An extruder head has a mandrel (5) with a centering tube (21) and inner (23) and outer (25) tubes outwardly around the centering tube (21). An adjustment assembly is provided for adjusting spacing between the tubes. The adjustment assembly has a first adjustment member (33) which provides simultaneous corresponding adjustment of the inner (23) and outer tubes (25) and a second adjustment member (35) which provides adjustment of the outer tube (25) independently of the inner tube (23).
