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MG Midget and Sprite Technical - Brake pipe route - Frogeye

I am fitting all new brake pipes to my rotisseried Sprite. The main fore and aft pipe needs to end at a fixed bracket at the back of the rear bulkhead. This is where the rear axle flexi hose will join up.

I know that on the Frogs this main pipe runs under the car, following the seat runner reinforcement. But I might decide to run it inside the transmission tunnel whilst it is easy to get at, where I think it will be better protected. Either way, I need a location for the position of the bulkhead attachment bracket as my shell doesn't have one.
I guess it goes on the offside of the tunnel hole, but could do with latitude and longitude coordinates if someone has an easy access shell. And a tape measure. Please.
GuyW

Is this any help?


Dave O'Neill 2

Found on this website

http://www.kstanden.plus.com/photos.html
Dave O'Neill 2

Guy, I've looked under my shell and can't see a bracket for the brake pipe, only for the handbrake cable. I think the brake pipe just uses metal clips, probably welded to the chassis originally but I can't remember precisely where the flexible hose attaches. Despite making notes and taking pictures when stripping I don't seem to have been precise enough.......
Bill Bretherton

Bill,
I think the weight of the flex hose and the male/female couplings need some sort of robust fastening against vibration. I don't think simple clips will provide enough support so I am going for a bracket at the position in the photo that Dave has helpfully found.

The only thing is that having painted the underside I don't want to weld it on so I will make a braket and fix it by a coupe of self tappers.
GuyW

Ah, do you mean the axle end of the flexible pipe? In that case it attaches to the axle casing via a bracket.
Bill Bretherton

No, I do mean the body end of the flexible pipe which also needs the support of a decent bracket.

The axle end connects to a 3-way brass connector, which fixes with a bolt to one of the external webs of the alloy part of the diff casing. So no bracket is needed there.
GuyW

The brake pipe on my 63 shell runs down the seat reinforcement and is held in about 3 places with the little Spring clips that you find holding the brake pipe under the steering rack. If you look, the reinforcements have holes in the side and the clips just push in, quite a neat and easy solution.

Bracket is in the position shown above.
John Payne

OK, I attach a picture of my old shell's underside. Although not very clear, you can see the junction of the brake pipe and flexible pipe. It might help for location.


Bill Bretherton

Yes, that's it Bill. Pretty well as on the shell in Dav's photo. Possibly slightly lower down.

The pipe on my later car ('71)is fastened to the side of the seat reinforcement rail with things like split pins with a somewhat oversized eyes for the head.
GuyW

TBH the bracket does seem a bit high in the photo that I posted.
Dave O'Neill 2

Those are the things I mean Guy - I think!

I'll try and post a pic tomorrow.
John Payne

Even better, I've found them in the Moss catalogue!




John Payne

does this help


S G Macfarlane

Thanks, that's a good shot of one with the pipe routed inside the tunnel. Did you modify it like that, or were some cars assembled that way from the start?
GuyW

given I can see a lamda sensor plumbed into the exhaust pipe it must be a well-modified car!
David Smith

All the Frogs I have ever seen had the pipe routed externally. I think the pipe was moved internally in the later Spridgets. certainly the RWA I owned in the late 70's was like that
Bob Beaumont

not my car - unfortunately - just a picture from a favourite website
http://www.britishracecar.com/derekchima-mg-midget.htm
S G Macfarlane

1965 Midget


James Paul

Bracket made, painted and fited. I used SS self tappers as I don't want to do any spot welding there at this stage!


GuyW

Now, another thread problem!
The main pipe running forwards from my new bracket ends up at a 4-way brass union on the front O/S inner wing. This fitting also takes the pressure switch, and this then is the question.

The switch and its recieving tapping in the brass union have a different thread from the brake pipe entries. It's difficult to tell but they appear to be 19 tpi tapered threads, I presume BSP threads. But the sizes are such that the switch only screws in two full tuns before it tightens up really firmly. With only two threads engaged I am doubtful that it won't leak brake fluid. I would rather expect it to screw into the fitting to either bottom out the thread to seal, or to take a copper washer under the head.

