‘A Guide for BSA and Triumph Oil Pressure Switch Identification’
By Dave Porter, Technical and Customer 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.
-Dave
BSA and Triumph Oil Pressure Switch FAQs
How do I choose the correct oil pressure switch for my vintage Triumph or BSA?
Identify the thread type fitted to your engine rather than relying only on the motorcycle’s model year or a parts-book listing. BSA and Triumph used both tapered NPT and straight NPS oil pressure switches, and replacement timing covers can change which switch your motorcycle requires. Compare the existing switch’s thread shape and measurements before ordering.
What is the difference between NPT and NPS oil pressure switches?
NPT stands for National Pipe Tapered, while NPS stands for National Pipe Straight. The switches discussed in the article both use a nominal 1/8-inch pipe size with 27 threads per inch, but an NPT switch’s threaded diameter tapers toward the tip. An NPS switch has a constant threaded diameter. Matching the nominal size and thread count alone is not enough to confirm compatibility.
How can I tell whether my oil pressure switch has tapered or straight threads?
Use a digital caliper to compare the outside diameter of the threads near the switch body with the diameter near the tip. The article’s 60-2133 NPT sample measured approximately 0.410 inches near the body, tapering to 0.3795 inches at the lower threads. Its 60-3719 NPS sample measured approximately 0.3795 inches throughout. These sample measurements help illustrate the difference between tapered and straight threads.
Which Triumph and BSA oil pressure switch part numbers have tapered NPT threads?
60-1943, 60-2133, and 19-6504 are tapered NPT replacement switches. However, historical listings can be confusing: Triumph retained the 60-2133 part number during a period when production moved to straight-thread switches. Verify the actual thread form rather than treating an old part number as conclusive proof of fitment.
Which oil pressure switches have straight NPS threads?
Triumph 60-3719 and BSA 19-6508 are straight-thread NPS switches. BSA and Triumph twins generally used straight-thread switches from 1970 onward. Because timing covers may have been changed during a motorcycle’s life, check the installed components before selecting a replacement.
Can I install a tapered NPT oil pressure switch in a straight NPS timing-cover hole?
Do not treat tapered NPT and straight NPS switches as interchangeable. Although the switches discussed share a nominal pipe size and thread count, their thread shapes differ. The article specifically warns against threading a tapered NPT switch into a straight NPS bore. Confirm the thread type before attempting installation.
Why does my Triumph parts book list an oil pressure switch that does not match my motorcycle?
Triumph retained the 60-2133 part number through the 1973 model year despite changing from tapered to straight switch threads during production. In addition, interchangeable timing covers on unit-construction Triumph twins mean a motorcycle may no longer have its original cover. The actual cover and switch fitted to the engine are therefore more reliable fitment clues than the model-year listing alone.
How does the oil pressure warning switch work on a classic British motorcycle?
The oil pressure switch uses a small internal plunger to respond to oil pressure and grounds through its threaded connection to the timing cover or crankcase. A spade terminal connects it to the warning-light circuit. The switch allows the warning light to illuminate when there is no oil pressure or when pressure falls below its preset switching threshold.
Should the oil pressure warning light come on with the ignition on and the engine stopped?
Yes. With the ignition switched on and the engine stopped, there is no engine-generated oil pressure, so an illuminated warning light is expected. The switch is also designed to illuminate the light if oil pressure falls below its switching threshold while the engine is operating.
At what oil pressure does a vintage BSA or Triumph warning switch operate?
A switching range of approximately 3–7 psi for the switches used on these vintage British motorcycles. The exact threshold depends on the switch. This is the warning-light activation threshold, not a specification for the engine’s normal operating oil pressure.
What do the numbers on an original Smith’s oil pressure switch mean?
Original Smith’s oil pressure switches usually have an identification number stamped across the flats of the switch body. The article explains that the number can identify the switching pressure: for example, 5300/1/07 denotes a 7 psi switch. These markings provide useful identification information, but the thread type should still be checked separately.
Did pre-1969 Triumph 650 motorcycles use the same oil pressure switch fitting?
Not necessarily. The article states that early unit-construction Triumph 650 models used a 3/8-inch–26 TPI CEI blanking plug, part number 70-2615, through 1968. This plug sealed with a copper washer and could be removed for lubrication-system servicing or fitting an oil pressure gauge. It is a different fitting from the later 1/8-inch–27 TPI oil pressure switches, so identify the timing cover before buying a switch or adapter.