Vacuum pump

Vacuum pump

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Dave James wrote this article in 1994.  We cannot locate him, so this publication is without his approval, should anyone have a contact address of phone number please let ISC know.

Installation of a vacuum pump Franklin 165

The installation of a Airborne 211CC vacuum pump on a Franklin 6A4-150-B3 or 6A4-165-B3 engine with Bendix S-20 series magnetos utilizes a standard Franklin right angle magneto drive P.N.18882 used on vertical engine installations in Bell 47 and Hiller UH12 helicopters. 

The right angle drive assembly must be used on the right side of the engine on the Stinson installation due to a section of the engine mount that would obstruct the drive if used on the left side. The drive assembly 18882 will retain the use of #18154 drive hub and 19313 one piece drive absorber, however the steel drive gear #18096 must be discarded. Replace this gear with aluminum gear #11045.

The right angle drive comes in several variations. The most common one has an oil inlet hole directly above the right angle gears. Oil enters this hole, flows over the gears, fills the gear cavity and overflows through an overflow opening back into the engine sump. Some right hand drives do not have this inlet andmust be drilled and tapped for a 1/8” NPT inlet fitting.

The magneto should be removed from the engine and the  gear  #18543 or #11045 removed from the magneto with the steel impulse hub and impulse spring. The male drive fitting TCM-Bendix #10-51333 is then utilized in place of the gear on the magneto and the impulse coupling spring must be wound up inside this fitting for installation on the magneto. There is a 1/4″ aluminum spacer plate #10134 mounted between the magneto and the accessory case of the engine normally, this is to be removed and discarded. The original aluminum magneto gear #11045 and 19313 absorber are now mounted to the input side of the right angle drive.

When the magneto gear is mounted on the right angle drive assembly and the assembly is mounted on the engine the magneto gear will be located in the exact position for proper engagement with the accessory drive gear in the engine. Install the vacuum pump on the drive assembly prior to installing the unit on the engine.

The two magneto hold down studs in the engine are rather long and you will find that the right angle drive/vacuum pump assembly will not clear the engine mount brace when you are attempting to mount the assembly to the engine there are several possible solutions to this problem.

  1. Unbolt the rear engine holdown mounts and lift the rear or the engine up about 1/2′ and you will be able to mount the assembly.
  2. Remove the original long studs from the engine and replace them with shorter ones. 
  3. Remove the original long studs and replace them with bolts. 

With the right angle drive/vacuum pump installed it should clear the engine mount brace by at least 1/2″. The timing of the magneto will be simplified if you mount the magneto on the right angle drive before mounting the assembly on the engine. If this is done you can set the magneto timing, set the crankshaft to the timing position, line up the red mark in the timing window of the magneto and without turning the magneto drive gear install the whole assembly on the engine . The timing may be off slightly since the gear has to align itself with the mating gear in the engine, but the adjustment on the magneto can be made in a normal manner to re-center the timing mark in the magneto.

The oil supply for the right angle drive is supplied from the oil pressure indicator line. A “T” fitting is installed on the oil pressure line where the hose from the engine is adapted to the tubing that passes through  the firewall. A hose assembly should be fabricated that will go from the “T” fitting to the threaded port in the right angle adapter. The fitting that connects the hose to the right angle drive must be silver soldered closed and then drilled out to .030″. This small hole avoids a large pressure drop indication on the gauge in the cabin and must be used. The right angle drive will be lubricated adequately since it has an internal oil sump that, over­flows into the engine oil sump .

The cooling blast tube for the magneto must be rotated and shortened approximately 1″ in order to direct cooling air to the re-positioned magneto.

The right angle drive and related gears are FAA certified parts built by Franklin Engine Company that was designed to last for the life of the engine in helicopter service. The total weight added is approximately 4.5 lbs. at station -32.0″ on a Stinson 108.

It may be found that on some installations one or two sparkplug wires will be too short and have to be replaced.

Be sure you do notuse a wet type vacuum pump on this installation. Either a Airborne #211CC or a EDO unidirectional dry pump will work. (I used the Unidirectional pump most satafactorily .