Try "toru", "to take".
There are a few rules of thumb. Katate-dori uses "-dori" because "katate-tori" would be difficult to say at natural speed even for native speakers. Unvoiced syllables often become voiced following a syllable made up of an unvoiced syllable, and retain their usual sound when following a voiced syllable. They also often become voiced following the "-n" sound. And I believe intonation plays a role, as well, as it makes some combinations easier (or harder) to say than others. There are many exceptions, though.
That's been my understanding. I'm surprised, though, that I come across "tori" more often than "dori." In our dojo we use the latter and I was thrown off when reading "tori" in a book once.
Learning Japanese helps a lot when learning the names of techniques and understanding what seems inconsistent like the case with "tori" and "dori."