Servicing the Allison V-12

by C B F Macauley

Allison engines power the famed Lockheed P-38 Lightning and three other US fighter designs.

The military air power of the United States, which is being impressively demonstrated around the world today, is not wanting, nor is it out of balance as to types of powerplants, largely because of the Allison aircraft engine. If it were not for this powerplant, which is manufactured by the Allison Division of General Motors, and which was developed through the encouragement and with the cooperation of the US Army Air Forces, America's contribution to the total of the United Nation's air strength would depend, except for US-built British designs, exclusively upon radial air-cooled motors. The wisdom of complementing an air-cooled engine program with a liquid-cooled one is fully attested to by the fact that today most of this country's fighter aircraft are powered by Allison. They are the Lockheed Lightning, the Bell Airacobra, the Curtiss Tomahawk and Kittyhawk and the North American Mustang, all of which have given excellent accounts of themselves in war combat areas around the world.

The Allison V-1710 C 15 was the production model when the Army in May, 1939, issued its first quantity order for this American-designed liquid-cooled engine. It was the fourteenth model of twenty engines, custom-built by hand, part by part, during an eight year development period. Today Allison has in mass production in its Indianapolis plants two basic types of V-1710 engines — the "E" and the "F" types. The "F" type engines are built both as right- and left-hand rotation engines, which eliminates torque reaction in twin-engine airplanes. The rated horsepower output has been increased more than 30% in these later models over that of the original Army quantity order model of 1939.

All Allison V-1710 engines are of the same basic design. These engines have 1710 cubic inch displacement. They are twelve cylinder 60° "V" type, high temperature, liquid (Ethylene Glycol) cooled engines, having a built-in single-stage supercharger. Identification of the basic engine type is determined readily by a characteristic short nose reduction gear housing, with a high propeller thrust line.

The Allison engine has two cylinder blocks of six cylinders each. The cylinder block consists essentially of three parts: the head, the cylinder barrels and the coolant jacket. The head is a one piece aluminum alloy casting. Carburized, hardened cylinder barrels are shrunk into this head. A one-piece cast aluminum alloy coolant jacket encloses the six cylinder barrels and is fastened to the head by a number of studs. The bottom of the coolant jacket is secured to each cylinder barrel by a nut threaded to the cylinder barrel, thus completing the coolant seal for the head-cylinder jacket unit.

Each cylinder-head-jacket assembly is mounted on the upper half of the crankshaft by fourteen stud bolts extending through the head. These studs clamp the cylinder barrels securely between the head and the crankcase, and in addition transmit all of the power stroke forces directly to the crankcase. This construction relieves the head-barrel shrink-fit joint of all operating loads and provides additional rigidity of the crankcase.

The combustion chamber is of the roof-head type, and provides the rigidity necessary to hold the shrink-fit over the cylinder barrel. Each cylinder is equipped with two intake valves, two exhaust valves and two diametrically opposed spark plugs. Each pair of valves is set at 22½° with the cylinder axis, permitting the use of a simple and compact valve actuating mechanism. The combustion chamber presents a relatively small area to the cylinder gases and the simple valve passages are adequate for high speed operation. Forged alloy steel valve seats which are faced with Stellite are used with the exhaust valves. The material of the intake valve seats is an aluminum-bronze alloy. The exhaust and intake valves are chrome-nickel-tungsten steel, sodium cooled, and have 44½° and 29½° seats respectively.

The valve operating mechanism of the Allison consists essentially of six rocker arm assemblies operated by a single camshaft on the top of each cylinder block. Each camshaft is driven through separate inclined shafts by bevel gears from the accessory housing camshaft drive gear, which in turn is driven through a spur gear train from the camshaft.

The camshaft is mounted centrally over the cylinder head in eight plain bearings, one of which is a large flanged bearing located adjacent to the camshaft drive gear. This bearing provides axial location for the camshaft and takes the reaction from the drive gear.

The camshaft gear is secured to the camshaft by means of seven bolts. This gear has 36 teeth, which, in combination with the seven bolts, provides a minimum angular timing increment of 1.4° of camshaft rotation.

The valve actuating mechanism is lubricated by oil under pressure received through the camshaft locating bearing from the inclined shafts. The oil flows through the hollow camshaft to all the camshaft bearings. In addition, a small hole in the heel of each cam furnishes splash lubrication to the valve stem ends, the cam follower needle bearings and the rocker arm bearings. The cylinder heads are provided with drains at both ends to return the oil to the crankcase sump.

The crankshaft is a conventional six-throw, seven-bearing. counter-balanced type, machined all over. The counterweights are welded directly to the steel forging, providing a compact design. At each end of the shaft is an identical nine-bolt flange. A pendulum type dynamic balancer is bolted to the flange at the accessory housing end of the crankshaft, to minimize the effect of two-node crankshaft torsional vibration.

