An apparatus (10, 100) comprising: a shaft assembly comprising: a first tube, a second tube (3 70) coaxially disposed within the first tube, wherein one of the first tube (3 52) or the second tube is configured to translate relative to the other of the first tube or the second tube (3 70), and a distally projecting tongue (354, 372) fixed to either the first tube or the second tube (3 70); and an end effector comprising: an ultrasonic blade (3 90) extending from the shaft assembly, and a clamp arm (3 82) pivotally coupled to the distally projecting tongue (354, 372) of the shaft assembly, wherein the clamp arm (3 82) is configured to pivot toward and away from the ultrasonic blade (3 90) in response to relative translation between the first and second tubes (3 70), wherein the distally projecting tongue (354, 372) is configured to flex relative to the first tube and the second tube (3 70) in response to the clamp arm grasping tissue, characterised in that: the distally projecting tongue (354, 372) has an outer surface defined by a first radius, wherein the first tube or the second tube (3 70) fixed to the distally projecting tongue (354, 372) has an outer surface defined by a second radius, wherein the first radius is smaller than the second radius. a shaft assembly comprising: a first tube, a second tube (3 70) coaxially disposed within the first tube, wherein one of the first tube (3 52) or the second tube is configured to translate relative to the other of the first tube or the second tube (3 70), and a distally projecting tongue (354, 372) fixed to either the first tube or the second tube (3 70); and a first tube, a second tube (3 70) coaxially disposed within the first tube, wherein one of the first tube (3 52) or the second tube is configured to translate relative to the other of the first tube or the second tube (3 70), and a distally projecting tongue (354, 372) fixed to either the first tube or the second tube (3 70); and an end effector comprising: an ultrasonic blade (3 90) extending from the shaft assembly, and a clamp arm (3 82) pivotally coupled to the distally projecting tongue (354, 372) of the shaft assembly, wherein the clamp arm (3 82) is configured to pivot toward and away from the ultrasonic blade (3 90) in response to relative translation between the first and second tubes (3 70), wherein the distally projecting tongue (354, 372) is configured to flex relative to the first tube and the second tube (3 70) in response to the clamp arm grasping tissue, characterised in that: the distally projecting tongue (354, 372) has an outer surface defined by a first radius, wherein the first tube or the second tube (3 70) fixed to the distally projecting tongue (354, 372) has an outer surface defined by a second radius, wherein the first radius is smaller than the second radius. an ultrasonic blade (3 90) extending from the shaft assembly, and a clamp arm (3 82) pivotally coupled to the distally projecting tongue (354, 372) of the shaft assembly, wherein the clamp arm (3 82) is configured to pivot toward and away from the ultrasonic blade (3 90) in response to relative translation between the first and second tubes (3 70), wherein the distally projecting tongue (354, 372) is configured to flex relative to the first tube and the second tube (3 70) in response to the clamp arm grasping tissue, characterised in that: the distally projecting tongue (354, 372) has an outer surface defined by a first radius, wherein the first tube or the second tube (3 70) fixed to the distally projecting tongue (354, 372) has an outer surface defined by a second radius, wherein the first radius is smaller than the second radius. the distally projecting tongue (354, 372) has an outer surface defined by a first radius, wherein the first tube or the second tube (3 70) fixed to the distally projecting tongue (354, 372) has an outer surface defined by a second radius, wherein the first radius is smaller than the second radius.