Pre ignition is when the spark occurs a few degrees before it is meant to. This will result in higher burning temperatures and peak cylinder pressure being reached before the piston goes over top dead centre. The result could cause detonation of some, if not all of the charge which is like an explosion occuring on top of the piston. This will damage the piston.
However, although Danny P may be right, if you had excessive damage to the piston, you would get no compression at all. If the bores are glazed, this means that they have become polished to a near mirror finish. This is also caused by hot burning temps or rich mixture. Polished bores will not allow oil to stick to them for lubrication and cooling purposes, and the rings will glide over the bores rather than do what they are supposed to and create a seal.
I still think that it is worth removing the head and find out for definate what is going on. The engine may be salvageable, but be prepared to change it if you discover something nasty.
You could remove the oil filter and cut it open and examine the element for signs of metal. If the pistons have been damaged, then you will have little white rocks in the element along with white flakes which will be bearing material. Particles that are attracted by a magnet is an indication that something may have gone through the oil pump.
For those not au fait with the workings of the internal combustion engine, take a little look at this.
http://en.wikipedia.org/wiki/Otto_cycle#The_Otto_cycleIt does not go into great depth, but it gives you an idea.
