Norton 4-Brush Starter Conversion
Why?
Norton fitted electric start to the 1975 (and later?) MkIII models
presumably to try and keep up with the more sophisticated Japanese machines of the period.
It seems to be generally accepted that even when new, the electric start was not really up
to the task. Our own experience with our MkIII was that it would electric start
“fairly” reliably when warm but would never start when cold. This seems to be
fairly typical of how they worked. We decided that the Norton experience would be
much more enjoyable with a reliable electric start. Fortunately the process of improving
the starter has been well understood for a long time and a large amount of information and
resources are readily available. Using the information available we were able to improve
our starter to the point where it will reliably start the engine even when cold.
4 Brush conversion – what is it?
Brushes are carbon blocks that make an electrical connection to the
rotor or armature of the starter motor. In the standard Norton starter, a pair of brushes
feed power to the armature and also to a pair of field coils. It is the magnetic field
generated by the armature and field coils that drive the motor.
The starter used by Norton was made by Prestolite. Prestolite
made starters for many applications in different sizes and configurations. Some of these
starters share similar or common components. In the case of the Norton starter, it shares
components with Prestolite starters used with some outboard motors and also some Harley
Davidson motorcycles.
For some reason Norton chose to use a Prestolite starter
fitted with only one pair of brushes and field coils (2-brush) while similar starters used
in other applications were fitted with two pairs of brushes (4-brush) and field coils. The
starter in the 4-brush configuration will produce considerably (double?) more torque than
the 2-brush version so an obvious path to improving the performance of the Norton starter
is to add a second pair of brushes and field coils.
Two approaches are possible to convert a Norton starter to the
4-brush configuration. One approach is to fit the missing parts (coils and brushes) to the
original starter. The second approach is to swap parts from a compatible starter that
already has the 4-brush configuration.
2-BRUSH CONFIGURATION
4-BRUSH CONFIGURATION
Information
and Resources for the 4-brush conversion
The process of converting the Norton starter to the 4-brush
configuration to get improved starting performance has been well known in the Norton
community for a long time. Information is available from a number of sources regarding
this conversion. Also several suppliers provide the parts and services to do this
conversion. The following is a sample of some of the resources available by Internet.
http://www.noc.co.uk/ (information)
http://listserv.indiana.edu/archives/brit-iron-l.html (information)
http://www.captain.norton.clara.net/page1.html#page1%20top
(information)
http://www.oldbritts.com/ob_start.html (complete conversion service)
Conversion #1 – Harley Starter
The Prestolite starter used in some early Harley’s is very
similar to that used in the Norton except that it has the 4-brush configuration. It is
possible to swap parts from a Harley starter to convert the Norton starter to the 4-brush
configuration. Note that the Harley starter is not interchangeable with the Norton
starter. However although some differences exist, the casing, field coils and brush
assembly can be used.
The Harley starter rotates in the opposite direction. A Norton
starter rotates in a clockwise direction when viewed at the drive end. The direction of
rotation can be reversed by rotating the brushes by 90 degrees in either direction. In
some cases, the wire leads attached to the brushes may not be long enough to allow this
rotation and may have to be extended.
The casing of the Harley starter has a larger inside diameter
where it mates to the drive end plate. This will require some kind of very thin bushing or
collar to maintain accurate alignment. If the end plate is not accurately aligned, the
armature will contact the field coil poles and possibly jam, burning out the windings. At
very least it will make a bad noise.
NOTE: It is important to get a donor starter with the field
coils in good condition. If the field coils are not in good condition and need to be
replaced it would be less expensive (and about the same effort) to add the coils to the
Norton starter.
Conversion #2 – Norton Starter
It is possible to add the missing parts to the Norton starter.
However be warned that this involves quite a bit of work and some basic
electrical/soldering skills will be required.
The Norton starter casing is fitted with four field poles but
only two have coils fitted. The first step is to remove these poles and this may prove to
be difficult, as they will be rusted in place. A single large screw secures each pole. It
will probably take a combination of heat and an impact screwdriver to loosen these screws
without damaging them.
One end of each of the new field coils needs to be connected
to the motor casing. This connection would normally be made with a spot welder but most of
us do not have access to one of these. It may also be possible to make a solder connection
but I was concerned that the amount of heat required to make a good join would damage the
coils so I did not try this. I drilled and tapped two holes in the casing, drilled holes
in ends of the field coil conductors and used small 4-40 brass screws to make the
connection. This had the advantage of being able to easily remove and install the coils
while trying to get things to fit.
Things are a very tight fit and I had to do a number of
modifications to get everything in place. These modifications are necessary because the
field coils and brush holder are designed for a motor where the input terminal is offset
by 45 degrees and where the direction of rotation is reverse to what is needed for a
Norton.
In a 4-brush casing, the power terminal is offset by 45deg
relative to the field coils. In the two brush housing the terminal is adjacent to the
field coil so the following steps are required to make things fit.
1)
Grind down the plastic base of the input terminal to clear the field coil. This is
necessary because the terminal is adjacent to the coil.
2)
Re-attach the field coil brush to clear the input terminal
3)
Extend one of the brush leads attached to the input terminal so the brush will
reach the holder.
Care must be taken when installing the field coil poles to
ensure that they are correctly aligned or they will foul the armature.
I purchased the required parts from the following
sources -
1) Field Coils - 04483 Accel Field Coil Set (standard),
MotorcycleGoodies, $29.95. Note it is possible to purchase “high torque” field
coils but these are not recommended.
2) Brush Assembly – 57-SR104 4 Brush Starter Repair Kit
(outboard), IBOATS, $29.71.
Prices do not include shipping.
STARTER REBUILD KIT
ACCEL
FIELD COIL KIT
Conclusions
1) Converting from 2 brush to 4 brush configuration makes a
large improvement in cranking power and if the rest of the electrical system is up to it,
will result in reliable electric start.
2) The quickest and easiest conversion is to obtain a four-brush
starter (Harley/Outboard) in good condition and swap parts. It is possible to add the
missing parts to the Norton starter but more work is required.
IMPORTANT: Regardless of the starter configuration, the power to
crank the engine ultimately comes from the battery. The battery, solenoid and cables must
be in good condition or it will not work. In my case, the MkIII air box has been removed
and the battery tray from an earlier model fitted. I used the largest capacity battery I
could find that would fit in the space. The standard battery is typically similar to the
YB14L-A2 rated at 14 Ah. The battery I used was a YB16-B that is rated at 19 Ah.
