expansion stroke moreover the channels 122 and 124 serve to guide and transfer the discharge products of pre chamber combustion away from the center of the upper surface of the piston 126 and to the pockets 108 and 110 where a large portion of the air in the main combustion chamber is located at that point of time eventually as the piston moves downward in the cylinder combustion will continue first by using the air in the squish volumes over the piston and then extending to use of the air remaining in the exhaust pocket 112 fig8 shows a modified piston head 128 similar to the piston head shown in fig3 but with the two main combustion pockets 130 132 independently from each other and offset to the left from the centerline roughly corresponding in position to that of the swirl pockets of the comet system this configuration is similar to the earlier engines as previously described above as background in this instance the associated cylinder head similar to that shown in fig2 is provided with one intake valve and one exhaust valve also located in similar central or near central positions with this arrangement the flame spreads outward rather than being concentrated at the center of the piston top but the valves can only have very shallow relief pockets formed on the piston top or no pockets at all limiting the possibilities for improved valve timing events a further modification is seen in fig9 which is similar to the modification of fig8 but.