Home       Aeronca    Stinson                                                                                                               ... by John Baker

Continental C85-12 Tachometer Drive Oil Seal Replacement

Added 6/12/13  Click on photos for a larger view.

I recently noticed oil dripping from the tachometer cable where it enters the accessory case on my Aeronca Champ.   It was time to change the seal. The engine is a C-85-12, with provisions for a starter.   The seal is contained within the tachometer drive housing on the rear accessory case.  My first step was to gather information and parts.   I re-read the appropriate sections of the Continental Service Manual several times to make sure I understood how things went together.  I consulted my Aeronca friends.  Many thanks to small Continental expert Harry Fenton who provided tips on how to replace the seal, and to A&P mechanic Mike Roe who provided further guidance and supervised my work. Parts needed included the tach seal itself and the housing gasket.  I purchased both from Aircraft Spruce.  In the photo above, notice the hole at the bottom of the tach cable attach collar.  This should be located towards the bottom.  It allows oil that seeps past the seal to drip out rather than drain into the tach cable shaft.  Before I replaced the seal, I had  oil dripping from this hole.


Since this is a C-85-12 engine, the tach drive housing is different than the housing on the "A" series or the -8 "C" series. The tach drive housing on the -12 (and O-200) accessory case is attached with three studs and nuts over a gasket (PN 35019) it shares with the generator mount pad. (This contrasts with the -8 engine that has a housing that is screwed into a  left hand thread in the cover boss.)  I don't have a generator on my plane, just the cast cover plate.  The tach drive housing is located between the magnetos, the oil screen housing, and the generator pad.  Access is okay as long as the generator is not installed.  If installed, it would need to come off anyway because of the shared gasket.   A 7/16" socket is used to remove the nuts holding the tach drive housing.  A 1/2" socket is used to remove the generator pad cover.

 In this photo, both the tach drive housing and the generator pad cover plate have been removed.  The tachometer drive shaft is the slotted end of the oil pump impeller DRIVEN shaft. (On the -8 engine, the tach drive shaft is off the end of the oil pump impeller DRIVE shaft, which explains why the tachs on the -8 and -12 rotate in opposite directions). The steel-cased oil seal, PN 642714, is pressed into the front end of the housing. The rubber center portion of the seal surrounds the non-slotted portion of the shaft, adjacent to the accessory case.

This is the gasket that is shared by the tach drive housing and the generator.  The Continental Part Number is 35019.  For a gasket sealant, I used a very thin layer of silicone grease on both sides.

This photo shows the housing with the new seal pressed into place.  I forgot to take a photo before installing the new seal. The old seal was damaged during removal.  I had read several cautionary tales about removing the housing and the seal.  Some of these precautions applied only to the housing on the -8 engine that is screwed in (rather than attached with the three studs and nuts.) There were several cautions to be careful to support the "ears" of the cast housing so they are not broken during removal of the seal.

I used a method to remove the seal suggested by Harry Fenton.  I used a vise opened sufficently to clear the seal but support the housing.  An altenative would be to use a block of hard wood with a large hole to clear the seal.  I inserted a small diameter punch from the tach cable end of the housing and gently "tapped" around the inner circumference of the seal, a little at a time,  working my way around the circumferance, until the old seal popped out.  Another method suggested is to pry the seal out with needle nose pliers.  Harry says the seal usually pops out with very little effort.  If it is tapped out, Harry says to be careful to support the ears on the casting as they can be broken off.   In the photo above, the housing is not quite in position. 

As Harry promised, installation was just as simple.  Notice how the seal is installed.  The "innie" or concave side of the seal faces forward towards the accessory case.  The "outie" or flat side of the seal faces aft towards the tach cable.  Next, I placed the seal on a flat surface, lubricated the outside diameter with a bit of isoproppl alcohol (to help the seal slide into the housing), and tapped the housing over the seal using a plastic hammer.  With the seal flush with the bottom of the housing, it was seated. (Don't be fooled by the perspective in this photo - both the old and new seal are the same size.)  After cleaning all of the mating surfaces carefully, I was ready to reinstall the tach drive housing.  Harry says, "Be careful not to polish the tach shaft off of the oil pump gear.  It should be free of any build up of oil carbonand free of any nicks. But, if is is polished with crocus cloth, it will be too smooth and allow oil to seep out."  One option is to use red scotchbrite and leave a bit of a cross hatch on the shaft.  A very slight cross hatch actually breaks up the path for oil to migrate out and down the shaft.  Mike Roe suggests that the pattern could be a spiral opposite of the direction the shaft turns, so that oil is driven back towards the engine rather than towards the tach cable.  My shaft looked good so I decided to leave it as is was.  Harry says, "Take care when installing the housing/seal assembly over the tach shaft as the slott can knick the seal lip. A bit of grease on the end of the shaft helps the seal to slide on. Usually no problems, just slide the tach housing on."

The tach housing is secured with 1/4" lock nuts with washers.  The generator cover uses 5/16" nuts and washers.  Use the proper torque for the size of the nuts.  Insert the tach cable shaft, alligning it with the slot in the tach drive shaft.  Secure the cable collar to the housing and you are done.  Harry says, "It is really a simple job, so don't get to stressed about it."  Lastly, do an engine run to check for leaks.  Don't forget to have the appropriate log book entry signed by an A&P mechanic.

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