We assume everyone knows what threads are. No they are not lengths of thin cotton that hold our cars together! A thread is usually (but not always) a 'V' shaped groove wrapped around the cylindrical part of a bolt or other cylindrical part that may require it, (a male thread that is, more about sex later).
When the grooves are small and there are lots of them per inch of length T.P.I. (threads per inch) it is a fine thread, there follows when the grooves are big and fewer fit in an inch, it is a coarse thread.
Back in the days when someone realised useful objects could be made in metal as well as stone, another someone thought what a good idea it would be to make metal items disassemble which was quite a no/no with stone things, so he invented the thread, made a bolt and nut and became the first engineer.
Unfortunately the government didn't get into the act and declare that he should be the only engineer in England (if that is where it happened, and the Church didn't pass a decree proclaiming him to be the product of divine benevolence and, forbid anyone else from jumping on his bandwagon. The outcome being the industrial revolution, and the onset of the chaos that still persists to this day.
Digressing slightly. There has been a question burning in my breast for 50 years and if someone can answer it I will be forever grateful. It is this: How was the first thread made to build the first lathe that made the first thread? Please don't tell me about chickens and eggs.
Engineers and workshops sprang up like mushrooms all over the countryside, blacksmiths went from artisans and tradesmen to professionals overnight just by changing their signs from Smithing to Engineering. On a small scale what's in a name? But in the big time it was a different story. Every outfit that built something that used threads and employed more than 10 men and a company cat, invented their own thread.
This of course was so the mug customer would have to return to them for service and spares. It still goes on today Just ask a C.F.A. (Country Fire Authority) person who has been around for a while.
I won't go into specialist thread systems which rapidly evolved, sufficient to say they run into hundreds if not thousands and tend to make the waters of rational sanity very murky.
Came the motor car and many early examples were built by companies that already made other things, like 'BSA' into bicycles, 'Wolseley' shearing machines, 'Swift' sewing machines, 'Alldays and Onions' pneumatic engineering, and other famous names. We will disregard the continent, no doubt they had their own problems! I don't know how they did it but by the outbreak of WW-2 the English had their act reasonably together with the Whitworth BSW (British Standard Whitworth,) coarse thread and the BSF (British Standard Fine) threads using the Whitworth spanner sizes, which refer to the diameter of the bolt. Then the threads began to unravel again.
The British war department had at least some of their vehicle suppliers use off standard threads so (as I was told by my father who was for a brief period the unhappy owner of a disposals W.D. (War Department) 10 hp Austin utility) to deprive the enemy the use of standard spare parts! Just to add to the confusion the W.D. also decreed there were economies to be had by downsizing the Whitworth hexagons one step so that none of the spanners fitted the bolt sizes marked on them.
Whilst all this was happening on the old world side of the Atlantic, the Yanks would have none of it. Ever since they threw boxes of tea and the Poms into Boston harbour, they went their own ways. First they invented tea-bags, drank coffee, and had a good long look at British engineering, particularly motor cars. They went back to the ideas of the industrial revolution and designed their own thread, well almost!. These were conjured up by the Society of Automotive Engineers (SAE) and became the SAE, and SAE coarse threads. SAE has more TPI's than BSF, but SAE coarse has the same TPI's as Whitworth except 1/2 inch and 1 inch (I think). The hexagons are just different enough so that if you picked up a Pommy spanner whilst working on a Yank car or vice-versa you were (and still are) meant to simultaneously chew the corners off the hexagon and ruin your spanner. The SAE spanner sizes refer to the across the flats (AF) dimension of the hexagons and have almost nothing to do with the bolt size. There was a pathetic effort to trick the Poms and us by introducing the Unified National system, which was only the S.A.E. lot under a different name with revised tolerances.
