Brake light switch Jaguar XKs

Brake Light Switch Jaguar XKs


Jaguar used a hydraulic brake light switch on all of their XK models (and many other types as well). This switch is screwed in the Outlet Adapter of the Brake master cylinder.

Brake switches (screw-in type) in general, use different threads which may lead to confusion when looking for the correct replacement. The thread used by Jaguar on these early hydraulic switches is rather special. Finding the correct version is of the utmost importance to avoid leakage or damage to the thread.

All XK brake switch types had an 1/8″ x 27 NPTF (external, male) thread. This is a special screw thread derived from the US National Pipe Thread. NPTF is an abbreviation for National Pipe Taper Fuel and “Oil-dry-seal” thread. This conical thread is designed to provide a leak-free seal. NPTF threads have the same basic shape as NPT but the thread is adjusted to obtain an interference fit, eliminating leakage along the thread. All NPT threads have a taper of 1 : 16 on the outer diameter of the thread, which equals an angle of about 1¾ °.

NTPF thread Male NPTF thread on brake switch

Please note that the corresponding (internal, female) thread of the adapter is 1/8″ x 27 NPSF, which is a straight (non-tapered) thread.

  • The outer diameter of the 1/8” x 27 NPTF is 10.3 mm and the pitch is 0.94 mm.
  • The modern version of this brake light switch has a metrical thread coded M10 x 1 and it is therefore easy to get confused and use the wrong version on an XK. Always check whether the correct NPTF thread is being used!!
  • A third type of brake light switch has an UNF 3/8”x 24 thread (with an external diameter of 9.5 mm). Also avoid this version as (again) the thread will be destroyed by installing this UNF thread.

The following hydraulic brake light switches have been used by Jaguar for their XKs:

1. Jaguar XK 120

  • Early cars manufactured until January 1953 (chassis numbers up to 660979, 669002, 67048, 679621, 667000 and 677000) had a brake light switch produced by Lockheed: Jaguar code C3901 and Lockheed part number 23388. The switch has long screw studs for the electrical connection and can be easy recognised.
Early XK 120 brake switch  Early Lockheed brake switch  Lockheed Brake light switch 23388
  • With the introduction of the Tandem brake cylinder type, the XK 120s changed to a Lucas brake light switch with Jaguar code C5218 and Lucas code 31082/B.
Lucas 31082 brake switch                                                        Lucas switch 31082/B to F; photo right original Lucas; photo left possible later version

2. Jaguar XK 140

The XK 140 continued the use of the Lucas brake light switch with Jaguar code C5218 and Lucas code 31082/D, mounted in the Outlet Adapter of the Master Cylinder.

3. Jaguar XK 150

The XK 150 also used the same brake light switch but the Lucas number 31082 had meanwhile received suffix F. The switch was mounted in the Outlet Adapter of the Master Cylinder.

Replacement alternatives:

This type of brake light switch can be or was used on many Jaguars : besides the XK120, XK140 and XK150, also on the Mk VII, VIII, IX, the Mk 1, Mk 2 and successors, the E types and even the XJ types

It will be very difficult to find an original C3901 Lockheed switch, but fortunately the later C5218 versions fits without a problem and is still available as an aftermarket version (see list below).

Brake switches for later Jaguar types (code C16062 and Lucas 31802/D) used Lucar connections instead of screw connections, but are otherwise similar.

Lucas 31802 brake switch Lucar Jaguar C16062 (Lucas 31802/D) with Lucar connectors

The generic Lucas replacement brake light switch for the 31082 and 31802 is coded Lucas SPB400 but has Lucar connectors.

Other alternatives part numbers: (not all versions have been confirmed)

  • BMC/British Leyland 3H1894
  • Bosch 0 344 004 011
  • Bremi 10760/13
  • Bremi 10760/19
  • Intermotor 51600 Lucar connectors
  • Intermotor 51620 Screw terminals
  • Lucas 34542
  • Lucas 34765
  • Lucas SMB 423

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