I have disconnected my brake light switch on my 1970 Series 2 GHC and replaced it with a mechanical switch on the brake pedal. But I have noticed that the original switch is leaking fluid. I don’t think that it is coming from where the switch screws into the hydraulic union I think that it is switch itself:-
You can see in the photo that the previous owner has used thread tape to install the switch but I would think that the switch should screw right in with a copper washer under the head to seal it. So I’m a little worried that maybe the threads in the hydraulic union are partially stripped.
II would prefer to simple replace this switch with a bolt and copper washer. To that end does anyone know the thread size?
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The switch it an NPT thread, unless I’m seriously mistaken.
The repro switches are notoriously poor quality, so filling that port with an NPT bung would be an acceptable resolution.
I don’t think it’s NPT, more likely 3//8-24.
I recently installed a high mounted stop light, which allowed me to observe the brake light for the first time. I was shocked at how slowly these pressure switches react. The delay may only be a few tenths of a second, but that matters in an emergency stop. I left the standard switch in place, and wired a mechanical switch in parallel. The mechanical switch is mounted at the pedal box, and reacts very quickly to foot pressure. So by all means, if the switch leaks, plug it and install something better.
A quick search of the archives indicate it’s an 1/8” NPT. What would provide the seal if it were a straight thread?
Well all I can say it that I am surprised that some believe the hydraulic union has a 1/8 ins NPT which is a tapered thread. I have never seem a tapered thread on any brake system I have ever come across. But this sure explains why there is thread tape on my switch. Then again the pictures of the brake switches you can buy sure look like they have straight thread like 3/8 ins x24 as someone has suggested. In this case the seal is made with a copper washer just like on, for instance, the flexible hose that takes the fluid from the rear brake line to the union mounted on the rear frame :-
It’s a more interesting question that it seems…1/8 NPT isn’t too different from 3/8-24 And the VW switch which is often noted as compatible is 10mmx1.5. Just to make it more interesting, Terry’s lists the size as 3/8 BSP and they include a copper sealing washer (although I think they actually meant 1/8-29 BSPP.) There’s a thread on the British E-type forum which suggests that the thread size depends on whether the union is original or reproduction. So I have to throw in the towel, and say I don’t know. All of these threads are similar, but not identical, in diameter and pitch, so that may explain how the fitting became chewed up.
NPT brake pressure switches certainly exist. I’m still fairly confident it’s an NPT in this application, but none of the vendors seem to list the thread.
Does the Factory Workshop Manual show it?
https://www.amazon.com/Brake-Line-Port-Pressure-Switch/dp/B07B8W46PC
I have the S1 switch in my hand and it is definitely 1/8-NPT. Unless the threads have been mangled any 1/8-NPT male plug should work fine. You can also get the 1/8-NPT tap and chase the threads. If you are determined to use a straight thread with a copper washer bear in mind the thread pitch is 27 TPI so 3/8-24 will not engage fully.
Rick OBrien
65 FHC in FL
If I didn’t have my arm in a sling at present after rotator cuff surgery I would go out to the garage and check on the 3.4 and 3.8 engines that are sitting and take out the switches. Just thinking, as the cars were manufactured in the UK wouldn’t the prevailing threads be BSP(P)?
Me too - it’s an original Lucas switch date coded 9-67 and it’s tapered thread mic’d at .390" to .405".
I plugged mine with an 1/8 inch pipe plug. It fit just fine. Use Teflon tape or your favorite thread sealant.
Using teflon tape on any brake line fitting just sounds wrong. I also find it unlikely that the original union would have an NPT type thread. I can’t see anyone at Brown’s Lane specifying NPT. As @Michael_Frank has indicated, measuring an aftermarket switch (like the VW one, or some other flavour) may be misleading.
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I agree it is very unlikely to be NPT. To be honest I would have thought that it wasn’t a tapered thread but some people how replied say it is. So I have ordered a 1/8 ins BSPP plug but could it be 7/16 ins UNF?
I have no idea on what it is supposed to be, but wouldn’t it be logical that it is BSPT and not NPT or BSPP? The difference with NPT is 28 vs 27 TPI which isn’t real noticeable and would let either fit, although not ideally.
When threads are so close in the fit it is hard to tell what it is. Today I had to thread my new slave cylinder with a 7/16 NF tap because it came threaded 3/8 NF. It worked out fine and the feed pipe bottoms out and tightens. I would think if it was a tapered thread the nut might tighten before the pipe contacts the angled seat. I’m also putting a spring on it because after 10000 miles my TO bearing is at 1/8 inch, and I thought I’d never have to pull that motor again. LOL…
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As I said, I just don’t know…the information is conflicting. the Terry’s part is BSPP, the Rockauto part is 3/8-24, as are most of the brake line fittings. The VW part is 10mm, which seems unlikely. And there seem to be many folks that are successfully fitting 1/8 NPT. It could also be 1/8 BSPT. Or even 3/8-26 BSB (bonus to anyone who can identify where BSB fittings are used in E-Types.) Some of these threads are close enough to be “wrench corrected” into soft brass. And to further confuse the issue, the junction could easily have been replaced by an aftermarket part or retapped. Finally, I remind everyone that Jaguar doesn’t make brakes, the components were designed and manufactured by Lockheed or Girling. So use what works. It’s a more interesting question that it seemed at first.
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In our continuing series of “no topic too small and no horse too dead”, I offer the following.
My 1/8-NPT tap seems a perfect match to me. There is roughly 1/3 inch of engagement shown. If it was 28TPI instead of 27 I would expect a 1/3 thread mismatch, but I’m not seeing it. You can also make out the 7 64 build date on my switch. My car build date is 10 64, so that would seem to match. But, like everyone else, I am surprised Lucas would have been using NPT in England in 1964.
At any rate, it certainly seems that a standard 1/8" pipe plug would fit and seal just fine.
Rick OBrien
65 FHC in FL
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I’ve got my brake line and switch fitting, and new and old switches in my box of parts so I pulled them out.
I can easily thread my 3/8-24 thread chaser all the way into the two brake line ports, but only part way into the port for the brake switch.
Brake line port #1:
Brake line port #2:
Brake switch port:
Here you can see how the 3/8-24 thread chaser doesn’t match up well with the threads on my old brake switch (or the new one, for that matter). New one is C16062U from SNG.
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In hindsight, my last post wasn’t that clear. The new switch I bought from SNG has the same threads as my old switch, and they both thread into the brake switch port just fine. I don’t know what thread they are, but I do know they’re not 3/8-24. I suspect they’re 1/8-27 NPT as the switch you’ve shown. I was just trying to add more data points to the OP’s post.
Here’s the switch I bought. The “spade” connectors don’t match the “bullet” connectors of my stock one, but none of the other ones I’d found at the time did either. Will the switch you linked plug into OEM wires connectors if the screws are removed?
