Lucas Kill Switches and Cutouts
Table of Contents
In this technical blog I will cover some of the functions and applications of the more common cutout buttons, or “kill switches” supplied by Lucas during the 1950s and 1960s. The function of the various cutout switches evolved from single-wire grounding switches, as fitted to magneto ignition machines, to later versions with two wires in either “normally open” or “normally closed” condition, as seen on 1960’s era motorcycles. I’ll break the categories down by Lucas Model-Type, including the 4A (76204), SS5 (35601 and 31071), and finally the 151SA (35835).
4A Cutout – Kill Switch: 762080, 76200A, and 762204
The 4A cutout switch carried several Lucas part numbers, including Lucas kill switch 762080, 76200A, and 762204. While the 4A is used primarily on magneto and distributor-equipped models, it can also be used as a horn-push switch. Since magnetos are always “on”, as long as the armature is turning, the method of turning the ignition off is to press the button, grounding the wire attached from the switch to a stud on the end of the magneto. 76204 is the most common part number associated with this cutout switch, and it is still available today.
SS5 – Lucas Kill Switch: 31071, 31376
When Lucas began transitioning from magneto and distributor ignitions to battery and coil type ignitions, a revised type of cutout button was offered, the SS5 type. The earliest part number associated with this two-wire kill switch is 31071, which is a normally-open switch, also carrying part number 31376. When the button is pressed, the ignition coil primaries are connected together, essentially short-circuiting the ignition. This type of cutout was mounted on either the nacelle or on the handlebar in a prominent chrome cup.
From 1963-1967, this cutout button was fitted to the battery-less AC Magneto, or “Energy Transfer” ignition Triumph T100C, TR6C, T120C, and T120TT competition models, as well as the BSA A50W and A65H models 1966-1967. The black hard plastic button was housed in the 97-1669 cup, known as the “mammoth” and retained with clips on the handlebar. The Lucas 31017 is currently reproduced and available in today’s marketplace.
SS5 – Kill Switch: 35601
“There is lingering confusion about the differences between the 31071 and 35601 cutout switches, even after Triumph issued Service Bulletin No. 13 on January 29, 1966.“
There is another version of the two-wire SS5 kill switch, the 35601, which debuted in 1965 for the battery and coil ignition T100, TR6R and T120R models, continuing through the 1967 model year. This normally-closed switch was housed in the same large chrome cup, but had a brown button which was intended to differentiate it from the 31071. It functions differently by interrupting both ignition coils, immediately turning off the ignition when the button is pressed. This version is compatible with both the ET/ AC Magneto ignitions and battery and points ignitions, but is not currently being reproduced, so originals are scarce and highly prized by restorers.
There is lingering confusion about the differences between the 31071 and 35601 cutout switches, even after Triumph issued Service Bulletin No. 13 on January 29, 1966. Apparently, some of the brown button switches stamped 35601 were produced in a normally-open condition, like the 31071 black button switches, so a continuity test with a multimeter is in order if there is any question about the internal wiring arrangement before putting one of these vintage 35601 kill switches into service.
Triumph Service Bulletin No. 13, January 29, 1966
151SA – Kill Button: 35835
Once the updated 6CA Lucas contact breaker points ignition was introduced for the 1968 Triumph T100C and TR6C models, a new Lucas cutout button was fitted, the 151SA type, which was listed in the parts books as 35835. This switch is a two-wire normally closed type, with a rubber covered plunger that simply interrupts the power feed wire to the coils from the ignition switch.
Like the earlier 37071 and 35601 kill switches, the 35835 is contained in the chrome cup (97-2259) and retained on the handlebar with clips (97-2034). This arrangement was original specification for the 1968-1970 Triumph T100C and TR6C models, as well as the 1969-1970 Triumph T150 and BSA A75 three-cylinder models. This cutout button is generally recognized as the preferred switch for use with electronic ignitions such as the Boyer-Bransden and Pazon systems, as it doesn’t earth the coils or contact points, but simply breaks the connection in the power feed to the electronic ignition module. The 35835 cutout switch is currently reproduced and available from any number of suppliers.
One last observation is that the 1966-1967 Triumph 500 and 650 battery ignition models were supplied with the 35601 brown cutout button housed in the “mammoth” chrome cup, but an illustration in the 1963-1970 Triumph factory workshop manual shows a 151SA (35835) cutout button assembly mounted to a handlebar under Section H16, page H26. The accompanying under the Fig. H27 illustration indicate the ignition cutout button, as specified for the TR6 and T120 models, however this combination was only offered on the 1968-1970 T100C and TR6C models, as the 1968-1970 T100, TR6, and T120 road-going models were no longer equipped with a kill switch.
One wonders if the engineering department at Meriden was ever completely settled on how to discern the differences between kill switches on the unit-construction twins in the 1960s. No wonder there is so much confusion about the applications for the 31071, 35601, and 35835 kill buttons. Thanks for reading this article, and I hope it helps to serve as a guide for the mysterious Lucas cutout/kill switches.
Triumph Service Manual Section H16
