What work was done prior to this problem? Did you split the cases, or were you doing a top end and had the cylinder off?
The coolant could be making it's way into the cylinder from another area, is why I ask. These are the only places I can think of where coolant could make it's way into the combustion chamber, unless you have a cracked head or coolant passage: 1) at the case halves if you had the cases apart and via the crank case 2) at the bottom of the cylinder at the base gasket 3) head gasket.
Are the two O-rings you are installing new, with no obvious flat spots, or deformation? The O-rings can be a bit tricky to install, as they tend to pop out of the grooves in the cylinder. I typically stretch them a bit and add some grease in a few spots to keep them in place before quickly installing the head.
Also make sure that the surface of the head is not warped if the bike was overheated at any point.