Atom Minor Mk III
by Jan Huning
8. Cylinder Head
The head was machined from a piece of aluminium bar (6082T6, or HE30 in old UK specs) rather than the casting supplied. The combustion chamber, spark plug thread and bottom two fins were machined at one setting. I machined the head for a 1/4" x 32 tpi spark plug rather than the 3/8" x 24 tpi shown on the drawing.
The chucking piece left from the cylinder machining was modified for holding the part machined head. A spigot replicating that on the cylinder was machined, and a M3 hole drilled and tapped. A short length of 1/4" x 32 threaded bar (actually part of an old glowplug) was drilled through 3mm, screwed into the part machined head, which then allowed the head to be clamped to the chucking piece using a 3mm cap screw. All the rest of the external turning could then be completed at this setting.
If I had thought ahead, I would have first part machined the head. After machining and parting off the cylinder, the chucking piece could have been left in place and modified to hold the head without having to reset it in the chuck.