Abstract: A casing scraper for well casings in which the scraper blades are positively held on the scraper body. The blades are provided with retainer arms which engage a series of splines formed integrally with the scraper body, and slide up the splines to a cutting position. The blades are held in place by a retainer ring which has two sets of threads, with a gap in between the two sets. The lower set of threads normally engages annular threading on the scraper body to bear against the lower end of the blades and hold them in place. If the ring becomes partially unscrewed, the gap between the two sets of threading tends to resist further disengagement, and the ring remains sufficiently in place to prevent the blades from unloading. The orientation of the ring threading and of the annular threading is also reversed from the scraper's direction of rotation in normal use, so that under normal rotation the ring is urged to rotate more tightly against the blades, rather than to unscrew.