functions:
Separating air and vapor from the coolant is a vital function. The separation of air and vapor is obtained by means of two cylinders through which the coolant liquid passes immediately after entering the tank. Centrifugal action forces the liquid to the outer portion of the cylinders and the separated air is led off from the central portions through a tube. Header tanks with leaks or damaged internals lose the ability to separate air as well as a maintain a back pressure in the cylinder bank assembly and may lead to damage of the cylinder liners and heads.
To understand the header tank one must consider it in context to the P-51 engine cooling system. This 16.5 gallon system is unconventional in the respect that it operates at pressures of up to 50 pounds per square inch. Otherwise it is of the conventional type consisting of components normally found in a liquid cooling system. The normal cooling medium is circulated at a relatively high velocity by an engine-driven centrifugal-type pump mounted on the bottom of the engine. The coolant flows from the pump to the exhaust lower side of each cylinder block, passes to the cylinder heads through brass, rubber-sealed tubes, then out through the manifolds attached to the intake side of each cylinder head. The manifolds discharge the coolant into the header tank, mounted at the front of the engine. From the header tank, the coolant flows directly to the engine cooling system section of the radiator, then to the pump for another circuit of the system. A concentrated, high-velocity flow of air through the radiator dissipates heat from the coolant as it flows through the radiator tubes. On all coolant lines which are located in the areas not readily accessible, install double hose clamps.
The header tank was originally designed and engineered by North American as P/N 73-46002 on August 2, 1940, and approved about two weeks later. This initial tank was used on the P-51A, A-36, and earlier models, including the experimental mustang subtypes.
In November 1942, the design for a subsequent variant of header tank was drafted for the Merlin V-1650 powered P-51 Mustangs. This tank P/N 102-46002 was approved one month later and was installed on NA-102 and the first 400 NA-104 Inglewood California built P-51B models. It was also installed on the first 600 NA-103 Dallas-built P-51C models. This tank design didn’t last long and was inactivated for future use on April 30th, 1943. This tank did not include the scroll assemblies to remove air and create a back pressure in the cylinder banks.
The most recent tank design, P/N 106-46002, by NAA was drafted for production on June 22, 1943. This new tank had four subsequent production changes indicated by the addition of a dash number (106-46002-50, 106-46002-75, 106-46002-100, 106-46002-150). For simplicity we will focus on the makeup of the 106-46002-150 tank since it is the most recent tank manufactured for the P-51. Improvements on this tank from the initial 106-46002 design include the following:
The 106-46002 drawing also indicates that the completed assembly to be chromic anodized, though the external anodising of the shell is optional, and that the tank is to be pressure tested at 50 P.S.I.
Header tank variants 106-46002 are interchangeable, but use caution as design improvements were made and prior production lots were deemed inactive for future use. The propensity for pin holes is increased on earlier variants due to the thinner aluminum used on the aft shell.
The 106-46002-75 & subsequent header tanks included a dent on aft side of the tank near the filler port. This dent, added on July 8, 1944, provided clearance for the crank case vent line.
The -600 & -700 series Merlin engines require the header tank to two indents to provide clearance between engine heads and banks and the tank.
Transport Merlin Engines – Toward the latter end of the Merlin’s military/civil life, some variants of the engine had been developed to a very high degree of longevity. The engine developed for the civil DC-4, the Tudor and the Argonaut were the -600 & -700 series Merlins. Engines where longevity was of more importance than ultimate performance. When Transport Heads and banks are indicated it means that the cylinder blocks, cylinder heads, cams and operators are to this later specification and give an aspiration of longer life than normal over a stock Spitfire or Mustang engine. 1
1 Glenn Wegmann
Use the instructions outlined in Tech Order – Maintenance Instructions for F-51D, F-51M, ZF-51K, and TF-51D, T.O. No. 1F-51D-2 (AN 01-60JE-2), 30-Nov-1956
Removal of the propeller (though not necessary) will provide greater accessibility for header tank removal.
See the diagram below for appropriate steps:
This heavily worked component on the P-51 Mustang requires frequent inspection, borescoping for internal cracks, maintenance, and attention to prevent failure, particularly if operating a 70+ year old tank.
Some key questions in determining if your Header Tank needs to be inspected / tested / replaced / repaired:
The Nov 30th 1956 Maintenance Instructions for F-51D, F-51M, ZF-51K, and TF-51D, T.O. No. 1F-51D-2 (AN 01-60JE-2) details the following steps for testing the engine cooling system header tank
AirCorps recommends these additional inspections.
