A drilling assembly (2) comprises upper (4) and lower (6) drill strings with a fluid passage (80) therethrough for distributing fluid to a bottom hole assembly (10). The upper drill string (4) has an upper end (11) connectable to a drilling apparatus (13) and fluid source (16) and a lower end (18) with an attached casing expander unit (20) that communicates the fluid passage (80) of the upper (4) and lower (6) drill strings. An upper end (22) of the lower drill string (6) is formed from a casing string (24) telescoped over the casing expander unit. A constriction (31) connects the casing string (24) with the rest of the lower drill string (6). A flow restriction device (35) in the lower drill string acts to control fluid flow through the fluid passage. In use, the flow restriction device creates a pressure in the fluid passage that acts at the constriction to advance the lower drill string past the upper drill string while simultaneously expanding the portion of the casing string moving past the expander unit.