Stators and Armatures - Part 2: Stator Repair and Disassembly

Besides electrical faults and component age you’re probably not going to find a lot of problems with your stator. After all, it sits in an almost hermetically sealed environment behind a big hunk of metal in a sheltered part of your vehicle.  Rust and oil contamination are probably the other most frequent issues, unless your engine has been apart for some time and exposed to the elements.  Let’s start with the most likely candidates and work our way down:

Positive brush holders:

As I mentioned last time, the positive brush holders need to be totally isolated from the metal frame. This is done by using a series of fiber washers, gaskets, and tubes (you can buy new sets from the UK club). The washers separate the holder from the frame and the screw that attaches everything. The gaskets isolate the sides and bottom of the holder from the frame. The tubes isolate the screw from touching the electrically charged parts, and a tube also isolates the spring that provides brush tension from the metal frame.  The picture below illustrates the insulation scheme using these components:

Heinkel Positive Brush and Brush Insulation

Heinkel Carbon Brush Schematic Showing Tubes, Washers and Insulation
(Thanks Paul Cowan!)

The gaskets around the positive brush holder must be in good shape as well. It is mandatory that no part of the brush holder touches the metal case or the coils when they are attached. There’s a well where the carbon brush spring sits to provide tension. The bottom of this must be insulated. The sides of the well have an insulating tube. This sticks up about 1mm proud of the case and the brush holder may sit on top of it.

The best way to check these is with a continuity tester. Clip a lead to the metal frame and use the probe with a light to carefully check all around and inside the brush holder.

The "Well" for the carbon brush spring

Stator Metal Frame with 3 of 4 brush holders removed. Top left brush holder
in poor shape and was fixed by wrapping in PTFE tape

Wires to frame:

Sometimes the fat wires can be loose where they've been soldered to coils, or have cracked insulation, causing the wire to short against the frame. If this is the case you should replace the wires. I've found the gauges below to be suitable replacements, and they are available in spools at your local car parts store:

1.       Large wire: use 10 gauge as a replacement2.       Middle wire: use 12 gauge as a replacement
3.       Small wire (which you will have to splice): use 16 gauge as a replacement

When selecting replacement wire make sure the wire is composed of very thin copper strands and can be easily bent. A lot of automotive wire seems to be made of relatively thick strands and is difficult to bend at the angle required when the stator is installed back into the motor.

Large wire replacement can be a little tricky. The wires are soldered into a metal loop and then the insulation is wrapped around the coil. I tried using a micro torch and an industrial soldering iron to remove these wires. The torch started to degrade the coil insulation, and the soldering iron could not melt the old solder. I finally cut through the metal loop and insulation and then drilled out the wire.  I used a Dremel with a diamond wheel to make my cuts. It’s a lot like a dentist drilling and filling a cavity; once I made a suitable channel I soldered a new wire in. 

Drilled out channels (bottom left of picture) awaiting new wire

Then I took fabric, soaked it in epoxy, and made a new wrapping to secure the whole assembly.

Coils against the frame:

If the coils have frayed insulation they might short against the metal frame. Once you take the coils off of the frame it would seem possible to re-wrap them, although I didn’t have take this step.

Large flat wires that connect the starter coils:

These have shellac insulation. It’s unlikely but possible that the shellac has scraped away and a ground can happen, especially if something metal or wet was ever introduced to the environment (Shellac has limited moisture resistance). A quick visual check for tarnished copper will usually indicate an insulation problem. You can use standard hardware store shellac to recoat these wires. Two coats about 10 minutes apart (it dries quickly) should do the trick.

If you haven’t found the problem yet, your next step is to dismember the stator.

The coils are screwed on to the frame with 6mm slotted screws. IMPORTANT: These screws have smaller heads than regular screws and aren’t readily available, so hold on to them! They may be hard to remove and you may wreck the slots. I gently heated the metal frame, then had a helper hold the stator assembly while I used a square shafted screwdriver and wrench for extra torque to remove the screws.

Finessing the screws loose without distorting the shaft

The metal pieces hold coils in the assembly. Once you remove all of the screws you can unlace the coils and get a good look at the coil circuits, wires and insulation.

Four coil holders (removed from coils) and Heinkel stator frame

Large (starter circuit) and smaller field coils on Heinkel stator

At this point look for frayed wires and scraped off shellac. The insulation on the wires is probably 50 years old and brittle. Properly fixing the wiring between the small coils would involve disassembly and splicing. Since this is impractical, you can use electrical tape. Generally speaking you would not want to use tape in a place where the heat would melt the adhesive and have the tape unravel. However, in this area everything is tight together and I ended the wraps in a place where they could easily unravel.

Taped wires on coils before reassembling the circuits

It might be overkill but I also wrapped the frame with tape to keep any grounding of a coil from occurring (Note: I untaped it after I realized the armature scraped against the stator because of the reduced clearance.) The screw that hold the coil to the frame still provides a ground for the coil holders.

Taped Stator frame (don't do it- it won't fit!)

Gaskets on the Stator Wires:

There are 2 gaskets on the stator wires: a large one that slides into the case, and another that provides a seal between the case, the cooling tins, and the elements. The only way to replace these is to slide them off and on via the ends of the wires.

Heinkel Stator wire gasket- slide the small, medium, and large wires into their respective holes

The other gasket that slides over the wires. Note the wire shrink wrap between
the coils and the gaskets. This provides extra insulation where the wires bend
and make contact with the engine case.

If you are replacing the wires you  should plan ahead for this step It can’t be fixed after the fact unless you snip wires or the gaskets themselves (thereby negating their value as gaskets!)

The Solution to My Problem:

After all of this work I reassembled my stator, ran all my tests and was in the clear. I’d been at this for about 10 hours over a few weeks and I was happy to have it behind me. Then I tightened all of the coils and found out that I still had a tiny grounding problem. Ultimately the problem was when I tightened the screws that pushed on the insulation of the positive brush holder and caused a fault.

I re-insulated the brush holder and fixed the problem once and for all (I hope).

Stators are only part of the electrical producing equipment. Stay tuned for the final installmen, this time on armatures.