As reference, the Aircraft Inspection & Maintenance Guide – P-51, 00-20A-2-P-51, 7-Nov-1947 outlines a detailed inspection of the coolant system that should happen during pre-flight, after flight, daily, and at 25, 50, 100 hour inspections.
The near the inlets scroll assembly P/N 106-46149 – SCROLL ASSEM – COOLANT HEADER TANK TWIN INLET.
How to inspect – Water level check, may be possible with borescope
Tolerance of damage or wear : Zero
Solutions offered: Replacement of Scroll Tubes
The internal tube assemblies P/N 106-46147 are prone to cracks in a number of areas and require consistent inspection. Experience in inspecting header tanks has identified the propensity for cracking in a few select areas:
At lower corners of welds installing P/N 102-46150 – PLATE—COOLANT HEADER TANK INLET FACE
How to inspect: Pressure Testing
Tolerance of damage or wear : Zero
Solutions offered: Welding repair
Separation of line P/N 106-468103 – LINE – COOLANT HEADER TANK TWIN SCROLL BLEEDER LEFT from P/N 106-46147 – TUBE ASSEM – COOLANT HEADER TANK TWIN INLET LH.
How to inspect: Borescope through filler port or center casting
Tolerance of damage or wear: Likely will not affect tank performance but thorough inspection of rest of tank necessary
Solutions offered: Weld during other repairs
Cracked weld attaching lugs (three) to scroll tubes attaching to P/N 109-46094 – FITTING—COOLANT HEADER TANK OUTLET.
How to inspect: Borescope
Tolerance of damage or wear: Repair necessary – scroll tube damage immanent
Solutions offered: Welding
How to inspect – Pressure Test, Borescope
Tolerance of damage or wear: Repair necessary if tank does not pass pressure test
Solutions offered: Replace shells
The most common failure on the header tank is due to pitting and corrosion of the forward and aft outer shells. This corrosion is the combination of factors that include lack of internal anodizing, poor coolant types being used, contamination of the system and frequent environmental factors such as heat and cooling. Pinhole leaks are common in old tanks and require frequent removal of the tank and welds. These pin holes commonly develop on the inner surface of the shells and create leaks at random. Aircraft operating with a pre-106-46002-150 variant tank have an increased propensity to develop pinhole leaks due to the thinner material used.
If these small leaks are occurring, it is commonly sign that the integrity of the tank is compromised. The only way to know the full scope of the corrosion is to split the tank in half by cutting it at the weld.
AirCorps recommends, Following guidance given in Military T.O. 01-60JE-2 (Maintenance Instructions for F-51D, F-51M, ZF-51K, and TF-51D, T.O. No. 1F-51D-2 (AN 01-60JE-2), 30-Nov-1956)
INSTALLING ENGINE COOLING SYSTEM HEADER TANK
Proper bleeding of the coolant system is necessary to release any trapped air and must be repeated after running the engine until all trapped air has been released. Continual release of air from the bleed line can indicate failed scroll tubes or chambers inside of header tank. Bleeding is done for both the coolant system and aftercoolant system at the door assembly located on the left wing root fairing P/N 109-10007 – FILLET ASSEM. – WING TO FUSELAGE – REAR.
When installing the avimo connections it is important for the tank to be lined up with the engine outlet connections and the tank not be forced into position and then clamped. A straight tank of the proper outside dimension should have no troubles lining up. If the Avimo connections do not line up loosen main tank straps and position for proper alignment. Remember to clock the gap in Avimo shell 90 deg to the gap in the clamps and verify that shells have captured the final ring on both the tank and the engine castings.
The following parts are available from Vintage V-12’s.
P/N: 616588 or 604867 – Avimo Shell (4)
P/N: 616478 or 604866 – Avimo Seals (2)
It is important to remember to not use knives and hard picks or other sharp tools to remove the Avimo clamps and the outlet connection houses. It can be quite easy to damage the relatively soft castings that make these connections.
To maintain a clean and properly functioning coolant system it is necessary to properly ground all coolant tubes and the engine. Grounding reduces electrical activity and corrosion. If left ungrounded the cooling system becomes a warehouse for this stray electricity. This charged coolant is constantly searching for a ground or a way out of the system. When it finds a material it can attack (the path of least resistance), the coolant goes to work “eating through” material that makes up the engine and tank.
According to the parts manual there is a hard line connection P/N 102-46892 – LINE – COOLANT HEADER TANK CROSS FLOW between the two interconnect nipples on the bottom of the tank. This hard line is then clamped to the engine. We have now seen on multiple occasions cracks develop at the base of the nipple due to stress from the hard line connection. We recommend eliminating the hard line for a single hose between the two nipples. This prevents stress that is likely to break the nipples and cause a coolant leak.
