Blackspur 115mm BB-AG100 Angle Grinder: Excessive Sparking, Overheating, Smoke

I am trying to fix a 500W 230V 50Hz Angle Grinder.     (Blackspur 115mm BB - AG 100)      Problem - much sparking, overheating and smoke.
There is a lot of sparking on the commutator (appears to be only one of the brushes) , and after only a few seconds of light running, the motor quickly becomes very hot, the heat becoming manifest through the plastic casing some seconds later.  
A few more seconds of running, and smoke starts coming out,   (The outer plastic casing still feels very hot a couple of minutes later.)
I inspected the carbon brushes, they looked slightly scored, but plenty of length.  The commutator looked slightly blackened, but cleaned  up nicely.
The gearbox and bearings are smooth and free running.
I checked the 24 segment commutator with an ohmmeter - readings across opposite segments: (meter leads subtracted)1-13    4.8 Ω2-14    4.5 Ω3-15    4.1 Ω4-16    2.9 Ω5-17    2.5 Ω6-18    2.5 Ω7-19    2.7 Ω8-20    2.7 Ω9-21    2.8 Ω10-22    3.8 Ω11-23    4.5 Ω12-24    4.8 Ω
Can any motor expert please tell me if this is normal variation, or if it indicates a short circuit in the windings ?
Many thanks for any help you can offer,All the best,Frank.

What about the field winding?If it is a shorted turn(s) then an ohm meter will not detect it, you have to hit the windings with a pulse and look at the decay. I would say that you have lost a winding either on the armature or the field, they are usually detected by looking at the coulour of the winding, sometimes the varnish is burnt off completely, and the copper changes colour where it has oxidized due to overheating.I have hand rewound field windings, had to make up a plywood jig with the correct profile and shape, but it took some time, and you have to be able to source copper wire with high temperature insulation, transformer winders have it, but an armature is in a class of its own. They can be rewound professionally.
Cheers,Richard

Further to my comments, there is a bit of gear that used to be used to check motor armatures, see testing using a growlerCheers,Richard