There is an STC for installing the 0-320 in the Stinson 108. The STC SA10365AT allows any O-320, but hasto be a conical mount which leaves you the early model 320’s to put on. The B models are 160hp.

Vernatherm types

There is a Flat head and a Top Heat type

Note, if you are using a New STC’d Pacific Oil cooler, The flat top vernatherm will not work with the new POC oil cooler. I already tried it and the bevel inside the oil cooler will not allow the vernatherm to go down far enough to seat.

I am using the top hat version, and it apparently has a similar problem.  On a test ground run of 5 min I detected no oil leak, but .7 in the air it was obviously leaking oil at the gasket.  No way I’m going to spend another $800 for a new Vernatherm.

Light weight starter

Sky tech sellls a lightweight starter as a replacement for the original Delco starter. The weight difference is 12 pounds lighter. Chris at Airwork can get a discount on them.

Vacuum pump

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 .

Timing Magnetos

Timing the Mags on your engine

What you will need:

1.       Straight edge

2.       New mag gaskets, 

3.       “Never seize”.

4.       Plug wrench 

5.       15/16” open end wrench, 

6.       Screw driver 

7.       1/2’ socket or suitable to loosen the nuts holding the mag in place

8.       Mag timing light; borrow one or better yet take the plunge and spend $42.95 for the Action Air Parts Inc., Inductor Dual Mag Synchronizer (P/N 12-16901 from Spruce), since you should check the timing each annual it will quickly pay for itself.


Removethe upper cowl, and if you have split your nose bowl, remove the top half as well. (If you have not split the nose bowl you may want to think about is, unless you are a purist or Luddite, splitting the bowl makes access especially easy. I will cover this in a future Technotes. 

Loosen the mag retaining nuts just enough to be able to advance or retard the timing by moving the mags in the slotted flange. If your mags are stuck tight, you will have to pry/tap them loose and replace the gaskets well coated with Never Seize. (If you do this, read Installing Mags below) Remove the top plugs, rotate the engine in the normal direction until you have TDC on #1 cylinder. You will hear the impulse couplings go off as you approach this position

(Place your finger over the spark plug hole and rotate until you feel pressure build to maximum). There is a special tool to find TDC that you really don’t need. On a 220 you can see the piston travel and verify the position by looking in the sparkplug hole. On all engines there is also a mark on the prop flange showing 28°, 32° and TDC. Connect the timing light by putting the green lead on the left mag, and the red lead on the right mag, you will note a thick braided wire exiting the bottom of the backplate of the mag, this goes to the noise suppressor, also attached to the mag. A wire is screwed into the suppressor that disappears into the firewall; this is the famous “P” Lead.(Connect the green and red alligator clips to the screw that attaches the P lead to the noise suppressor). Black lead to airframe ground, turn the ignition switch to BOTH and switch the timer ON.

 On Franklin’s both mags have impulse couplers. Now rotate the prop backward about 1/8th of a turn, place the straight edge along the engine crankcase joint, see diagram 1, and then slowly rotate the prop fwd. To the timing mark for your engine. (150 are 28°, 165 is 32°, 220 is 28°). On the timer both lights will be ON, as you arrive before, at or just past the timing mark the red and green lights will go out. Carefully align the straight edge with the XX° mark, do this by moving the prop in the normal direction of rotation to the mark. Do not jiggle back and forth on the prop, as there is a lot of lash in the drive gears and you will take up the slack only and so the engine will not be in the correct position. If you had to remove the mags and replace the gaskets, which I think is a good idea any way….

Installing the Mags

 With the engine set at the XX° position, remove the little plastic screw cap on the top of the mag, and rotate the drive gear until you see a red marked tooth inside the hole that you removed the plastic cover from.  The mag will rotate freely in one direction and in the other it will come up against the advance coupling, turn the easy way. If you want to test the advance coupling make sure you have not got your finger in one of the plug lead holes! With the gaskets in place and well lubed with that sticky silver stuff that gets everywhere and is a pain to remove, insert the mags into the drive holes, keeping the little red tooth exactly in the center of the inspection hole, and gently finger tighten in place.

Setting the timing

 With the straight edge and the XX° mark aligned, the green light on the left mag should go out and there should be a continuous beeping sound emitting from the timer. If this is so, tighten left the mag in place. Turn the right mag in the slotted hole until the red light goes out and the pipping sound becomes a continuous whine. What you are trying to do here is have both green and red lights out and a continuous whine from the timing box, when the engine is in the XX° BTDC position. That’s all there is to it.

See also pages 33 to 39  in service manual.