The Roush Aviation Coolant Header Tank Pressure Cap is produced as an FAA-PMA component that is a substitute for P/N 99-46141 – CAP – COOLANT HEAD TACK FILLER NECK this cap contains a schrader valve in the cap to pre-pressurize a cold system to perform a maintenance leak check. Pre-pressurization of coolant system with shop air to 14-15 psi prior to operation will increase the boiling point of the coolant by 45° F (25°C). This will reduce the risk of having localized boiling in the cylinder head which leads to cylinder head cracking. This cap is available with an o-ring type seal that requires an alteration to P/N 73-46008 – FLANGE – COOLANT TANK FILLER NECK. This cap is also available with oversized threads to repair tanks with damaged threads.
The header tank assembly was provided from the factory with clamps, cap, and a pop-off valve P/N SA3817-C30 produced by Standard Aircraft Products. We recommend using a fresh new old stock valve with seals that have been replaced and tested to meet AN 03-1-21, Operation, Service and Overhaul Instructions with Parts Catalog for Coolant Relief Valves (Standard Aircraft).
Operators are able to keep tabs on critical cooling system pressures before they become an issue with this AirCorps Aviation Coolant Pressure Gauge kit. The installation kit provides a pressure sender, wiring harness and 1 1/4″ gauge for convenient mounting in the cockpit. Also can be provided with 2 1/4″ gauge if there is an empty hole in instrument panel to mount in. Includes a tank mounted sender which requires removing the tank and tapping a 1/8″ pipe thread hole to install sender. Installation by 337 and field approval.
Originally these tanks were welded using a gas welding process which does not create stress in or near the welds as the modern TIG welding process can. Due to the fact that the tank is made from multiple aluminum alloys we can’t stress relieve the tank using heat after welding.
AirCorps uses a mild subharmonic vibratory stress relief process called Meta-Lax that stress relieves the tank after fabrication and welding. This process, done in addition to the rigorous testing, improves the tank and weld quality. While it is a technology that wasn’t present in the 1940’s embracing it today ensures we are delivering the best tank possible to owners, operators, and maintainers of the P-51 today.
If you’re looking for a replacement header tank, ensure airworthiness prior to purchasing by inspecting for cracks, dents, finish, and by testing pressurization. While replacement header tanks are quite difficult to source right now, AirCorps is available to perform inspections, answer questions, and perform services related to header tanks and attaching parts, caps, and valves related to installation.
In repairing and inspecting numerous tanks over the past seven years, AirCorps has established a standard process of handling these increasingly rare, high-value components. The importance of recording serial numbers, inspecting for airworthiness by checking for cracks, corrosion, wall thickness, and finishing, if necessary, cannot be understated
All work begins with inspection of each individual part of the tank. A concise estimate for repair is prepared, and work is executed to the original North American Aviation drawings and processes. Tooling, fixtures, and certification of welders are all necessary to allow AirCorps and others to complete the work to FAA airworthiness standards..
All header tanks can be provided with or without indentions for transport heads and are pressure tested and chromic anodized per the NAA drawings.
Contact Eric Trueblood for more information – firstname.lastname@example.org
Vice President of Restoration
Mark Tisler achieved his Airframe & Powerplant in 1985 after graduating college with a degree in Agricultural Mechanization. In 1988, Mark began working with Gerry Beck at Tri-State Aviation in Wahpeton. His love of warbirds didn’t stop at just airframes, his humble and technical expertise make him a favorite of operators, veterans, and aircraft owners. Tisler has worked on 13 major P-51 restorations and everything from a Stearman to a Corsair. This AirCorps co-owner helps lead the award winning Restoration & Maintenance departments which work not only on resurrecting historic aircraft but also prepping restored aircraft for flight readiness. This capable team does inspections, light and heavy maintenance, and component overhaul / repair for a variety of aircraft.
The prior month saw Mark troubleshoot a Corsair engine in Canada, assisting in the structure assembly of a P-51K in Fargo ND, troubleshooting issues on two P-51D Mustangs in the Midwest, delivering a truckload of P-51 parts including a completed P-51 lower end to Midwest Aero Restorations. Today Mark is back in Bemidji working with Jose Flores from Vintage V-12s who is in town to assist in some V-1650 test stand work.
Contact us for more info on ways we can assist with your project!
AN 03-1-21, Operation, Service and Overhaul Instructions with Parts Catalog for Coolant Relief Valves (Standard Aircraft), 25-June-1945
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