40 is to be introduced into bore 32 the diameter of rock head 40 has to be reduced this is accomplished by moving or pivoting the body 60 a of each drill bit 60 about retaining pin 80 fig6 a as body 60 a pivots the first and second ends 82 a 82 b of spring 82 are compressed toward each other by the movement of front wall 66 toward inner wall 76 this movement causes roller cone 60 b of each drill bit 60 to move inwardly toward the central region 52 of base 42 thus causing the overall outer diameter of rock head 40 to be reduced from u201c a u201d to slightly less than u201c b u201d the pivotal movement of body 60 a also places springs 82 under compression and allows wheels 70 to be brought into contact with the interior surface 24 a of pipe 24 as boring machine 10 advances along tracks 12 toward soil face 18 it causes auger 22 to move through pipe 24 pushing rock head 40 through the bore 32 thereof fig8 fig7 shows the configuration of rock head 40 traveling through bore 32 of pipe 24 fig9 and 10 illustrate how the rock head exits pipe 24 it can be seen that while the wheels 70 remain in contact with interior surface 24 a the rock head 40 is in its first position with the roller cones 60 b of drill bits 60 disposed proximate central region 52 of.