The brake lamp switch for the later Elan +2 is threaded 1/2"-20 UNF. The part number for this switch is A036 J 6154, indicating first use on the Elan so I assume that the late +2 and the S4 use the same switch. Unfortunately my workshop manual and service parts list for the Elan do not cover the later switch at all so I am left to educated speculation. On the +2 the switch threads into a nut welded to the pedal box and no lock nut was used by the factory. This always bothered me because that left the electrical wiring to the switch as the only "locking" device that would prevent the switch from rotating out of adjustment with respect to the brake pedal. I made a very thin lock nut to keep the switch in place after adjustment. The lock nut had to be thin ( about 1/8") to avoid using up too much of the threaded length of the switch which is needed for adjustment. Obviously the lock nut can't be tightened much as the plastic, fine threads of the switch body are easy to damage. I hope this helps.
Elan +2S (1971)
Elite S2 (1962)