No, not hiding Alan's keys for Churchill, nut the things made of steel, brass, et al that hold things together. There are 5 main types of self locking.

Only the first three could be found properly on springs, (castles are rare on these) Normally now only Nylok. Precrushed ones are difficult to spot. Also you could use a locknut but this needs more thread. A thin first nut is put on then a thicker top nut butted up to it. (believe it or not, I was told this is the correct way. Else 2 thick nuts can be used. Obviously a fan of the late K.N.Harris, a Model Engineer of note, and world class curmudgeon, who, despite the fact that just about every machine drawing from about the year dot-and-a-bit showed the thick nut on first, maintained it had to be the other way round. His argument was that the thin nut was put on first to position the component, and it was the thick one that took the strain so it had to go on last. Everybody else said that, yes, the thick nut took the strain, so put it on first and tighten it down, then the thin one was just there to lock the other one. On a practical level, if you do it Harriswise you need a thin spanner to hold the nut to stop them both turning together when you remove them. Or come to that when you are doing the assembly. And it looks ! better, too, particularly on marine engine eccentric straps. My 1904 copy of "The Marine Steam Engine" (newly updated to take account of the steam turbine) shows the thin nut on top. The authors, Sennet and Oram, were both Engineers-in-Chief to the Royal Navy, and I would think, knew their stuff rather better than K.N.H.

Also you could go into spring or better still tab washers or drill and wire the nuts (SEE lr DIESEL ENGINES). The first 3 should only be used once. (I know there are tricks but when a con rod nut falls off, you won't ever do it again!