The driving mechanism for all accessories mounted on the rear of the engine is connected to the crankshaft through an internal spline in the dynamic balancer hub.

An internally splined coupling is bolted to the flange at the propeller end of the crankshaft to provide a drive connection for the reduction gear assembly.

All crankshaft journals are hollow and are fitted with removable aluminum alloy plugs. These provide a passage for lubricating oil, and also permit the collection of foreign particles and possible oil sludge in the crankpin plugs.

The crankcase consists of two rigidly constructed aluminum alloy castings parted on the horizontal centerplane of the crankshaft and a magnesium alloy oil pan. Fourteen long studs set in each deck of the crankcase upper half hold the cylinder blocks in place. Long studs are set in the parting face of the crankcase upper half and are located in the main bearing webs on both sides of each main bearing bore. These studs pass through the lower crankcase half and are used to clamp the two crankcase halves over the main bearing shells. Hollow dowels through which the main bearing studs pass locate the two halves with respect to each other. The cases are sealed completely around the outer parting flange by a series of closely spaced studs. Two engine-mounting bosses are located on each side of the crankcase upper half.

All seven main bearings are flanged steel shells, made in halves, and lined with a thin layer of lead- bronze. The bearings are located axially by the steel flanges and are prevented from rotating by a dowel in the lower crankcase at each bearing. The center main bearing is provided with bronze-faced flanges which bear on the center crank-cheeks and provide axial location for the crankshaft.

The magnesium alloy oil pan casting is bolted to the bottom of the crankcase lower half and provides breathing passages between the crankcase compartments.

The connecting rods are of the forked and blade type, each forged to an "I" section column and machined all over. The bearing consists of two halves of a flanged steel shell with a bearing alloy lining which bears on the crankpin journal, and a bearing alloy overlay which acts as a journal for the blade rod. The bearing shells are clamped in the forked rod by two caps and four bolts. A short dowel in each rod cap prevents rotation of the bearing. The blade rod fits around the bearing alloy overlay and is held in place by a single cap and two bolts. A bronze piston pin bushing is pressed into the small end of each connecting rod. The connecting rod bearings are lubricated by oil under pressure from the crankshaft. The piston pins are splash lubricated.

The pistons are machined from aluminum alloy forgings, and the underside of the piston heads is grid ribbed to facilitate cooling. Each piston has three compression rings above the piston pin and two oil rings in a single groove below the piston pin. The piston pin floats in the piston and is retained by two snap rings, one at each end of the pin.

The "F" type engines have an integrally mounted reduction gear assembly, while the "E" type engines are equipped with a reduction gear box which is located outboard of a long extension shaft.

The shaft of the "E" type is composed of two flanged shafts each 2½ in in diameter and 48-9/16 in long, and supported at the center by a ball bearing mount. The reduction gear box consists of two aluminum alloy castings which support the propeller shaft, thrust bearing, reduction gear and pinion gear.

The reduction gear is an external spur gear mounted by bolts to a flange on the propeller shaft. The propeller shaft is supported at the front by the thrust bearing and at the rear by a large roller bearing. The pinion gear is mounted between two roller bearings and is driven by the extension shaft through an internally splined flexible coupling.

The reduction gear teeth are lubricated by an oil nozzle supplying three jets of oil directly on the teeth. A combined pressure and scavenge oil pump is mounted on the front of the reduction gear housing and provides the oil pressure to the oil nozzle and scavenges the case. Oil must be supplied from an external tank (not supplied with the engine) for the reduction gear system.

Drives are provided on the rear face of the reduction gear box for two gun synchronizers, one hydraulic oil pump and a propeller governor.

The "F" type engine with its integrally mounted reduction gear provides a 2 to 1 reduction between the crankshaft and the propeller shaft and like the "E" has external spur gears. Both the reduction gear and pinion as well as the propeller shaft are encased in a short nose section consisting of two aluminum alloy castings which are stud mounted to the front face of the crankcase.

The reduction gear in this type is bolted to a flange on the propeller shaft. The propeller shaft is supported in the front by a ball thrust bearing in the nose of the reduction gear housing, and in the rear by a large roller bearing.

The pinion gear is mounted between two plain bearings and is driven from the crankshaft through a flexible splined coupling. The propeller thrust line is located 8¼ inches above the center line of the crankshaft. A front coverage oil pump is located inside the reduction gear housing. A propeller governor drive is provided on the rear of the housing in the "v" between the cylinder blocks. The housing is also provided with oil passages to supply both governor and engine oil pressure to the propeller shaft for hydraulic propeller operation.

The teeth of the reduction gears are lubricated by an oil nozzle supplying three jets of oil on the outer of the mesh side of the gears.

