Adjustable isolation sleeve assembly for well stimulation through production tubing

A fracturing system (17) includes a fracturing spool (18) that mounts onto a wellhead assembly (10) for injecting fracturing fluid into a well (12) beneath the wellhead assembly (10). An isolation sleeve (42) is included with the fracturing system (17) that couples to the fracturing spool (18) and extends into the wellhead (10) to isolate and protect portions of the wellhead assembly (10) from the fracturing fluid. A seal (52) is between the isolation sleeve (42) and bore (49) of the wellhead assembly (10), which is threaded to the isolation sleeve (42). Manipulating the threaded connection between the isolation sleeve (42) and seal (52) selectively positions the isolation sleeve (42) to designated axial positions within the wellhead assembly (10).