Also when choosing brushes, look at the suppliers images, ie for similar springs (number of coils (12), thickness of wire compared to the old spring (0.5mm), diameter of coil (5mm), free length 20mm and shape of electrical contact (can be filed down- see next section).

Ideally, the carbon brush should incorporate a stop (pink arrow), such that when worn out, the spring and other hard parts cannot contact and damage the commutator. The stop is formed by a groove along most 80% of the edge of the new carbon brush such that the other 20% forms the 'stop',

There is a corresponding protuberance in the brush liner which fits into the groove (orange arrow).

The old brushes may not have had stops, and although this type may last slightly longer, it may have caused the commutator damage.

Using a brush with a stop means that it can only be inserted into the liner in one direction ie from the back which is why the liner is removed. Another advantage of using a brush with a stop is that it wont fall out during reassembly.

Of course if there is no lug, indentation or protuberance in your brush liner, there is no point buying a brush with a stop. A plain brush would suffice. (No significant difference in price).

Measure the inside of the brush liner to find the required dimensions of the new brush, such that it moves unrestricted in the liner (5x8mm in my case). If slightly stiff it can easily be filed down.

I think the length of the brush should probably be about 50% of the length of the liner. I choose 12mm long for a 22mm long liner. Ensure it fits in the liner with the spring compressed. If too long the carbon brush can easily be filed down.