Appts. to alter the stroke length of a needle carrier (12), which has a reciprocating motion to inject a pickling agent into materials to be pickled, has a lower dead point which is dependent of the stroke length. The needle carrier (12) has a linkage connection to a swing intermediate lever (24) which moves round a fixed axis (26), in turn linked to a rotating crankshaft (18) through a connecting rod (22). The connecting point (34) between the connecting rod (22) and the intermediate lever (24) is set at a guide (34), which runs at the lower dead point setting of the intermediate lever (24) concentric to the rotary axis of the crankshaft (18).
