Abstract: A blind fastener capable of providing a strong fastening force on workpieces whose composite thickness may vary from minimum to maximum within a predetermined range. It comprises a tubular sleeve and pulling mandril therewithin, such mandril having a head which is pulled against the blind end of the sleeve to cause the same to be crimped or upset against the blind side of the workpieces. The sleeve is formed with a tapered end portion which varies from maximum wall thickness at the blind end thereof to minimum thickness intermediate the length of the sleeve. The mandril is formed with a complementally-shaped taper which varies from the head of the mandril to a point intermediate the length thereof.