There are often questions about which way the Prestolite starter motor turns on a MK3 Norton Commando.
To cut to the chase, it is Clockwise as you look at it from the primary case.
There is an excellent article from Fred Eaton over on the Old Britts website, which goes into rebuilding the MK3 starter in good detail.
You can access the article by clicking here:
I have “borrowed” one of the pictures from Fred’s superb technical article and drawn some clear arrows over it, just to make it crystal clear about the direction of rotation for each gear.
A popular task is to renovate and upgrade the Prestolite starter using one from a Harley Davidson or a boat (I think the Mercury engine ?) as a doner.
Of course the Norton version is no longer in production, so you are unable to get a like for like replacement.
The great thing about the Harley unit is that it has four coils and four brushes (compared to the Norton’s two) so this represents a pretty good upgrade – making the starter motor significantly more torquey!
The Harley one spins in the other direction though, so you need to rotate the brushes by nintey degrees (which is very easy to do)
Here follows two pages from the Norton Workshop Manual containing information about the electric starter:
Electric starter mechanism: General arrangement.
Diagram of starter gear reduction (and drive track)
Categories: motorcycles, Tech Articles, Troubleshooting & Diagnostics
The later Harley-type starter I bought from Norvil UK spins the same way for both positive and negative earth, which was useful when I custom wired my Mk3 special.
A field coil type motor (as we see on the MK3) will spin the same way for positive earth and negative earth.
It is the relationship/angle of the brushes to the stator field windings which governs the direction of rotation on this motor type.