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MG Midget and Sprite Technical - Nipple Design

Not strictly MG related, but I am seeking knowledge about nipples. Or rather, the detail of how a bleed nipple actually seals. Someone here with a proper engineering background rather than my normal shade-tree approach will know.

The point is this (there's a pun). A bleed nipple has a pointy end that seals in the bottom of the hole that the nipple screws into. But must the angle of the pointy bit match accurately to an angled base to the hole so that there is a broad conical contact surface.(A)

OR, (B) does the point just act as a self-centering device so that the tip enters the hydraulic gallery, the seal then being achieved by the lip of the round section gallery contacting a corresponding diameter on the cone of the bleed nipple?

I can see either method could work, but there must be a preferred and more commonly used engineered design

Guy W

slippery when wet ??? ... is more interesting to me

Sorry... I cant offer anything of factual value,

Prop and the Blackhole Midget

Prop, I wonder why I expected you to be the first to answer!
Guy W

Guy - can you not see an indent or mark on a removed nipple? If there isn't one it may be a butt-fit (A), or if there is one some distance up the point it would suggest B.

Excellent opportunity thread for innuendo by the way (I am a nipple man rather than a butt man).
L McInally

Being a nipple kind of guy I would suggest that anywhere the nipple touches should feel ok
They should be checked regularly though by sight and most important have a feel and make sure they are nice and firm
If you come across a loose one and there is a nice firm one available, I'd lean towards the firm one myself

William Revit

Yes William, I quite agree, firm ones are much better. Its no good if they flop about.

The problem I am dealing with, as I am sure will have been guessed, is a broken off bleed nipple. I am planning on drilling it out and retapping. Same size if possible, but could go up a size if necessary. The question is though, is the angle of the tip of he drill bit critical in creating the correct mating surface at he bottom of the hole.

I cannot check the end of he nipple as it is seized in position. But I think usually they show a wear ridge at the contact point which would suggest that they seal by centering in the hole, rather than against a broad conical surface. Hoped some proper engineer would know!
Guy W


A left hand drill may be helpful as they'll likely get to a point where they just back out the broken nipple . I wouldn't expect the drill point to get in contact with the sealing face of the nipple recess as the nipples usually have a short straight section after the thread and when the thread goes the end will just spin preventing further drilling. Regarding the sealing I would have expected a line contact seal rather than a full surface mating but don't know for sure.
David Billington

Idon't think it would cause a problem either way
full seat or slightly differing angles It would still seal up ok
We have pressure packs of a product called Free It here
It is freezy cold ,a bit like the spray for burns only way colder
With a disc caliper we get it as hot as possible by pouring boiling water on it for a while then give the nipple a good freezy squirt and most times they screw out with any easyout
Might be worth a go willy
William Revit

Sorry just been for a looksy and I gave the wrong name for the stuff It's actually Chill Zone

Finish Line - Chill Zone - Bolt Loosening Spray

You get it from racing bicycle shops

William Revit

Guy. Are you are speaking of a bleed nipple, as used on brake calipers and wheel cylinders, or a grease nipple, commonly called a zerk fitting?

The bleed nipple (used on my MGB caliper) is a threaded item with a lower end of smaller diameter than the threaded area and forming an insert which seems to fit into a hole at the end of the threaded hole within the caliper. The very bottom end of the bleed nipple is angled and fits into an angle, left over from the drilling, within the caliper. (The two tapered pieces mate with each other forming a fluid tight seal.) There is a hole drilled partly through the long axis of the bleed nipple with intersects with a hole near the upper end of the smaller, non-threaded portion of the nipple. When the nipple is opened, the bottom is raised off of the sealing surface, allowing fluid to flow up the minor diameter (bottom extension)and enter cross wise hole and flow outwards through the hole along the center line.

If you have a spare bleed nipple, you can examine it and see what I am writing of. You can, also, measure the diameter of the lower portion of the bleed nipple and the diameter of the threaded surface. It should be possible to measure the thread pitch of the new nipple either by using a thread pitch gauge or by simply counting the number of threads in a quarter inch segment and multiplying by four to find the threads per inch. With that information, you are ready to remove the broken part.

To remove the broken part, by drilling, you always start by soaking the area with a good penetrating oil (NOT WD-40) and letting it sit for a bit. (Over night with another spray of oil about every six hours works well.)

Then, determine the "tap drill" size for the thread diameter and pitch of the nipple. This is the largest diameter drill bit you can use without guaranteed damage to the existing internal thread. I would start by drilling the existing hole a couple of sizes larger and then try a broken screw extractor (eazy-out) to see if the nipple remains can be removed by that method. If not, you keep drilling the central hole larger, trying not to go any deeper than the factory hole depth, until you have removed most of the body of the nipple. At that point, the little bits left can be removed using a dental pick and, if necessary, the remains of the stub end (lower end) of the nipple can be blown out of its seat with hydraulic pressure.

