A fastener including: a first load receiving portion and a second load receiving portion; a strainable portion between the first and second load receiving portions; a measuring end including an alignment protrusion; a cavity extending from the measuring end of the fastener, the cavity including a reference pin root shoulder located within the strainable portion of the fastener; a reference pin having a root end and a reference end, the root end of the reference pin being fixed to the reference pin root shoulder in the cavity and the reference end of the reference pin including a reference datum; and either a) a strain datum on an outer end of the alignment protrusion, or b) a strain datum sleeve having a strainable portion datum shoulder contacting end and a strain datum end, the strainable portion datum shoulder contacting end for contacting a strainable portion datum shoulder included in a bore in the cavity and located within the strainable portion of the fastener, the strain datum end including a strain datum, the fastener being characterised in that the alignment protrusion has an engagement portion having an engagement width perpendicular to an axial engagement length, the axial engagement length being at least 0.9 times the engagement width or at least equal to the square root of a cross-sectional area for non-circular outer profiles and the engagement width of the alignment protrusion has a tolerance of no wider than ISO 286-1:2010 c11. a first load receiving portion and a second load receiving portion; a strainable portion between the first and second load receiving portions; a measuring end including an alignment protrusion; a cavity extending from the measuring end of the fastener, the cavity including a reference pin root shoulder located within the strainable portion of the fastener; a reference pin having a root end and a reference end, the root end of the reference pin being fixed to the reference pin root shoulder in the cavity and the reference end of the reference pin including a reference datum; and either a) a strain datum on an outer end of the alignment protrusion, or b) a strain datum sleeve having a strainable portion datum shoulder contacting end and a strain datum end, the strainable portion datum shoulder contacting end for contacting a strainable portion datum shoulder included in a bore in the cavity and located within the strainable portion of the fastener, the strain datum end including a strain datum, a) a strain datum on an outer end of the alignment protrusion, or b) a strain datum sleeve having a strainable portion datum shoulder contacting end and a strain datum end, the strainable portion datum shoulder contacting end for contacting a strainable portion datum shoulder included in a bore in the cavity and located within the strainable portion of the fastener, the strain datum end including a strain datum, the fastener being characterised in that the alignment protrusion has an engagement portion having an engagement width perpendicular to an axial engagement length, the axial engagement length being at least 0.9 times the engagement width or at least equal to the square root of a cross-sectional area for non-circular outer profiles and the engagement width of the alignment protrusion has a tolerance of no wider than ISO 286-1:2010 c11.