a block 35 for setting the stitch length the block 35 is hinged at the bottom at 35b to the connecting rod 32 and has at the top a head 35a to which there are hinged at 35c upper arms 31b fig1 of the feed dog 31 the arms 31b being radiused together by a cross web 31c the feed dog 31 has a middle head 31a having a bottom side 31d positionable at a maximum oscillation distance a fig1 from the top side 35b of the head 35a an oscillation setting screw 47 having a head 47a and abutment end 47b is threadedly mounted for adjustable movement inside threaded hole 35e formed in head 35a by increasing the oscillation of the articulated block 35 the shorter will be the stroke of the feed dog 31 and therefore the shorter the length of the stitches the maximum distance a is attained by loosening the screw 47 until its abutment end 47b reaches below the top side 35b of the block head 35a see fig2 with the abutment end 47b in abutment with the bottom side 31d of the head 31a see fig4 a 4b 4c the cross web 31c will abut against the head 35a and the block 35 and feed dog 31 form accordingly a rigid piece in practice the range of length variation of the stitches is from about 10 11 mm rigid linkage condition of fig4 a 4b 4c to about 3 mm free linkage condition of fig2 to reach the screw 47 with a screwdriver a hole 48 is provided in the bottom 4a of the chamber 4 and a.