One note. Most of the bleed nipples I have examined have not had a precision hole drilled through the long axis--it is slightly off to one side by about .010" to .020". If you have to drill up to tap drill size, you have a good chance of doing some damage to the internal threads. How much would be hard to say. Sometimes, sufficient to require that you open up the hole to be retapped or have a thread insert fitted. Given the design of the bleed nipple, opening up the hole to the next larger size might prove somewhat more complex than average due to the requirement to open up the smaller hole for the lower end of the nipple. Best to keep it all original if possible.

Good luck with the project. Les
Les Bengtson

Thanks Les,
Yes I am talking about brake bleed nipples ('though not MG ones). I think it was clear from my first posting.

And you have very accurately described the form of the bleed nipple itself. What still isn't clear is if the angle at the bottom of the drilled hole - which is determined by the angle of the cutting face of the drill - is critical to the sealing of the nipple. Observing a conical seat and a conical point on the nipple itself, the temptation is to conclude that they are matched. In fact I suspect not.

Functionally a more reliable seal would be obtained simply by the pointed cone end entering the opening of the fluid gallery, and self centering as it is tightened up. In manufacturing it would be an easier design to replicate and there is also less likelihood of a poor seal resulting from a bit of contamination getting stuck on the flat mating surfaces. Well that's my thought-based assessment, but I was looking for a wise engineer to confirm - or otherwise!

I have also come across this product which looks like a good practical solution. Scroll down a bit and take a look at the video if you are interested.
Guy W


I don't know the answer to the A or B question, but in either case I think that the surface in the bottom of the hole in the caliper is machined more carefully that that which would be produced by a drill bit. That is, I don't think it would be smart to ram the bottom of the hole with a drill bit regardless of the sealing mechanism.

However, isn't it true that the threaded portion of the hole and nipple end well before the bottom of the hole? If so, you should be able to drill the entire threaded portion without ever touching the sealing surface.

C R Huff

Guy. The bleed nipple I removed shows a marking on the bottom, conical area where it mates with the drilled hole in the caliper. Thus, I suspect the marked area represents the sealing area of the nipple. One thing to note is that the extended (non-threaded) area is smaller than the minor diameter of the threaded area of the nipple. Hence, if you drill out the threaded area, the extension should come out with little problem. (Hopefully.)

The repair kit sounds like an interesting idea, with an oversized, threaded insert with a smaller than original size bleed nipple installed into the insert. But, I would, if possible, rather remove the old nipple and install a new, original sized, nipple when possible.

Les Bengtson

although that site doesn't make it very clear, the replacement bleed nipple is a 6mm - so the same size as the original in my case. Ridiculously small though - no wonder they are prone to breaking off!

I rather liked the style of his presentation. I think it is genuine, but it could just be a smart bit of professional video marketing!
Guy W

Guy, don't know for certain but am fairly sure the seal is only line contact not full taper contact. However to be sure (I assume you have a new nipple to hand) why not grind the point of the drill to the same angle as the end of the nipple and if you haven't got a replacement yet take one of the others out to measure.

T Mason

yes I could try that. But thinking about the geometry of it: If the point is intended to enter the end of the hydraulic gallery, sealing as a circular line contact, then to be effective the angle of the bleed point needs to be more acute than the end of the drilled hole to create a continuous high pressure contact line.
Guy W

Guy, I take your point but I tend to think the opposite, as if the nipple has the more acute angle the seal would be between the point and the shoulder of the machined vee, whereas with your way the seal would occur in the actual hole below the vee. Either way could be right but without checking a cylinder I'm not sure.

Thinking as I type this though, I'm starting to lean towards your theory, in which case the taper in the cylinder is just to step down from the thread diameter to the cylinder hole diameter and would therefore not be very critical. Need to dig out an old cylinder now just to satisfy my curiousity.

T Mason

Interesting Question.... What brought this on?
Did you get some special calipers or wheel cylinders... (Not an MG project?)

Are they somthing you cant replace cheaply?

I drilled two broken bleeder screws out of a set of corvette caliper style brakes with out damaging the threads, it can be done! :-) The vette was circa 1974ish.
Steven Devine

General engineering practice - certainly used by Lockheed is for the nipple to seal on the cone against a smaller hole in the casting - see the attached picture.

Aftermarket items may of course be different!

Chris at Octarine Services

OK, here is what I should have drawn right at the start, instead of trying to describe in words! Just that words are quicker!

