By Dave Porter, Customer and Technical Support at The Bonneville Shop
The topic of which oil pressure switch to select regularly comes up in discussions, and still remains somewhat baffling as to why there are so many inaccuracies abounding regarding this simple switch. From late 1968 onward, BSA and Triumph used an oil pressure sending unit which has a small internal plunger that senses oil pressure levels. The switch is grounded to the timing cover or crank case, where it is threaded into a tapped bore. There is a spade terminal at the top of the switch which, when connected to a power wire to the warning light on the headlamp shell, will illuminate the light when there is no oil pressure present, such as when the ignition switch is in the ‘on’ position without the engine running, or when the oil pressure falls below a pre-set level. The switch point can be as low as 3 psi and as high as 7 psi on the switches equipped on vintage British motorcycles. On the original Smith’s oil pressure switches, as supplied to the British automotive and motorcycle industry, the switch point was denoted in the part number for each application and usually stamped across the flats of the switch body. An example would be 5300/1/07, where the switch point is 7 psi. Seems pretty straightforward, right? This is where things start to get a little murky regarding these switches.
“The first Smith’s oil pressure switches specified for early 1969 Triumph T150 models (from May 1968) had a tapered 1/8-27 TPI thread, or National Pipe Tapered (NPT).”
The first Smith’s oil pressure switches specified for early 1969 Triumph T150 models (from May 1968) had a tapered 1/8-27 TPI thread, or National Pipe Tapered (NPT). In fact, some accounts suggest this switch appeared at the very end of the 1968 650 model run, sometime in the summer of 1968. Triumph listed this part number as 60-1943, and it was supplied in the NPT condition bearing the Smith’s designation 5307/05 on the flats. The 1969 and later BSA A75 models used a 5300/1/07 switch and was assigned a part number of 19-6508, which had the straight 1/8-27 TPI thread, or National Pipe Straight (NPS). The early 1969 Triumph 500 and 650 models are believed to have left the factory with a tapered thread 60-2133 NPT switch, also bearing the 5300/1/07 Smith’s number. It appears that well into the 1969 model year, Triumph started tapping the timing covers on the twins to fit a 1/8-27 TPI NPS switch. Oddly, although there was a switch to a straight thread NPS switch, the part number for the switch remained 60-2133 through the 1973 model year. BSA also had a running change that occurred where the 1969 A50 and A65 models were specified with the 19-6504 NPT tapered thread switch, showing a Smith’s designation of 5307/05. For 1970, the BSA twins changed to the 19-6508 NPS straight thread switch and was used through the end of production in 1972. It is safe to assume that BSA and Triumph twins were specified with straight thread NPS switches from 1970 onward. But the story doesn’t end here…that would be too easy. The timing covers need to be discussed to fully appreciate the need to measure the existing oil pressure switch before selecting a replacement switch.
The early unit Triumph 650 models were fitted with a 3/8-26 TPI CEI blanking plug through 1968. The plug carries the part number of 70-2615 and was sealed with a copper washer. The plug could be removed for lubrication system servicing, or for fitting an oil pressure gauge. The timing cover had a part number of 70-4567 from 1963 through 1967. The number changed to 70-8797 for the 1968 model year, perhaps due to the change in thread form of the contact breaker plate pillar bolts. From 1969 through 1977, the part number for the timing cover remained 70-9246. This is telling, as it suggests the drilling for the oil pressure switch was tapped for the 1/8-27 TPI NPS switch, which was originally Smith’s 5300/1/03, and is referred to as 60-3719 in the 1974-later parts books. Some refer to this condition as the “Veglia-type”, although Smith’s made all of the oil pressure switches up to 1978. The Triumph unit-construction timing covers are interchangeable, so it becomes as much about which timing cover your 650 or 750 twin has fitted, as it does about what the parts books and suppliers list for correct fitment.
“…it is strongly recommended to measure the threaded length of the oil pressure switch you are replacing.”
In summary, it is strongly recommended to measure the threaded length of the oil pressure switch you are replacing. A digital caliper will give you an accurate enough measurement when measuring along the parallel plane of the threads. While the taper is subtle, it will be visible. We have measured our 60-2133 NPT switch in this manner and have found the top of the threads measure approximately .410” tapering down to .3795” at the bottom threads. The same measurement of the 60-3719 NPS switch reveals a constant diameter of .3795”. So, while the parts books may list the 60-2133 as correct fitment from 1969 through 1973, the measurement will tell the truth about which switch you are replacing, and which timing cover you may have on your engine. Threading a tapered NPT switch into a straight NPS bore will yield undesirable results.
NPT Switches: 60-1943, 60-2133, 19-6504, 19-6504
NPS Switches: 60-3719, 19-6508
I hope you have enjoyed this article about oil pressure switch selection for your vintage BSA or Triumph motorcycle.