For present-day drilling operations in deep and challenging well sections, a technology called casing while drilling can be used. It involves isolating and ‘casing’ a formation while drilling, where the casing itself is used as the drill pipe, which is cemented in after a certain depth is reached. By doing this, the need for separate casing runs will be eliminated and well stability is improved. The concept has become increasingly popular since the beginning of the 21st century, due to an increase in technical possibilities and use of this technique, which makes it more profitable. To enlarge a wellbore past its original drilled size, to enable lowering of the casing, an underreamer is used. Under reaming can be performed in a reaming while drilling (RWD) configuration. To perform these operations, the bottom-hole assembly (BHA) containing the underreamer is lowered in the casing string to a depth just below the casing shoe. Afterwards, drilling fluids are used to fill the casing and pumps will be engaged to drive a hydraulic piston in the underreamer assembly that opens the cutter blades. Hydraulic pressure is used to fold the blades outward and prohibits closure of the tool. The width of the reamed borehole is not only determined by the outer diameter of the casing couplings that have to fit thru, but there also has to be a clearance for the drilling mud. The annular space between the borehole and casing is relatively small which causes relatively high friction losses during current operation. Because of this, the dynamic pressure loss of the drilling fluid along the outside of the casing becomes too high. To lower the pressure, the borehole needs to be enlarged further than possible with underreamers currently available. There are also limitations on the length of the underreamer. From tests carried out by Huisman, it is found that an increase in length has adverse effects on the steering behaviour of the BHA. Therefore, the length of the reamer should be kept at a minimum. The DRILLSTAR Z600H design that is currently being used does not allow for a diameter enlargement. New concepts must be devised to enable this. A solution to this problem can provide Huisman Well Technology (HWT) with the opportunity to perform the drilling of more and deeper wells. A solution to the problem was found by splitting the piston and spring internally. This enabled the spring to be placed in the top part of the underreamer while the piston remains in the lower part. The blocks are located in the middle and fold out eccentrically to reach a hole diameter of 228.6 mm (9 inch). The angle under which the blocks fold out has been reduced from 60 degrees to 45 degrees to allow the underreamer to be shorter by 66 mm.