KK
Short Piston
The shorter piston of the KK variant is used in industrial sewing machines (often overlock, coverstitch, or specialized leather and buttonhole machines) where space on the needle bar is very limited. The shortened piston end allows for additional fabric thickness during the stroke in some machine classes.
Since the distance from the piston end to the needle eye is identical in both variants, a 134 KK can be used in many standard machines instead of a regular 134. However, it usually doesn’t work the other way around: If you insert a standard size 134 needle into a pure “KK machine” , the piston—which is too long—hits the top of the needle bar, causing the needle eye to sit too deep and resulting in skipped stitches or a broken needle.