A centrifugal coolant pump located on the bottom of the accessories housing supplies coolant to each cylinder block at two inlets one located at the coolant jacket. and the other at the rear of the cylinder head. The outlet scroll of the pump terminates in a "T" with two flanged ends, and is connected by pipes to the dual inlets of each cylinder block. The coolant is admitted to the bottom of the cylinder block through an inlet manifold which is cast the full length of the jacket. These manifolds have an orifice at each cylinder barrel which meters the coolant flow. The inlet at the rear of the cylinder head provides a direct rapid flow over the combustion chambers.

The moving parts throughout the engine are supplied with oil from a pressure lubrication system. Constant pressure is maintained by a single pressure pump in combination with a pressure sensitive relief valve. All oil to the engine passes through a Cuno disc-type strainer which incorporates a safety by- pass valve. Provision for scavenging in both propeller-up and propeller-down positions is made by locating the main scavenge pump at the rear of the engine and an auxiliary scavenge pump in the reduction gear housing. The main scavenge pump and the pressure pump are arranged as a unit on the lower right hand side of the accessories housing. All pumps are of the conventional spur gear type.

Oil supplied to the pressure pump from an external tank is delivered to the exterior of the Cuno oil strainer through a spring loaded check valve, which prevents oil flow from the tank to the system when the engine is stopped. A pressure of only one pound per square inch from the oil pump side of the valve is necessary to provide check valve response in operating. Static oil pressure from the outlet of the Cuno strainer is maintained against the piston of the adjustable spring opposed balanced relief valve, which bypasses excess oil directly from the outlet to the inlet of the pressure pump. This arrangement maintains a constant oil pressure in the engine, with increased strainer restriction within the capacity of the oil pump. The relief valve is accessible for cleaning or pressure adjustment without removal of the pump.

From the strainer outlet, oil is distributed to the moving parts of the engine. A large tube in the crankcase upper half connects with a drilled passage in each main bearing web, conveying oil to the main bearings. The main bearing and crankpin journals are fitted with aluminum alloy plugs and are all interconnected to carry oil to the connecting rod bearings, where it is thrown to lubricate the cylinder walls and the piston pins.

A branch from the lead to the crankcase tube carries oil to the inclined shafts of the camshaft drive and to the magneto drive shaft bearing. Oil is carried through the inclined shafts to the camshaft locating bearing, where it enters the hollow camshaft for lubrication of the camshaft bearings and the valve mechanism from a hole in each journal and in the heel of each cam.

Three oil passages distribute oil from the Cuno strainer outlet to the supercharger and all accessory drives contained in the accessories housing.

Oil drains through passages at both ends of the camshaft compartment to the crankcase. In level or propeller-end-up positions all oil drains to the oil pan and is scavenged by the main scavenge pump from the accessories end of the oil pan. The second scavenge pump is located in the reduction gear housing and is driven by the oil plug of the reduction. Its inlet is located low in the forward position of the reduction gear housing so that oil will be scavenged in near vertical positions. The discharge from the forward scavenge pump is carried to the outlet of the main scavenge pump so that there is but one oil outlet to the engine.

Forced induction system used

A forced induction system is used to supply the fuel-air mixture to the cylinders. Carburetion of the fuel is obtained by use of a single Bendix-Stromberg two-barrel injection carburetor. The carburetor consists of a throttle body, a regulator unit and a control unit mounted on the supercharger inlet cover. An external pipe carries the fuel from the control unit to the injection nozzle to the rear of the supercharger elbow, where it is injected directly on the supercharger impeller.

The supercharger is contained in the accessories housing. The impeller consists of two parts, one part having fifteen radial blades with strengthening webs between each blade, and another part made up of fifteen matched curved guide vanes. The parts maintain their matched relation through a common spline on the impeller shaft. The fuel air mixture flows through a diffuser passage having six curved vanes and then through a scroll to a single supercharger outlet in the "V" of the cylinder blocks. A branched manifold system distributes the mixture to all twelve cylinders.

To prevent backfire flames from reaching the supercharger, a backfire screen is incorporated in the manifold "T" leading to each cylinder block. The screens are made up of alternate layers of flat and corrugated bronze, mounted in steel frames.

All accessories, with the exception of the propeller governor drive and the front oil pump are mounted on the rear of the engine. A cast magnesium alloy accessories housing is stud mounted on the rear face of the crankcase and contains the supercharger and drives for the starter, oil pump, camshaft drive, fuel pump, generator, two vacuum pumps, two tachometers, magneto and the coolant pump.

