Pole Spacing Notes
Pole spacing determined by the spacing of the strings and string spacing is, in turn, determined by the width of the nut and the bridge. As the bridge is wider than the nut the strings will fan out in the direction of the bridge. This is why you will generally find that the pitch of a bridge pickup is a little wider than its matching neck pickup.
In an ideal world, the strings will pass over the centre of the poles of both neck and middle pickups. However, in practice this is seldom precise and as long as the string passes reasonably close to the pole centre performance will not be adversely affected.
A pickups pole spacing is measured between the centre of the bottom (6th string) and the top (1st string) poles. Gibson have traditionally used 49mm for both the neck and bridge positions and, over the years, most USA guitar makers who have fitted humbuckers have used this spacing which is commonly known as standard spacing. This is why you will find that the strings do not pass perfectly over the pole centres of both pickups. Fender guitars have a wider string pitch at the bridge, and their bridge pickups are typically 52mm. This is commonly referred to as F-spacing.
Common practice today, especially in Asian factories where most guitars are now made, is to space the neck pickup at 50mm and the bridge at 52mm. This provides the best compromise on almost all modern guitars and this is that spacing that Entwistle pickups now use. Most of the best known replacement pickup manufacturers have their pickups available with this spacing.
The wider 52mm F-spacing at the bridge is also a proper match for guitars which are fitted with Floyd Rose style trems.
Some customers who want both neck and bridge pickups to have the narrower neck spacing of 50mm, which is very close to Gibson's 49mm, have used neck pickups in both the neck and the bridge positions but this is only suitable with pickups which have the same electrical specifications, notably the same DC resistance. The bridge pickup is sometimes wound a little hotter than the neck pickup to compensate for the drop in output and bottom end as the pickup is moved closer to the bridge.
There is lots of comment about pole spacing on Internet forums and we suggest that you Google on the subject to find out more.