Photo shows how far in it screws. Both parts are new from the same supplier sold as a combined package so one might reasonably expect them to be compatable



GuyW

Guy,

IIRC the brake pipe fittings are 3/8" UNF but I thought the size of the pressure switch was too small to be 19TPI BSP which would 1/4" or 3/8", I thought the pressure switch was 1/8" BSP but that would have 28TPI pitch. I can't access mine at the moment so maybe someone else will confirm
David Billington

David,
You are quite right! Or very nearly so. I had measured various other threads out in the garage and got my notes muddled up. I think this is a 1/8" X 28tpi fitting. Or very close. My 28tpi guage sort of fits the thread, but doesn't quite nestle down into the threads as it should when held up to a backlight from behind. But neither do any of my metric pitch guages either.

If the switch should screw further into the fitting - more than 2 threads that is - then perhaps the tapping in the fitting dosn't go far enough up the cone of the taper?
GuyW

When using a tapered tap, you obviously have to use the correct part of the taper to match the taper of whatever is being screwed into it.

By its very nature, a taper has different diameters at any two points along its length.

It sounds like another case of the manufacturer not being given the correct spec.
Dave O'Neill 2

Guy,

I've cut many threads with dies and single point cutting with a lathe and IIRC the cutting tool is always perpendicular to the access of the part even with tapered threads so the pitch has to be measured parallel to the part axis, I've never cut any tapered threads in a lathe yet. The taper would make checking the pitch of a pipe thread more difficult as you will likely lay the thread gauge down against the thread which will give an error due to the 1:16 taper of the thread resulting in a slight error in the apparent pitch.

Dave,

BSP thread (ISO pipe) have defined standards and gauging details so it shouldn't be a matter of being given the wrong spec although in my experience there is often quite a bit of variation in the fit of parts with nominally the same size.
David Billington

David,
Thanks for that. As I checked with the gauge it very nearly fits the 28tpi blade, but just not quite perfectly. I suspect your explanation is exactly the reason.

If the male thread on the switch only engages the first two turns of the female union then it seems to me that the tapered thread tap that was used hole wasn't cut in far enough. Given that its a blind ended then I would imagine the plug tap needed to be truncated slightly more at the leading end.

Can anyone confirm my opinion that just two threads of engagement before it is fully tightened isn't enough?
GuyW

Guy,

See http://en.wikipedia.org/wiki/British_Standard_Pipe and gauge length. IIRC the gauge length is half the thread length as my 1.5" BSP die is 1" thick and the others follow per chart. I would expect at least that gauge length engagement at a minimum so I don't really see 2 threads as sufficient.
David Billington

Guy what is the part number of your new switch,
Andy Tilney

Guy, almost all my 'treasure' is from 1275cc cars (so could be different) but FWIW I've just separated two original examples of 4-way and switch, both were screwed in slightly more than 4 turns.
David Smith

Guy, the original brake light switch thread is 1/8" NPT (see extract from an article I wrote for MASCOT some time ago).



J.E. Davies

Thanks all.
So it is confirmed that two threads engagement isn't sufficient and there is something wrong. If the switch is a NPT thread then it may be that the supplied union is the wrong one and has some other thread - possibly BSPT?

The easy solution is to return both parts to the supplier, but in the meantime I am still inquisitive!

The switch is stamped 27/17 Might this refer to a 27 tpi pitch? I don't have any data tables on NPT threads.

Andy, the other number stamped on it is 51600 so that maybe the part number. It was sold as a combination of the union and the switch as 17 H 7108 / C16062A

John, I was aware of the switch contact quality issues but intended to deal with that by adding a relay. This is what I did with my '71 car when the first replacement switch failed in 2009 and it has been fine ever since.
GuyW

Just found this (table below) lists a 1/8" NPT thread as 27 tpi. And in column 7 headed Hand Tight says 4.36 turns. So I guess my 2 turns should be 4 and 1/4 turns before a spanner is applied.


