This is a replacement plunger for a cheap, metal desoldering pump (internal diameter of tube is about 16mm). The pump is well made except for the plastic plunger which will break in half after about 30 actuations, because the plastic is thin where the two metal rods are pressed in.

The metal rods won't just push in, so heat them slightly and push them into the holes for a good fit.

Fusion 360 file is included.