About this time another someone invented the 'King Dick' knuckle buster, then the 'Crescent' adjustable shifting spanner and made a fortune! Another thing the Yanks didn't like about the English threads was the rounded top (crest) and bottom (root) as that required an extra step in the making called chasing, so they left theirs flat. The angle of the British thread is 55 degrees which means nothing to anyone but the Brits; so they (the Yanks) made theirs 60 degrees which is a much easier number to remember and divisible by 2. American grease nipples will only screw into an English BSP (British Standard Pipe) hole 2 turns before the slight differences in TPIs cause both male and female to disagree, so watch it when you are modernising your treasure. Since I have mentioned male and female threads and we are all grown up and wondering what it's all about. Let's talk sex. The male thread is the 'V' groove cut into the outside of a cylinder. If the cylinder is tapered, you have a tapered thread, commonly found on grease nipples, differential, gearbox filler and drain plugs. These are used to seal liquids without the need for a gasket or sealing washer. As far as I can recollect our treasures don't have these and only use parallel threads with fibre or copper washers. In order to serve a useful purpose the male thread must have a mate. Here comes the seamy bit. The partner is, you guessed it a female thread. This is a tubular hole with a 'V' groove created on its inner surface into which the male thread screws. (To think I used to snigger when I was a kid when my Dad would refer to one of his files as a coarse bastard.) Both male and female threads can be cut on a lathe and often are, otherwise the male is cut with a round hardened steel nut-like gadget called a die which is held in a holder, usually with long arms called a 'stock' (Fig 1). The female thread is cut into the wall of the hole with a hardened steel object like a headless bolt with 3 or 4 grooves along its length called a 'tap' (Fig.2), this is held and used with a 'tap wrench' (Fig.3) or 'tap holder' (Fig.4). (They look quite different). Just like all things female, great care, a gentle touch, and lubrication, (except in brass and cast iron) are necessary when using a tap, otherwise they are likely to snap and you are instantly in deep trouble. One could write a book how difficult and expensive it can be to remove a broken tap.
Back in the 50s I was drawing motor truck parts and we had to write our own specifications which were then typed and put into the complete vehicle spec. Our typist was a devout, and innocent, lass about 20 who taught Sunday school, did an excellent job and kept a dozen randy young draughtsmen very much at arms length. Sex was never to be mentioned. What did rock the boat was when the specifications called for male female couplings, connections, threads or fittings. Nipples in any form also offended her deeply. I seem to recall one of the older male specifications clerks used to type these pages for her until one day our chief design engineer took her aside and convinced her that we were not agents of the devil endeavouring by some covert means to seduce her by writing dirty. Now some A&Fs about threaded things. A stud is a headless bolt with a thread both ends, the most common being those that hold the cylinder head on. When removing or replacing a stud take careful note that quite often the thread on one end is coarse and the other end fine. The coarse end usually screws into a large object (let's call it the host) such as the cylinder block or crank case and the other end has a nut screwed onto it. To remove a stud the first thing that springs to mind is to grasp the unthreaded portion with pliers. This almost never works, then multi-grips, still no go. A mole grip wrench sometimes works and by now is chewing up the stud. Then a stilson wrench either shears off the stud or is successful in both removing and mutilating it beyond recognition.
There is a special tool for removing and installing studs without damage which slips down over the stud and grips the unthreaded portion with an off-centre roller. I haven't seen one in 30 years. Someone in the club may have one. What you and I have to do is this: Put 2 nuts on the stud. Align the top face of the top nut with the end of the stud, hold it there and screw the lower nut back firmly against it. Give the top nut and stud a sharp hit or two on the end with a largish hammer. Now tighten both nuts against each other as tightly as you dare. If you are lucky you may be able to get a ring spanner on both nuts at the same time, otherwise use two spanners and try to unscrew both nuts and the stud as one item. At the first sign of movement stop and carefully apply some WD-40 or equivalent to the host, being careful not to lubricate the nut end of the operation or the nuts will promptly let go and unscrew themselves. If as sometimes happens the two nuts keep letting go put them on the stud as described using 'Loctite'. Loctite sets in about an hour, but for best results allow 2 hours or better still overnight. To remove the nuts from the stud use heat. (More about Loctite later).
Let us dwell for a while on Studs. A common problem with studs is the failure of that end screwed into the host. What usually happens is the female thread in the host is destroyed. The most common cause in old vehicles being corrosion or electrolysis caused by the dissimilar metals, ie. steel stud and a cast iron or aluminium host, or brute force and soft metal as with the crankcase to cylinder block studs on an Austin Seven.
There are a number of ways to fix this as follows: (1) Tap the host to a bigger size thread, drill the stud holes to clear and fit larger diameter studs. (Tootle's head studs are 3/8" but were 5/16"). (2) Tap the host to a bigger size and fit step studs. These have a larger thread on the end that screws into the host but are stepped down to the original size. CAUTION! When fitting step studs (Fig, 4.) the step (or shoulder) must be below the surface of the host otherwise the bolted on part will be holding on the step of the stud instead of the host's surface. (3) Next there is the 'stud-saver' or insert (Fig 5.) This is a threaded sleeve of brass or steel which has the original thread size on the inside and bigger stronger thread on the outside. These are locked into the host with Loctite, but can only be used where there is plenty of metal around the stripped hole. (4) The thread insert is another option. The only one I am familiar with is the 'Helicoil' (Fig 5.), though I do know there is at least one other on the market under another name. The thread insert is like a closed coil or tension spring with the same number of coils per inch as TPI of the original thread. Using a special tap and inserting tool, the hole in the host is tapped and the insert screwed in. This is the most attractive repair, particularly for spark plug holes in aluminium heads, but it doesn't come cheap and can only be done by someone with a somewhat expensive kit Most motor engineering workshops have them. (5) Probably the most drastic fix is to totally weld up the stripped hole and re-drill and tap a new original size thread. For technical reasons (heat distortion and crystallisation) this is invariably the last resort.
