Folks, I solved my alternator problem today.  Was a rather unusual fix, but
it solved my problem for a very low price (only about $50), and I can't be
more happy about that.

Background:  2100D alternator not charging shortly after engine rebuild.  I
initially suspected my voltage regulator was bad, so I tried two different
type replacements, neither of which fixed the problem.  I was getting 13.5
VDC at the positive and negative ends of the voltage regulator, but it as
soon as I hooked the ends up to the battery, the multimeter would
essentially read the battery voltage, indicating that there wasn't enough
current to charge the battery.  In addition, the battery gauge (in the
cockpit) would "barely" tick up/down when I turned the alternator on/off.
I'm talking like 12 --> 12.1V, instead of the usual 12 --> 14V like
normal.  Obviously something was wrong, so I elected to pull the rotor and
stator and figure out what was going on.

Anybody who has seen one of these old Revmaster 20 Amp systems up close
knows that there is very little room between the outer portion of the
stator and the inner portion of the rotor.  When I inspected mine, it was
clear that the stator had suffered serious heat damage over the years from
rubbing up against the rotor.  Because of this, the rotor was also
scuffed/scraped badly and had almost no magnetic field left (it could
barely hold a machine screw).  Basically, this alternator was toast.

Looking for a replacement, I contacted Joe Horvath at Revmaster, who
informed me that they no longer keep the 20A system for the 2100D in stock,
and that creating a new rotor would cost approximately $600. I also
contacted Great Plains to see if their 35A system would work with the
2100D, and was told it would not.

Now I was seriously in a hurt locker; what the heck was I going to use?  It
wasn't until my wife out of all people, goes "well babe, couldn't you just
use an automotive alternator?"  Initially, my first thought was "you're
crazy, that wouldn't work", but after thinking it through, the more and
more I came up with a plan, and the more and more it made sense.

Since I'm currently running an electronic ignition, and the back of the
engine is left unused, the secret was to connect the magneto gear from my
old Bendix D3000 to the alternator, then mount the alternator to the
magneto drive on the flywheel, using it to spin the alternator.

So I got to it.

The first step was simple; find an alternator.  I chose this model from
O'reillys, since it was only $38, had an internal voltage regulator, and
was 60 Amps, WAY more than I'll ever need.  Believe it or not, all the
20-30 Amp alternators were in the $300-400 range, the most absurd being
$2100 (not a misprint).

The next step was to remove the old magneto drive from the D3000 and grind
down the bottom portions of the drive gear (which were originally meant to
activate the impulse coupling).  I then bought two 5/8-inch "thick" washers
from ace hardware, which fit PERFECTLY into the drive gear for the magneto
(this helped extend the magneto drive a little further along the alternator

>From here, I removed the lock nut on the alternator (which required the use
of an impact wrench), removed the fan on the alternator, then from the
bottom up installed the two thick 5/8" washers, the magneto drive gear,
then tightened everything down with the lock nut. After tightening the
bolt, I installed the magneto drive cushions, and the metal plate,
completing the entire assembly.

The last step is simple, but time consuming.  I took the alternator,
aligned the magneto drive, and seated it into the engine.  From here I made
two marks with a permanent marker in the flywheel flange to drill and tap
some holes for mounting.  Afterwards, I simply installed some bolts to
mount the alternator, wired the alternator battery leads with some 10 gauge
wire, and installed a 12V "exciter" wire to the alternator (replicating the
12V ignition wire in an automobile, basically to turn on the alternator).

I held my breath and started the engine.  Miraculously, it actually
worked.  As soon as I activated the exciter wire, the alternator kicked in
and the voltage jumped up from 12V to about 13.5V at idle, and then up to
15.5V at speed!  I initially thought the internal regulator was broken,
because 15.5V nearly maxes out my battery gauge, but I verified this with a
multimeter and the voltage held steady at 15.5V from about 1500-3200 RPM
(full power).

Despite this taking care of my alternator problem, and providing up to a
massive 60 Amps of power, it does have some disadvantages.

1.  Weight - for starters, this unit weighs a lot more than the old
stator/rotor unit for the 20A system.  I probably lost 7-8 lbs of useful
load, going onto my already nose heavy KR2S.  Not a huge deal, but it
should be noted.

2.  Uses the magneto drive - The 2100D only has one magneto drive, so if
you are running any type of magneto, obviously this solution will not

3.  Takes some engine power - As soon as I activated the alternator (turn
on the exciter wire), you can literally feel the engine bog down a little
bit.  I can't get a real accurate measurement since my SDS ignition only
measures in increments of 100, but you probably lose 50-75 RPM at idle when
the alternator kicks in.  That said, I guarantee it is more smooth than the
old clunky 20A rotor literally rubbing up against the stator.  With this
new setup, I still managed to get 3200 RPM static, and it's a toss up which
system takes more power to operate.  However since my static RPM is
basically unchanged, and this alternator can provide much more power, so
far I am impressed with it.

The goods are pretty self explanatory; a heavy duty, 60A alternator
installed in your plane with no need for an external voltage regulator.
This would be very good if you run a lot of electrical equipment, such as
landing lights, engine management units, Ipads, Garmin GPS's, etc.  For me,
I only run about 15 Amps at a maximum and just needed a quick fix, and
believe it or not, this seemed like the best option.  For nearly $50, I had
a brand new alternator.

I realize this article was pretty rough and dirty (finished it at 1am) and
that 99% of you will probably never need/never be able to use this fix
(since you're probably running a magneto).  That said, I did take pictures
if anybody is interested in seeing them.

Now that this is finished, I can quit slacking and focus on the Technical
Report for the Electric KR2S.


Sam Spanovich
74S, Anacortes WA
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