All accessories drives at the rear of the engine except the starter and oil pump are taken through a hydraulic vibration damper. The starter drive is taken oft the rigid outer member of the damper, and is unaffected by damper vibration. The oil pump is driven through bevel gears from the starter shaft. The outer member of the hydraulic damper is splined to the hub of the dynamic balancer on the rear of the crankshaft. An inner member is connected to the outer rigid member by a flexible quill shaft, and reacts against the outer member through a hydraulic member to minimize single node low frequency torsional vibration.

One spur gear train from the flexible member of the vibration damper drives the accessory housing camshaft drive mechanism and the supercharger. A second gear train is used to drive the generator, two vacuum pumps, the fuel pump and the coolant pump.

Ignition is supplied by a dual high-tension Scintilla magneto driven at 1½ times crankshaft speed. The voltage from the magneto is distributed to spark plugs through two separate engine-driven high-tension distributors. The magneto timing is fixed and tires the exhaust plugs 6° before the intake plugs.

Care and servicing considerations

Those concerned with the care and servicing of the Allison engine should bear in mind that an important feature of this American-designed aircraft motor, contributing to ease of overhaul and maintenance, is its breakdown into six major sub-assemblies, which are as follows:

  1. Ignition wiring harness with distributors.
  2. Cylinder banks, which includes cylinder barrels, cylinder heads, valve mechanism and coolant jackets.
  3. Reduction gears and housing, or crankcase front cover of the "E" engine.
  4. Accessory drives and housing, including super- charger, carburetor and accessory drives.
  5. Crankcase assembly — upper and lower halves and oil pan.
  6. Crankcase. including connecting rods and pistons.

In view of the fact that the method of cooling is the outstanding feature which makes the Allison unique among American-designed aircraft engines, its coolant system of necessity must be a subject of special study and interest to aircraft maintenance and service people. In "trouble shooting" for high coolant temperatures (over 125°C) here are important leads which should be followed: leaks in the system causing insufficient quantity in system; coolant pump not operating; clogged radiators or air ducts; shutters left in closed position; expansion tank line to pump inlet too small; vent lines too small or improperly located, resulting in air traps in system, clogged cylinder head outlets; engine allowed to idle too long on ground; incorrect ignition or valve timing, improper coolant solution; and expansion tank pressure relief valve improperly adjusted or stuck.

Checking seepage of coolant

If seepage of coolant from the coolant pump seal drain becomes excessive the coolant pump spring packing unit must be replaced. This can be done without removing the coolant pump from the accessories drive housing by using the following procedure:

  1. Remove cowling from beneath accessories drive housing so that that coolant inlet connections and coolant pump cover are accessible;
  2. drain coolant from lowest point in system into clean containers;
  3. disconnect coolant hose connections from coolant pump cover;
  4. remove the eight ¼" nuts and plain washers from coolant inlet cover flange on the bottom of the pump and remove the coolant inlet cover and cover gasket;
  5. remove the 3/8" slotted engine nut and impeller retainer washer from the lower end of the coolant pump drive shaft;
  6. remove the pump impeller from the splines of the pump drive shaft;
  7. press thumb against coolant seal disc and depress the packing spring at the same time removing the snap ring with a bent-point scriber;
  8. remove the detail parts of the coolant seal assembly;
  9. remove the seal disc, packing, packing retainer and pressure spring from the impeller sleeve; but Do Not remove the impeller sleeve from the impeller;
  10. place a new packing retainer in a new packing pressure spring and place a new coolant packing in the packing retainer cup. Insert this assembly, with the spring down. into the impeller sleeve;
  11. press a new seal disc against the packing until the spring is compressed sufficiently to allow the installation of a new snap ring;
  12. place Coolant Pump Packing Seal Assembly Tool No 2353 over the threaded end of the pump shaft;
  13. install impeller assembly on splines of pump drive shaft by guiding the coolant seal assembly over the end of the drive shaft so that the Packing Seal Assembly Tool will align the seal disc, packing and packing retainer as the impeller is started into position on the shaft. (Caution: Do not use force to seat the impeller fully into position because the bevel gear on the other end of the shaft may tend to climb into the bevel drive gear and damage may result.)
  14. impeller should slip into the splines sufficiently to permit removal of the Assembly Tool;
  15. place retainer washer on shaft and start 3/8"-24 slotted engine nut;
  16. tighten the nut and draw the impeller into position. Safety the nut with a cotter pin;
  17. install new cover gasket and install cover, using self-locking nuts or plain nuts and new pal nuts. Place plain washers under the nuts;
  18. install new coolant inlet flange gasket and install inlet hose flange connection;
  19. refill with coolant and install cowling.

    This article was originally published in the July, 1943, issue of Air Tech magazine, vol 3, no 1, pp 19-32.
    The original article includes 18 detail photos and 5 detail diagrams and drawings.
    One drawing is credited to Flight, all other illustrations credited to Allison.