Category Archives: 10. Brakes

Brake light switch Jaguar XKs

Brake Light Switch Jaguar XKs

Introduction

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

Factory brake servo kit for XK 140 Coupé

Form no. 1117

VACUUM SERVO BRAKE KIT PART NO. KL.72536 FOR JAGUAR XK140 COUPE

 IMG_0340 Original restored Lockheed  5½” Servo for XK 140

Service Bulletin 235 of  January 1958 announces that a servo kit is now available for the XK140:  “For XK140 owners who would prefer less effort to operate the brake pedal a servo kit is now available from the Jaguar Spares Department under Part number 7076.  Detailed instructions for carrying out this modification to the Fixed Head Coupe model are included with each kit.

This is a Jaguar specific Lockheed 5½” Servo Unit which was offered from 1958 onwards for retrospective installation in XK 140’s. It is identical to the Servo Unit for the Mk 1 and quite different from the XK 150 and Mk 2 version which was a Lockheed 6 7/8″ unit. The XK 140 / Mk 1 servo is not only smaller but also lacks the external “atmospheric” pressure pipe of the XK 150 / Mk 2 version.

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Description of Lockheed 5½” Servo for Jaguar Mk 1   

image Exploded view of the Servo unit

There are in fact 2 types of the Lockheed 5½” servo: , both with :

  • Jaguar part number C.11000 and Lockheed 82266 with a non-adjustable push rod, delivered until autumn 1957
  • Jaguar part number C.13821 and Lockheed 89368 with an adjustable push rod manufactured from autumn 1957 onwards.

Only the later servo with the  adjustable push-rod has been offered for the XK 140. This push rod has to be adjusted when the servo has been overhauled. The push-rod  tends to corrode but can be remanufactured (see drawing below) using the push rod of the 6 7/8″ servo of the XK 150 and Mk 2 as a basis.

Push rod Lockheed 88020 Adjustable Push-rod 88020

These servo’s normally have an aluminium tag on which the Lockheed servo number has been engraved. For the XK 140 servo kit this should read 89368 (see photo below).

IMG_0330 Tag reading 89368 and date 18 3 60 (?)

Service kit for vacuum piston & push rod

There are two different service kits: the earlier non-adjustable push rod version requires Lockheed kit 72521 whereas the later adjustable push rod version requires Lockheed kit 72524. Original (NOS) Lockheed kits are still offered sometimes offered at Ebay.

Servo repair kit KL72521 Lockheed service kit 72524 Lockheed service kit 72521  (non adjustable)         Lockheed service kit 72524 (adjustable)

Fitting the servo in the Jaguar XK 140

The following parts are required to install a servo unit in the Jaguar XK 140 models.

Brake servo installation XK 140

The Lockheed 5½” Servo has 3/8″ x 24 UNF brake pipe connections (for 3/16″ pipe) whereas the Jaguar XK 140 normally has 7/16″ x 20 UNF brake union threads (for 1/4″ pipe). Therefore two Adaptors (Lockheed 81087) are required (see also photo at the top and details provided further down the page).

Also note that Jaguar assumes that both front and rear brakes are served by the servo. It is however possible to only connect the front brakes with very good results as the contribution of the rear brakes is less than 10%.

Installation instructions

Jaguar provided an installation instruction with every XK 140 brake servo kit (see text given below). It proposes a lengthwise positioning of the servo in the LH battery box for LHD cars. But this seems impossible for the Fixed Head Coupe version: the depth of the battery box is significantly shorter than those of the OTS and DHC versions. Even if the prescribed (90 degree angle) banjo is placed at the end of the slave cylinder, space is still very tight. More specific, the higher the mounting position of the servo (as prescribed in the instructions) the less space is available. However, a transverse position is feasible with the vacuum chamber positioned in the direction of the outside of the car, even in case a second 6 Volt battery is used (as per original).

Installation, Coupe (LHD)

Servo

The servo unit can be fitted in the LH battery compartment above the battery. The exact position selected should be as high and as near the outside of the car as possible. By using the end cover of the servo as a template, mark and drill the fixing holes. The outside bolt should be fitted from the inside of the car to form a stud on which to mount the servo. It is necessary to fit two nuts to each of the mounting bolts as distance pieces. The unit should be fitted with the vacuum and hydraulic inlet connections pointing towards the engine, and the servo cover repositioned so that the air inlet slot is at the bottom.

Fittings

The front and rear brake pipes should now be removed from the master cylinder and connected to the 3-way piece. The rear pipe to the centre arm and the front pipe to the innermost arm. The lay of the pipes will now control the position for the 3-way piece which may be secured by a set screw into the base and positioned on the front wall of the battery compartment. Now remove the two armed banjo from the master cylinder and replace with single armed Banjo Part No. 7960 also replacing gaskets Part No. KL.44518 and KL.44522. The adaptor Part No. 81087 with gasket (KL.44516) should be fitted to the servo inlet.

Installation, Coupe (RHD)

Servo

The servo unit can be fitted in the offside battery compartment above the battery. The exact position selected should be as high and as near the outside of the car as possible. It will be found that the wiring harness is secured to the toe-board at this point and this bracket should be removed and the hole drilled to take the innermost securing bolt of the servo. By using the end cover as a template, mark and drill the outermost fixing holes. Drill also a hole for the wiring harness bracket which will now divert the wires round the servo. The outside bolt should be fitted from the inside of the car to form a stud on which to mount the servo. It is necessary to fit two nuts to each of the mounting bolts as distance pieces. The unit should be fitted with the vacuum and hydraulic inlet connections pointing towards the engine, and the servo cover repositioned so that the air inlet slot is at the bottom.

Fittings

The front and rear brake pipes should now be removed from the master cylinder and connected to the 3-way piece. The rear pipe to the centre arm and the front pipe to the innermost arm. The lay of the pipes will now control the position for the 3-way piece which may be secured by a set screw into the base and positioned on the front wall of the battery compartment. Now remove the two armed banjo from the master cylinder and replace with single armed Banjo Part No. 7960 also replacing gaskets Part No. KL.44518 and KL.44522. The adaptor Part No. 81087 with gasket (KL.44516) should be fitted to the servo inlet.

Breather

This is in fact an AC inlet for (filtered) air (under atmospheric pressure) required to operate the servo.

Description of parts

5½” Servo Unit 89368 (1x)

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Adaptor 81087 (2x)

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Banjo Bolt 30420 x 24 UNF (2x)

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Banjo 29588 x 24 UNF (1x)

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Gasket KL.44516 (3x)

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Gasket KL.44518 (2x)

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Gasket KL. 44522 (1x)

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Outlet 70742 (1x)

Description unknown
Banjo 7960: 7/16 x 20 UNF (1x)

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3-way piece 7965: 7/16 x 20 UNF (1x)

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Non-return valve 91607 (1x)

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Hose 70353/36 (1x)

Rubber vacuum hose 36” (90 cm) long.