Mark on Maintenance – P-51 Header Tanks

P-51 ENGINE COOLING SYSTEM

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.

VARIANTS OF HEADER TANKS

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 aft shell half (102-46002-7) was thickened from .064 to .091
• The two lower outlet castings (109-46094) had a minor design change to alter the location of the lugs at the base of the casting.
• The center casting (102-46150-50) had modification to decrease the size of the center port from 2.250” to .750”. Additionally, AirCorps has an approved change to increase the thickness of the web around the center port from 3/16″ to 7/16″ thick. This modification allows secure installation of a pressure sensor which transmits critical data to the pilot.

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.

INTERCHANGEABILITY

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.

RELIEF DENTS ON STOCK AND TRANSPORT HEADS

SINGLE DENT TANKS 

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.

DOUBLE DENT TANKS 

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

REMOVAL OF ENGINE COOLING SYSTEM HEADER TANK

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:

INSPECTION

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:

• Is the status of tank unknown?
• Has the tank recently been inspected?
• Do you suspect cracking on internal scroll tubes (details below)?
• Have you frequently been welding pin hole leaks?
• Does the header tank in use have original manufacturer outer shells likely containing corrosion (details below)?

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

1. Flush tank with hot water
2. Remove relief valve, and plug all openings, except one
3. To this opening, apply a pressure of 50 pounds per square inch, and submerge in water
4. Mark location of any bubbles formed by leakage

AirCorps recommends these additional inspections.

• Tank end scrolls should be leak checked at each removal to verify there are no internal cracks
• Tanks should be checked for internal corrosion with borescope when removed
• Verify clamps and gasket material are in good condition and aligned properly (AirCorps requires customers to provide clamps for alignment in a fixture so tank is assured to line up with engine studs.)
• A simple inspection, in addition to borescoping the tank, is to fill the scrolls with water and verify the water level does not leak to the main tank. If the water level drops there are the cracks in the scroll tubes.
• Visual inspection by the pilot of external surfaces on each preflight as outlined in this discussion for all header tanks.

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.

HEADER TANK FAILURES

ISSUE 1 – CRACKING / BROKEN WELDS 

Cracking Location # 1

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:

HEADER TANK FAILURES

ISSUE 2 – CORROSION & PITTING

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.

PROPER INSTALLATION OF TANK

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

1. REINSTALL PRESSURE-SUCTION RELIEF VALVE
2. CAREFULLY POSITION TANK IN SUPPORTING CLAMPS
3. CONNECT AND TIGHTEN SUPPORTING STRAP TURNBUCKLE, AND SECURE BONDING BRAIDS
4. CONNECT ALL LINES TO TANK, BEING ESPECIALLY SURE TO CONNECT BOTH ENDS OF CROSS-FLOW LINE.
5. INSTALL ENGINE COWLING FORMERS.
6. REINSTALL PROPELLER IF PROPELLER HAS BEEN REMOVED
7. REINSTALL ENGINE COWLING.

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.

ATTACHING PARTS & RECOMMENDATIONS

AVIMO CLAMPS & SEALS

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 AirCorps Depot – links to product listings below.

P/N: 616588 or 604867 – Avimo Shell  (4 per aircraft)
P/N: 616478 or  604866 – Avimo Seals (2 per aircraft)

T.O. No. 01-60J-45, Inspection and Replacement of Coolant Header Tank Inlet Hose Clamps for P-51B, P-51C, P-51D, and P-51K, 22-Feb-1945

DAMAGE OF CASTINGS REMOVING & INSTALLING TANK

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.

GROUNDING

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.

FLEXIBLE HOSE ON INTERCONNECT TUBES

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.

PRESSURIZED COOLANT SYSTEM (ROUSH CAP)

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.

PRESSURIZED COOLANT SYSTEM (POP OFF VALVE)

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).

WHEN PURCHASING

In repairing, inspecting, and assembling 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.

Tooling, fixtures, engineering, auditing and certification of welders are all necessary to allow AirCorps to complete the work to FAA airworthiness standards and to support the industry with this safety sensitive and critical component.

AirCorps is available to perform inspections, answer questions, and perform services related to header tanks and attaching parts, caps, and valves related to installation. If you’re looking at an original or repaired header tank, ensure airworthiness prior to purchasing by inspecting for cracks, dents, finish, and by testing pressurization.

In committing to and completing the PMA of the header tanks AirCorps has ensured that this difficult component to source will no longer be a source of frustration for operators and maintainers.

Contact Eric Trueblood for more information – erict@aircorpsaviation.com

THE P-51 GURU

ADDITIONAL RELEVANT HEADER TANK TECH ORDERS & INFORMATION:

AirCorps Library – P-51 Mustang Resources

Aircraft Inspection & Maintenance Guide – P-51, 00-20A-2-P-51, 7-Nov-1947

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)

Maintenance Instructions – Cavalier Mustang – F-51D, T.0. 1F-51D-2, 27-Sept-1968

T.O. No. 01-60J-23, Modification of Coolant Expansion Tank Assembly for P-51B, P-51C, and P-51D, 18-Jan-1945

T.O. No. 01-60JD-20, Replacement of Header Tank Relief Valve Gasket for P-51B and P-51C, 23-May-1944

T.O. No. 01-60JD-5, Modification of Coolant Header Tank Relief Valve Locking Collar for P-51B, 9-Sept-1943

T.O. No. 01-60J-45, Inspection and Replacement of Coolant Header Tank Inlet Hose Clamps for P-51B, P-51C, P-51D, and P-51K, 22-Feb-1945

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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