Spark plugs

An old full size spark plug. An assembled 1/5 scale spark plug and the individual components below.
The spark plugs were made using 3.12mm OD 1.6 ID ceramic tube. A 14mm sleeve of filled teflon is pressed over this tube, it is drilled 2.8mm.This sleeve is turned on a 3.12 diameter mandrel to 4mm for 9.5mm, the remainder is turned 5mm diameter.
The body of the plug is made from 20mm long hex bar 7mm across the flats. It is bored 4mm for 19mm by drilling and using a 4mm slot drill to achieve a flat bottom. The body is next drilled 4.8mm and threaded 7/32 inches by 40 TPI for 6mm.The outside was threaded 1/4 inch by 28 TPI for 12.5mm. The end of the body was milled 3mm from the end to form the earth for the spark. The hexagon is turned to 7mm diameter leaving 2.5mm.
The center electrode is  low percentage silver solder. It is turned from 1.8mm to 1.5mm to fit the bore of the ceramic tube. This electrode is threaded 10BA and has a 1mm by 1.8mm head. The teflon sleeve  is pressed  onto the tube and the electrode held in place by a 10BA nut, this assembly is then screwed into the body. The 5mm section of the teflon sleeve deforms to match the 40 TPI thread at the same time gripping the ceramic tube.To adjust the spark gap it is necessary to remove the assembly and slide the ceramic in the teflon sleeve. A gland nut made from hexagon bar 5.5mm across the flats threaded 7/32 inches by 40 TPI is screwed into the body  to lock the assembly.