After a good deal of thought about this I think the design is version B (keeping with my original A and B descriptions) The seal being made where the pointed shape of the nipple contacts the lip of the feed hole.

If you look closely then Chris' diagram above I think this is shown at the bleed nipple (4)

Guy W

Yes - design B - if you think about it, any particle of dirt will stop design A sealing, whereas design A will not trap dirt between the nipple & seat and the seat will distort to meet the nipple ( or vice versa) if the match is not perfect.

As a matter of note, this is the same design of seat/seal used on the oil pressure relief valve.
Chris at Octarine Services

I still reckon the freezy method is worth a try
I've been looking for a link to it and found this Loctite product which appears to be the same as
Might be a bit easier to get

William Revit

Ive never seen that stuff before but it seems like a great idea. Have you used the stuff? What happens if you get it on your fingers? I know my question sounds abit joking but im serious.
Steven Devine

I know those without a proper engineering background are to be held at arms length on this thread but I'm addressing Steven now

I used a freeze and release spray, not Loctite, and for the job I was using it on I found it ineffective and gave the can away

for three decades I've used a good quality (not ordinary WD40) penetrating/releasing fluid, just clean around the area and apply, leave as long as possible overnight is good then try to tighten first to break the rust/crud seal - if this doesn't work then repeat procedure leaving overnight again

I've only once had to leave overnight for a second time and that was on a water connection that was buried underground

let the chemicals do the hard work whilst you're comfortable and doing other much more enjoyable things - of course it's not very macho or more complicated (and possibly unnecessary) engineering
Nigel Atkins

Hey Nigel!
I appreciate the feedback...My method is hit quickly with hot burst of oxy actyelene torch.
Just enough to turn the nipple end orange. Grab an oil soaked rag and cool the bleeder. The heat concentrates on the bleeder screw and releases the rust. My rule is to do these 3x or more. Ive always got the bleeder out no matter how rusty it was if it was undamaged to start with.

If the head or nipple is broken off I used a drill bit a littlebigger than the inside diameter of the broken off bleeder insert up to the body of the broken bleeder and drill. The hole in the body of the broken bleeder acts as a guide and helps keep your bit on target. I usually try reverse drill bits and up to the next size until enough material has let the offending bleeder be removed. Some times they spin right out. Key is take your time and let the drill bit do the work Dont force it. Use new sharp bits.

The liquid freeze method is the same idea imho but I wondered if there claims were over stated. Im glad you let me know what your experience was as I was a bit skeptical about the results. Any one else use the freeze stuff successfully?

I uploaded the kit in the picture area of oxy acetelyne Nigel so in case you havent seen them you will recognize them now.
These are small portables.

Steven Devine

Nigel, I also prefer the gentle approach - at least as a first attempt. On this occasion as the nipple is on the side of the caliper I built a little reservoir wall around it with plasticine and filled that with Plus Gas. And left it overnight. I was pretty confident that would work, but as soon as I started with a small 8mm ring spanner on it the end just sheared off.

I had more success with heating and cooling, using a propane torch to expand the alloy and then a cooling fluid from a spray can. The cooling stuff was with my plumbing kit - it is sold for temporarily freezing water pipes when changing fittings like radiators. I presume it is the same stuff sold by Locktight, just cheaper! Anyway that got one unscrewed.

Score so far is two sheared, one removed and one still resisting! One of the sheared ones now drilled out.
Guy W

Ive tried propane but my experience is that it takes to long to get the bleeder red hot....I always worried about heat transfer wrecking the seals. The acetylene was fast enough rise in temp to qwench down cylinder bodies fast. If rusty I would hit with a wire wheel first and hit with propane and qwench repeatedly wth oil. If theres anyone on the bbs that can get this job done Guy, My money is on you!

Good luck! Luck always helps!
Steven Devine

Guy. Yes, your letter B illustration is the one that is, most probably, correct. The standard drill bit has an included point angle of 118 degrees. The bleed nipple I examined yesterday has an included point angle of 90 degrees. Hence, with that nipple and a standard drill bit, the system would work as per illustration B. With the proviso that the depth of the hole (which, on a hole that does not fully break through the bottom surface) will have a significant impact on where the bleed nipple seals on its bottom cone. Only a few thousandths of an inch will make a significant difference in the diameter of the portion of the hole drilled fully through the work piece.

(Included angle--hold the middle fingers of out hands so that the tips touch each other. The angle between the palms of your hands is the included angle and represents the angle formed by such things as the tip of the drill bit and the bottom end of a bleed nipple.)

