HOW TO MAKE

MINIATURE SPARK PLUGS

Part one

By Graham Meek

I think it can be safely said that many have tried different materials over the years to achieve a durable small spark plug. Many years ago L C Mason wrote in Model Engineer magazine that he had made his own spark plugs. I can recall that the process involved ‘firing’ the insulator in a furnace and that he used an old thread chaser to form the ridges in the embryo insulator. I cannot, however, remember how he retained the insulator in the plug body. LCM also stated in his article he had successfully made plugs as small as 4BA, and one of these is shown in his book Model 4-Stroke Petrol Engines - Designing - Building - Running. He also mentions that Edgar T Westbury experimented with Mica, and was successful, but I have not seen this in publication. There is also mention in the same book that others had successfully tried Steatite or Soap Stone. A method of plug manufacture was published in an article Miniature Spark Plugs by Sam Rhodes in Model Engineer which involved the use of a piece of glass from inside a spent domestic light bulb which was melted to seal in the electrode in the Spark plug body and the external insulator which was a Fish Spine bead placed over the electrode while the glass was still molten and pressed down to the steel body of the plug to form the external insulator. Fish Spine beads are ceramic insulators used to insulate wiring on heating elements; I have included a drawing of the principle dimensions of the fish spine used in my plug manufacture. Others over the years have used PTFE rod to form the insulator complete and, either making the plug body such that the insulator is held in by a screwed gland as in the early days of real plug manufacture, or by swaging over the body. While PTFE will withstand the normal temperature of the combustion chamber, it can give off some rather nasty emissions should this be exceeded. For this reason I elected not to use it within the combustion chamber. For those who do not consider the use of PTFE as a complete insulator to be a risk and think I am being over cautious I have included a drawing of my plug with this fitment. Some years ago now while working for a small Electronics Company in Monmouthshire, I came across a machinable ceramic called Macor obtainable from RS (usual disclaimer). While this material is quite expensive it could be used for the purpose of plug manufacture, being easily machined with HSS tools. It was while looking for Macor in the RS catalogue recently that I again came across the Fish Spines as they are on the same catalogue page. The chance occurrence of this reminded me of an old article by Sam Rhodes in Model Engineer. Looking at the Fish Spine with its short taper reminded me of the insulator nose inside a full size spark plug. If I used one of these in the body of my plug then I remove the danger of the PTFE breaking down. Then PTFE could safely be used for the remaining insulator and because of its soft nature could be relied upon to seal the plug and not allow any combustion gases to escape. A search of the Internet produced the external dimensions of the commercial ¼” spark plug and the fact that a 10-24 UNC version was now available. Armed with this information I started drawing up the plug you see in the first photograph, the drawings for which are also included. A twin earth electrode was chosen on the plug body to ease the manufacture of the plug body and as an added bonus I would probably get an extend plug life; again inspiration was the chance finding of a pair of twin electrode plugs in my box of oddments. I have since designed a more conventional plug, what I call the long reach, but with a single earth electrode. I have also adapted the twin electrode body to single electrode, as shown. These long reach plugs were made to fit the Seagull engine. If it is intended to use standard or twin electrode plugs then you will need to reduce the length of thread in my modified Seagull heads should you decide to build one. While I have not reproduced the ridges in the PTFE insulator as in full size practice there is no reason why they cannot be machined in. I doubt that the presence of these ridges on the smaller plug would have quite the same effect as in the full size version of increasing the surface distance between the body of the plug and the electrode at the cap end, thereby reducing the risk of tracking across the insulator to ground. It has not been found to be necessary in practice after many hours of running. CLICK ON DRAWING ABOVE TO DOWNLOAD A good place to start the plug manufacture is with the plug body; I have made my plug bodies out of leaded bright mild steel (BMS). I have been toying with using Stainless Steel but as yet I have not been able to try this. Produce an embryo-profiled body with all the external features