GuyW

Guy that part number of 51600 cross references to the lucas part number SMB 423, which is 1/8" NPT 27 TPI, I have 2 of these NOS Lucas ones unused, if you would like to try one let me know,
Andy Tilney

Guy

Lucas parts are normally stamped with a 4-digit code for week number and year, so ostensibly week 27 of 2017.

I don't know if you have a Lucas branded switch, or a copy.
Dave O'Neill 2

The part number 51600 is the part number used by intermotor, so this must be an intermotor part, not a Lucas one, the original Lucas part number for this switch is 34542B which has now gone out to SMB423,as this switch has a tapered thread,you will not be able to screw it right in by hand, and when it is fully tight(USING A SPANNER) you will still have between 3 and 4 threads still visible, this switch seals by using a taper thread,
Andy Tilney

This is a picture of a Lucas SMB423 brake light switch, which shows how much thread is left sticking out after it is fully tight, this is exactly the same amount of thread that was sticking out of the one I removed which was the original one, fitted to my car 48 years ago at the factory.


Andy Tilney

I contacted the seller who was helpful. Suggested l try turning with a spanner using 'moderate' force. (his words). Sure enough this has taken it in to 4.5 turns which l am happy with for a taper thread seal, and is similar in appearance to Andy's photo.
GuyW

This is my 1965 midget brake light switch. Looks similar to Andy's


James Paul

Guy

It looks like a Lucas SMB423 is the same as a Land Rover Series IIA long wheelbase (109) brake light switch, Land Rover part number: 502097 (superceded by STC1689). And probably common to a few other British vehicles.

If you are not happy with your fix, there maybe some genuine NOS Land Rover switches out there, for example worth phoning Dunsfold Land Rover as they have large NOS stock (http://www.dunsfold.com/stockroom.php) Pattern part from Intermotor: https://bearmach.com/brake-light-switch-br-2086

I wish I had the Lucas paper crossreference parts manuals that they used to fish out for me when looking for LR parts!

Cheers
Mike

M Wood

Thanks, its always helpful when those with access to parts lists post alternatives. In this case both the switch and the 4 (-5) way connector seem to have had multiple applications. Consequentially, browsing alternative supplier's sites throws up a wide variety of prices for the identical item!

The two-way brass banjo union used on the frog rear cylinders for the bleed nipple seem to be less ubiquitous. So the price for the much simpler 2 branch one is higher than the 5 way one!
GuyW

The various Lucas numbers can get confusing but some Lucas catalogues here - http://www.northwestclassic.co.uk/lucascatalogues.html
Nigel Atkins

Too late on this one, Guy, but having got home from holiday yesterday, I saw the thread, and couldn't resist a picture to confirm what you've done.


Nick and Cherry Scoop

My apologies for hogging the BBS with my questions lately. I am concious that not everyone is interested in some of these obscure questions! But I do value the help here!

I have Horler, and Clausager and the Frogeye Manual. Despite dozens of photos, not one shot shows a top view of the frogeye pedal box and the arrangement of the brake and clutch lines along the top of the footwell. From the two m/c outlet connections they must loop around and go forwards but is it one either side? Do they cross over, or both go down one side?
GuyW

Picture 1

Rob


MG Moneypit

Picture2

Rob


MG Moneypit

Picture 3


MG Moneypit

Oh ! Thanks Rob. I wouldn't have guessed that! I assume then that is the original layout.

Very useful. Many thanks.

I am not doing the clutch line just yet, but it looks like it is 3/16" pipe as well. I note the anti-vibration coil position as well
GuyW

Yes, this was a local owners build. He was very pedantic.

Clutch pipe same size as brake pipe as the connection to the frogeye clutch slave is a small union.

Also, note the jaunty angle of the control box. Also as original.

Rob
MG Moneypit

This thread was discussed between 27/09/2017 and 14/10/2017

MG Midget and Sprite Technical index

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