Originally Posted by auchencrow

The problem these days is that good quality cotters are like hen's teeth and the cheap cotters don't have flat faces.
The so-called "flats" are rounded such that so they don't seat well, and tend to allow just point contact with the bb.
This is why I always reface/file cotters - even if it's only a little to reface the flat, to get a better bite with the bb.
(Of course maintaining the same angle as was extant on the cotter from the manufacturer is still essential for the aforesaid reasons).