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.

Valves

 Cylinders #1 and #2 with their valves and push-rod tubes in place.












Valves were made from 1/4" 304 stainless steel bolts 2 inches long. They were used in preference to bar as the bolts were work hardened from forging which made them easier to lap to size. Valves for cylinders #3 and #4 are shown below, they still need the retaining clip grooves machined.

The bolts were centred on the thread end first then the head was held in a three-jaw chuck and turned to 4.03mm. 0.03mm allows for lapping to parallel. The lap is shown with their silicon carbide inserts one course, one fine. The lap is held by hand as the valve spins in the lathe. Lapping took about five minutes per valve stem.Once the valves are lapped to size the head of the bolt can be turned into a valve head.

Here is an entire valve assembly from the #1 cylinder head. The valve guides are made from cast iron salvaged from a storm water grate (Trapezoidal in section!) and turned to round. The finished valve has a groove turned in the end to fit the c-clip. The c-clip for the keeper is made from a piece of 1mm silver solder (the long length of wire in the background). The keeper is stainless steel 10mm OD and the spring is commercial. To assemble, the spring and keeper are compressed on the valve stem and the clip pressed in the groove, it is held in place by a recess in the top of the keeper.







The inlet and exhaust ports have been counter-bored to accept the manifolds. The valves and rockers will eventually have a cover, this bolts to the plate shown on the left head.









Push-rod tubes have been installed. Valves in cylinders #1 and #2. The 'castings' at the bottom of the push-rod tubes have been turned from cast iron and serve as the bearings for the cam followers.

The threaded hole for each spark plug can be seen in between each cylinder's push rod tubes.