Spark plug

The spark plug is finally finished. The requirement for a custom spark plug is due to the design of the combustion chamber. In order to get the maximum port area the valves use the maximum area available. The off the shelf spark plugs are too big to allow such big valves and so a custom spark plug was required.

The body of the spark plug is made from stainless steel, which is turned, milled and threaded, and finally swaged to capture the insulation components and electrode. The thread is M5, and extra long thread section reduces the chances of wearing out the thread on the engine head. In the V8, I would probably opt to have a wire insert to reduce even further this risk.

The insulation materials are machined from Macor, which is a machinable ceramic. It machines quite nicely, but because of its brittleness one has to be extremely careful. Wall thickness's in some cases is <0,5mm. On the thread section of the spark plug, the diameter of the insulation is Ø2mm and electrode Ø1mm. There are two ceramic parts in this spark plug, with a flange that gets locked by the swaging process. To achieve a leak prof part, adhesive is added during assembly, which seals any void produced by machining tolerances.





A dedicated swaging tool had to be made to press the collar onto the ceramic insulation. There is not much pressure anyway, as the adhesive also helps keeping the bits together and did not want to crack the delicate ceramic material.

Another tool required is the special 4,5 mm spanner socket that is used to fit the spark plug onto the engine head. This is made form a M6 x 120 cap head bolt which cut down in length. The spanner socket for the spark is broached onto the stud section with a custom made broaching tool. The tool is quenched to achieve the required hardness.

All in all, only a few details are left to make: installation of ingnition system (CDI), test bench, carburator assembly.

The engine is looking good so far!