Brake light switch thread - BSP or NPT ?

[Andrew1967]

Hi,

Got to change the brake light switch on my '60 850.

Should the switch be an NPT thread or BSP with a copper washer ?

The one I took out was NPT but the new one I have is BSP, and with a copper washer fitted, there a small leak when under pressure.

Thanks

Andrew

[dklawson]

My comments are general, not strictly technical, and not Mini specific.

You said the switch you took out was 1/8 NPT. That could easily be confused with the slightly smaller 1/8 BSPT, both would have easily perceptible thread taper. If you removed a switch with the tapered threads, you really need to replace it with a switch having tapered threads.

In "that size", 1/8 NPT, 1/8 BSPP (parallel), BSPT (tapered), and M10x1 threads are similar in general size and appearance. In some applications you can force them to work together. In brakes... I would not.

If you cannot get the switch with the correct threads, there are some instances where you can use BSPP with its sealing washer in 1/8NPT applications. For it to work, the top surface of the T fitting will need to be completely clean, flat, and free of rust or pitting. If your brake T meets those criteria you should be able to tighten the BSPP switch in place and have its soft copper washer produce a suitable seal.

Again, like for like would be best but you can probably make this switch work if you have to and the condition of the T fitting washer contact surface is nearly pristine.

[Spider]

They are 1/8" NPT (27 threads to the inch).

As Doug has said, they seal on the Taper and the originals didn't have a Copper Washer.

However, there is a Banjo Adapter (29 in the Diagram below) that the Switch Screws in to and sometimes these come out with the Switch. They have a 3/8" UNF Thread which is parallel. It sealed with a Copper Washer (24 in the diagram below).



It seems that many of the new switches around these days don't last too long, I've now gone over to fitting these Hella Hydraulic Switches as a direct replacement;-

[Andrew1967]

Thanks for your replies Doug and Spider

I wasn't sure that the switch I took out was correct, although I did fit it some 15 years ago. The new one I fitted is definitely a straight thread so is not correct for the job.

I'll see about getting a replacement one. The Hella one seems a good option.

Thanks again

Andrew

[dklawson]

As Spider said, there are issues with the new switches not lasting, particularly if you are using DOT-5 brake fluid. I left the failed switch on my car's brake piping and wired a mechanical pedal box switch in parallel to it. The under bonnet brake system still looks original but the actual functioning switch is inside the car.

[ianh1968]

They are 1/8" NPT (27 threads to the inch).
As Doug has said, they seal on the Taper and the originals didn't have a Copper Washer.
However, there is a Banjo Adapter (29 in the Diagram below) that the Switch Screws in to
and sometimes these come out with the Switch. They have a 3/8" UNF Thread which is parallel.
It sealed with a Copper Washer (24 in the diagram below).

BRAKE LIGHT PRESSURE SWITCH

Top Washer (24):
OD 0.687" ID 0.437" TH 0.055"
NB: Although the thread is 3/8" UNF, the washer is a bigger ID

Bottom Washer, not shown but goes under (28) and on top of the flexible pipe bulkhead fitting:
OD 15mm ID 10mm TH 1.2mm (?5/8" by 3/8" by 0.050"?)
NB: This is what I use as they are "close enough"...

Ian

[ianh1968]

<SNIP>and wired a mechanical pedal box switch in parallel to it

Anyone know the official part number for the bracket for this switch?
I know that I could make one, but if I could buy an official one it would be easier.

I am one of the DOT5 brigade and am looking to do the same thing as Doug...

My current "emergency system" uses an old motorbike pull switch which is cable-tied
to the steering column, along with a long spring out of a washing machine that hooks
round the back of the pedal... I failed an MOT once, and went back an hour later
with this cobbled together setup and passed...


Ian

[Andrew1967]

Thanks for the replies gents.

Doug, that's a good idea to fit a later style switch inside as swapping out the switch is a bit fiddly and a little fluid ends up escaping.

Just changed the incorrect one out with one I found in the box of my pick up bits. Its the original to the pick up dated '75. Works perfectly

Andrew

[Spider]

Anyone know the official part number for the bracket for this switch?

Ian

Ian, I've never seen the bracket as a separate item. They are spot welded to the pedal box. All I've even seen (at least in these parts) is the pedal box that has that bracket with a different part no.

I've made them up and fitted them (though usually to the clutch pedal), when the box is out of the car. Would be a ripe bugger of a job in car, though some guys have managed.

I recon it's easier just to by a decent hydraulic switch (like the Hella one).

[Vegard]

What about wiring it through a switch on the dash like the Mighty Mini guys?