Part four by Jason Ballamy

The drawings include details for a home made spark plug to suit the engine so I had a go at making one. Well, two actually as the extra can be used in the Type D when I get round to making that.

The body started out being turned on the end of some EN1A bar and then screwcut 1/4 x 32 UNEF as I only have a tap that size. The large picture makes it look rougher than it is.


A suitable female thread was tapped into the end of a bit of scrap so the body could be held to finish the top end.


The insulator is a length of 3mm glass tube which cut very easily with a diamond coated disc in the Dremel, I used the slowest speed to avoid heating the glass and then just twisted the glass in my fingers against the disc to chamfer off the sharp edges


The central electrode is a piece of 0.5mm tig welding electrode and the final part is a small brass cap with a groove to accept the wire clip. The electrode is trimmed to length after assembly.

The plugs are assembled using Loctite 380 ‘Black Max’ adhesive. I could not get the plugs to work at first and it turned out that the adhesive had insulated the electrode from the brass cap, a quick spot of electrical solder on the end soon cured that problem and the engine ran better than with the standard length Rimfire plug that I had been using for initial testing.

Unlike a conventional plug the spark does not travel from the central electrode to the plug body but to a long screw that enters from the opposite side of the cylinder. The plug gap is adjusted by turning this screw and securing with a locknut.

I did not take many photos of the adjustable timing bracket. Above is the lever having had a 2BA thread put on the end; it was slid out of the chuck to reduce most of its length to 3/16" leaving a 1/4" collar for the contact.


The ‘hemi’ cylinder head was turned from some cast iron and, as I didn't have the large diameter ball nose cutter suggested, I first drilled to depth and then hand cut the shape.


It was then held the other face out in the soft jaws and the outer curve also shaped by hand turning with a flat ended tool.


Finally over to the mill to drill for the fixings


The fairly long piston can be turned from the chucking spigot that was cut off of the base casting but I chose to use some 25mm CI bar. After turning and cutting the groove for the quad ring it was held in a collet block to have assorted holes and notches cut which provide the porting for the Loyal cycle.

Part three here. Part five here.

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