finished turned with the exception of the ¼”-32 UNEF thread. Gripping on this unthreaded portion in a chuck if the run-out can be assured to be within about 0.03mm, (or 0.001” in old money). Better still a collet can be used to advantage, provided this is of the ‘draw-in’ type as the diameter is a little on the short side to use one of the ER type collets with any degree of safety. Failing that a split bush held in the chuck is a very good stand-by. Always provided attention is paid to the marking of the split bush with reference to say number 1 jaw, so that the bush is replaced in the chuck in exactly the same position as it was bored or reamed. Therefore, holding on this diameter all the internal diameters are finished to size, with particular attention being paid to establishing the length dimensions. Later on you will read how to assemble the plug components, there are a couple of faces that can be machined to assist assembly, however if you are wildly out with the plug body then you will only be making things more difficult for yourself later on. When making the single electrode plug the 3.8mm diameter hole will need flat bottoming. This can be done with a jobbers drill or a slot drill that has been ground to 3.8mm diameter in both cases a 0.25mm radius needs stoning on the corner. If you are making several plugs then the 7mm diameter needs to be pretty close throughout the ‘Batch’, as this bore is used as a tooling location. The last operation in the lathe at this set-up is to turn the 7.5mm Diameter by 1.7mm long portion that is swaged over to hold the insulator. The sides of the portion that will become the hexagon require chamfering to the intersection point with the respective diameters; the correct angle according to my Technical Drawing teacher and my Instructors during my apprenticeship is 30°, the flank of a screwcutting tool is ideal for this operation. I have seen many fine pieces of work ruined by the use of 45° chamfers on nuts and fittings. Let’s face it if you have to put a chamfer on there it is just as easy to make it the correct one. Again holding on the ¼” diameter in your means of indexing, mill on the 8mm A/F hexagon. Now turn up a short length of BMS 6.98mm diameter and not more than 5mm long that is an easy slide fit in the 7mm diameter hole in the plug body. Insert this into the plug body; this is to stop you crushing your precision-engineered component. Place them both in the milling machine vice with the electrode end uppermost, resting on a parallel to keep things square, grip the plug across the 8mm A/F. It is now possible to mill the electrode to size using a 4mm end mill and light cuts. The above milling procedure relates to twin electrode plugs. Single electrode plugs are slightly more involved. Using the 7mm diameter as a holding point a short length of BMS is turned a snug fit for this hole and the body held with Loctite. A photograph shows a double ended spigot mandrel resting on two embryo plug bodies. Please note the ‘felt tip line’ on the centre of the mandrel. This is to maintain concentricity when returning to the lathe for screw cutting. Again in the indexing attachment, gripping on the BMS spigot, mill on the 8mm A/F hexagon and then mill on the electrode, again using a 4mm end mill for this last operation. It is best to mill one side of the electrode first, taking light cuts of the order of 0.2mm using the side of the 4mm cutter. Following the same procedure mill the end of the electrode followed by the other side of the electrode (see photos below). Do not be tempted to try and mill this in one cut using a larger cutter, especially a slot drill, it is fraught with danger. There is a sequence of photographs to show this operation. Obviously, to speed the job along several BMS spigots will be needed if you are making more than one plug, the Loctite can be broken by heating after the thread has been finished and you will get a nicely ‘blued’ plug body in the process. There is no reason why twin electrode plugs cannot be produced in this way; it is just that the first method saves some work. All that remains to be done now is screwcut the thread. Many people at this stage reach for a button die, however as this is ¼-32 UNEF the cost of this die will probably be horrendous. Besides after lavishing all that precision work into this part I feel it deserves to be screwcut on a short mandrel, using an M2 capscrew end-on to hold the twin electrode body, or gripping the BMS spigot for single electrode bodies, this gives a thread that is perfectly square to the sealing face and great job satisfaction into the bargain. The choice here lies with the reader. SEE PART TWO HEREMiniature_spark_plugs_2.htmlMiniature_spark_plugs_files/Miniature%20Spark%20Plugs-Model-A3.pdfMiniature_spark_plugs_files/Miniature%20Spark%20Plugs-Model-A3_1.pdfshapeimage_2_link_0