BUILDING AN RMC TYPE ‘B’ ENGINE

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.

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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.

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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

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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.


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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.

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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.

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Finally over to the mill to drill for the fixings

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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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