Loctite is a trade name for an anaerobic adhesive and or sealant, this means that when it is trapped between two surfaces (nearly always metal) where the air is excluded, it sets hard and locks the surfaces to each other by means of the minute irregularities on the surfaces. It does not require mixing and is usually thin and runny. It is not a substitute or repair for a stripped thread. DON'T THINK ABOUT IT! It is not to be confused with super-glue or araldite adhesives which actually bond to the surfaces and have high tensile (pull apart) strength. Loctite is not a glue and except for special grades has very low tensile strength. Its virtue is the fact that it has enormous shear (sliding) strength. There is a grade (262) stud lock) made to hold studs in the host so the nut unscrews and not the stud. There are a wide range of grades available, a 10 ml bottle of 601 or 262 will probably do most jobs we would need and will last the average Joe-Blow half a lifetime. If you want to know more about them there is a ready supply of pamphlets to be had at the bearing, tool, and nut and bolt shops. If you have to undo a bolt, stud, or nut that you know has been loctited and wont budge. You can kill the Loctite with 250 degrees C heat which is not enough to affect the strength or temper of metal parts. A good guide is to put a drop of engine oil on the obstinate item and heat it until it starts to smoke, this is about 300C, still a safe heat, but only just!
To install studs in the host simply reverse the two nut procedure. When dealing with studs it is a good idea to draw a line across the end with a marking pen. This enables you to tell what is, and what is not rotating. When installing studs and bolts keep in mind the bottom of the tapped hole in the host into which they are screw, if it is a casting. It is all too easy with a blind hole even with cast iron, to push the bottom out. Another trap is to screw down into hole with scunge or grease in it and by hydraulic force crack the host, particularly if there is not much metal around the hole and it is aluminium.
Crossed threads: 'Nobody,' as distinct from 'Somebody' invented crossed threads, they just happen! They are an unfortunate condition that afflicts larger diameter and fine threads and are caused by a combination of ignorance, brawn, impatience, and inexperience. I inherited a doozy on the filler neck of Tootle's radiator. There are a number of prospective candidates on our cars, e.g. the aluminium grease cap on the front hub, the torque tube to cross-member adjuster, the horn button, the oil pressure indicator, and the rear hub from axle extractor thread, just to name a few. What happens is one thread is screwed askew onto the other, this causes the 'V' grooves to cross, thus deforming the metal making a new path which results in the centerlines of the male and female being at an angle to each other. The key word is deforming. That is why threads in soft metals such as aluminium and brass are most prone. Shallow fine threads also qualify. Unfortunately once crossed the damage is permanent. Prevention is the only way to go. Don't let it happen in the first place! When screwing any threads together always remember the golden rule "WHAT UN-CREWS WITHOUT EFFORT SHOULD SCREW UP AGAIN WITHOUT EFFORT" If any resistance is felt STOP and start again. Take care and see that the cap or nut is square and the surfaces are parallel before starting to turn. A good habit, particularly with fuel and radiator caps, is to rest the cap in position on the male thread and carefully turn it backwards (anti-clockwise) until you feel a slight 'click'. This is when the starts of the threads have passed each other and should be in line. Then start to screw clockwise if there is any resistance STOP and have a look, if the cap is at all askew note which the way it is leaning, and start again only this time apply a pressure bias to keep the cap square. It may be necessary to try half a dozen times before it screws on easily. Better that than a crossed thread.
What now? If you thought by now with men on the moon, probes in outer space, and computers small enough to fit up a mosquito's proboscis, they would have their act together on a simple matter of nuts and bolts.............WRONG. Laudable, and not before time, Sum Wun decided to rid us of a ridiculous system based on the a medi-evil king's stride, the length of his boot, and the width of his thumb, and replace it with something to do with the circumference of the earth and an angle of latitude. This has added confusion to total disarray and lumbered we peasants with yet another bag of worms!..........The Metric System. Let the politicians take note, No country using the Metric System has ever won a war!!
They have done away with threads per inch. We now have to remember how far it is from the crest of one thread to the crest of the next to a decimal point of a milli-metre, (which by the way is 3.96 thousandths of an inch). Now we have 6 mm which is nearly 1/4" but not quite, 10 mm nearly 3/8", and a heap of other lookalikes. At least the bolts have their diameters on the head, no such luck with the nuts. All I can say is keep your heathen froggy bolts away from your vestal Christian females or you'll be over-run with seized and stripped threads.