Narak, Naruka, & Nar(u)kuthûn
These three words showed up in Parma Eldalamberon XVII: Words, Phrases and Passages, pg 47, as part of the etymology of the Sindarin word Narog. For clarity, here is the relevant passage:
S Narog. This was in origin a Dwarvish name (of Petty-dwarves?): ? naruka > S narog. nargothrond is Sindarinized from nar(u)kuthûn.
[ ... ... Etym. NÁRAK- 'tear, rend', *narāka 'rushing, rapid, violent', ... ... ]
Apparently, this has been taken as saying that there is a Khuzdul word naruka from which was derived the Sindarin narog. Following that view, Nar(u)kuthûn is the Dwarvish name for Nargothrond (instead of Nuluk-khizdîn!) and incorporates the word naruka. Given the passage above and the similar appearance to other Khuzdul words, I can see where this view comes from.
However, if we look a little closer, we can see these are not Khuzdul words at all. In Vinyar Tengwar, Volume 5B, issue 45, pg 37, there is an entry for a Quenya root NÁRAK-:
NÁRAK- [after:] Q naraka harsh, rending, violent [add:] (of [?sounds]) [for:] Q narki [read:] Q narka.
On Ardalambion, Helge Fauskanger has a downloadable file of Quenya vocabulary, which lists words from the same root:
naraca ("k") adj. "harsh, rending, violent" (NÁRAK; according to VT45:37, Tolkien added a qualification that is not certainly legible: "of [?sounds]")
narca- ("k") vb. "to rend" (NÁRAK; the form "narki" in LR is a misreading for narka; see VT45:37)
At the top of the file, Helge explains why some of the words that have a <c> in them have ("k") in the entry:
The spelling used in the source is usually indicated; for instance, ("k") following a word indicates that the word is spelt with a k instead of a c in Tolkien's text.
So, the entries in his list naraca and narca- are given by Tolkien as naraka and narka-.
From all of this information, we can see narak, naruka, and Nar(u)kuthûn are not Khuzdul words. The note that starts the entry, "This was in origin a Dwarvish name...", is actually referring to ten pages earlier (pg 37) where Tolkien says:
The river-name Narog is probably of D. origin, Narâg.
If narak, naruka, and Nar(u)kuthûn were Khuzdul words, the entries would contradict each other.