Les Bengtson

you've probably got big engineer's hands not use to being delicate - you need lady size hands like mine more use to using small spanners

you also can't repeat the process if you break the fitting the first attempt, with a different sized spanner! :)

narh, pulling your leg, little brass fittings are a real pain if they're seized in, even my weak wrist has sheared the heads off brass screws with only slight effort
Nigel Atkins

Nigel, no, not particularly big hands! And I am a strong believer in using the right sized spanner. Not just one that fits properly, but one of the apropriate length, evolved over many years to provide the right amount of torque for the bolt/nut size.

But in this case the problem is a design one. The little bleeders are, um, little bleeders! So not much to them to start with. Then of course they have a hole drilled through the middle to weaken them further (think of midget rear wheel cylinder bleeders, for size) But the design problem doesn't end there. They are then fitted to aluminium calipers. So there is a bi-metal reaction, aided by damp, salt and many years of inactivity and the two components virtually become one!

And Les, thanks for the explanation of included angles. It must be getting on for 60 years since I learnt that, so probably high time in my second childhood for a reminder. ;-)

And its not even my car!
Guy W


although I must admit I've got a much shorter 8mm ring spanner than my 5/16" spanners
Nigel Atkins

Yes Nigel, 8mm, its a metric car.
Guy W

then next time try a 5/16" :)

no good deed goes unpunished - Sod's Law if you work on someone else's stuff things will go wrong
Nigel Atkins

What kind of car is it? can you just get rebuilt calipers?
Steven Devine

Sure you can Steven. For . But there's nothing wrong with the calipers. So why just throw money at it when a little care and thought will fix the problem for just a few
Guy W

Guy, kept out of this 'til now, there's enough real knowlege anyway but have you got enough threaded bit sticking out to let you fill the inside of a small nut with weld if you pop it onto the broken bit?

I have done this a few times now, first time amazed me but it worked perfectly. The heat of welding breaks the grip of the corrosion and say "hello Unca Bob".

And yes the taper fits into the hole,inside the thread as you and Chris and the rest said.

Hi Bill,
Both of the two that have sheared have broken off down in the bottom of a slight counterbore, so no residual bit accessible to weld a nut to. But I could try that on the last one if I do so before shearing it off. It may be the best policy, rather than stressing it any further with a spanner which method has failed twice already
Guy W


how about using a small nyloc upside down so the nyloc recess is in the hole and the hexagon is upsides

heat the nyloc first to drain out the plastic then see if that works

These things are proper buggers when they go like that

I probably have a couple of suitables if you need them

I'll look tomorrow

8mm diameter you say?

8mm spanner size, 6M thread and 6mm bleed nipple. But then there is a thinner neck to the bleed nipple and that is where they shear, maybe 4mm across with a 2mm hole down the middle!
Guy W

Got ya Guy, Yes if they are expensive its worth the effort. Any luck? What make is the car?
Steven Devine

I'm a bit slow getting back to you
I havn't used the Loctite spray as I don't think it's been out long I havn't seen it here but use the Finish Line one quite a bit
Your question about getting it on your fingers - DON'T
It's as cold or colder than refrigerant , you could do yourself some serious damage I reckon
I use it a lot assembling gearboxes Warming bearings and cooling shafts it makes things easy
It's great getting the Subaru boxes off them tight dowels just warm the bellhousing a bit to get some residual heat into it then squirt the ends of the dowels and the box just pops out instead of the big frozen solid dowel problem that they are so well known for
I've used it on calipers before and it works well
The main thing is to get some heat into the caliper first then after you spray the seized nipple the heat left in the caliper helps it expand away from the cold nipple it works

William Revit

Thanks Will,
I appreciate the feedback! :-)

I wonder how Guys Nipples are today? From his description theyve had quite a work out!

Ha ha ha! :-)
Steven Devine

You got the better of my curiosity so I had to have a look inside hope the picture is of some use.
P.S it is, was a midget caliper

Dave Pratt

Neat! A
Anthony Cutler

well done dave, thats certainly cleared that one up.
bob taylor

Excellent Dave! That matches my "B" version supposition. Its nice to get a definitive conclusion to it.

I just wish these bleed nipples were as chunky as the Midget front caliper ones!
Guy W

So Dave, do you think red or blue hermatite, would be best to use when reassembling that caliper? :-
Lawrence Slater

Hmmm I was thinking some no nails adhesive and good old gaffa tape would do the trick.
Dave Pratt

Well as long as you do the same to the other front caliper you shouldn't get uneven braking
Guy W

Is this a top secret military designed japanese caliper your working on?

Is that the reason non of us can know the make and model of this car? Wink, wink,nudge, nudge.
Steven Devine

This thread was discussed between 14/01/2014 and 18/01/2014

MG Midget and Sprite Technical index

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