jasco alternators on jacobs engines

If we can judge anything by the engines that have come into our shop the past few years, the Jasco alternator has largely (though not completely) replaced the generators with which many of the small radials were originally equipped.  It is understandable:  the Jasco weighs half that of a generator, charges just above an idle (as opposed to just under the red line of the engine in the case of a generator), and unlike the generator, uses a regulator that is easy to obtain.  There are a few hold-outs who have retained their generators so we sell and maintain a handful of them every year, but each year their numbers seem to be dwindling.  Most of the Jasco alternators we sell are for the W670 Continental, the R680 Lycoming, and the R755 & R915 Jacobs.  On the Continental you often find the same problem with the Jasco as with the generator, which is related to oil leaking back through the step up drive and into the generator or alternator.  This is not the charging unit’s fault and probably could be the topic for another thread.  My current comments are limited to the Jacobs application.

For all its advantages the Jasco has a few drawbacks, one of which is its outside dimensions:  the Jasco is larger in diameter than the generators that it replaces.  With the typical 35A generator there is plenty of clearance between the generator and the other accessories.  Unfortunately, if you remove that generator and try to plug in a 6555 Jasco alternator, you will find that the alternator collides with both the accessory drive on one side and the oil pump on the other.  The remedy is to mill a flat on the sides of both the accessory drive and the oil pump.

There is plenty of wall thickness on the accessory drive for this machining, but things get really thin on the outlet of the oil pump.  It is important to not machine more than necessary on the oil pump housing, and then to not tighten the outlet fitting excessively or it can crack the housing.  We developed a DER approved repair procedure for this machine operation, and perform it on all engine overhauls whether there is a generator or alternator to be installed.  Sooner or later the engine will probably get an alternator and it is much easier for us to do the alteration during an overhaul than to pull everything apart and machine it after it is in service.  

Though most Jacobs engines are magneto and battery timer equipped, there is a growing number of dual magneto engines.  That right magneto has to be shoe-horned into the spot where the accessory drive used to be, so it creates a new set of challenges that go something like this:  Since everything is so tightly fitted together, there is an order to which accessory goes on first, second, and third; Rubik’s cube style.  Unfortunately the right magneto is first to go on, last to come off.  Guess which accessory is most likely to need to come off.  You got it—the magneto.  For years this has been the way of things.  If you have a right magneto problem then you are going to need to remove your fuel pump, starter, and alternator just to get to it.

Some people have wondered if it would be possible to carve some metal away from the Jasco to make things a little easier to service, so we contacted Skytronics (manufacturer of the Jasco alternator) who assured us that it was possible and even necessary in some installations.  In fact, they said that sometimes in order to get their alternators to fit a specific application they have to grind so much metal away that they go right through the housing and have to fill the hole with RTV silicone.  Not good.

At our shop we have a dummy Jacobs accessory case with accessories that we use as a jig for grinding the aforementioned flats on the accessory drives and oil pump, so we looked at this problem with the right magneto.  By grinding some material from the alternator and magneto, removing the upper left hand alternator pad stud, and replacing the stud with a 1” 5/16-18 grade 5 cap screw and lock washer, it all worked as desired.  Any accessory can be removed at will without disturbing the others.  By cutting the 90 degree bend off a long reach ¼” Allen wrench and gluing a ¼” x ¼ drive socket to the top of it with a little epoxy; you can make a handy dandy tool with which to tighten that cap screw.  Just install the cap screw and lock washer in the appropriate hole in the alternator flange, set the alternator in place, and start the cap screw before putting any other washers or nuts on the studs.

When grinding the right magneto housing, check it often against the installed alternator and be careful not to go through the housing.  Unfortunately the magneto housing castings are not uniform, with some having a lot more aluminum in that area than others.  You would have to grind a long way before scrapping the housing, but there is no point in removing more material that necessary.  Getting the right magneto on and off is the goal.