Atom Minor Mk III
by Jan Huning
10. The Piston
The piston was machined from a piece of 1" diameter cast iron bar, long enough for the piston plus a chucking allowance so that all the turning could be completed in one setup. The OD was roughed to about 10 thou oversize and then the inside bored to drawing dimensions.
The undercut between the crown and the gudgeon pin boss was machined using a tool ground up from a broken 1/2" slot-drill. This provides a stiff tool without requiring too much grinding. Finally the outer diameter was turned to about 0.002" bigger than the finished cylinder bore and the piston parted off. It was turned around and held lightly in the 3-jaw to finish the top end to length, reduce the diameter for the length of the baffle and radius the corner.
The piston was held in a small milling vice, clamped lengthwise, for drilling and reaming the gudgeon pin hole. The vice is set parallel to the milling machine table, and an edge finder used against the side of the vice and against the fixed jaw to establish the position relative to the spindle. A depth micrometer is used to measure from the side of the vice to the side of the piston. The table is then moved across by this dimension plus half the piston diameter to position the axis of the piston under the spindle, and moved lengthwise to position the hole at the correct height relative to the bottom of the skirt. This procedure is quite quick and ensures that the gudgeon pin is in the correct position without needing to do any marking out.
The baffle was machined with the piston held in a small 3-jaw chuck clamped to the rotary table, using an end mill with the corners of the cutting edges radiused by hand. A piece of silver steel was used to set the gudgeon pin hole parallel with the table movement.
The piston was lapped to fit the cylinder using a split aluminium lap and 600 grit diamond lapping paste (rolled into the surface of the lap using a piece of tool steel). All the corners of the lap are rounded off to avoid damage to digits if (when) the lap grabs the piston. For the same reason, make sure the clamp screw is short enough so that it does not protrude from the outer surface of the lap. The piston is held on a simple fixture for lapping. The dummy gudgeon pin is made from aluminium so that any lapping paste that may find its way between pin and piston beds into the aluminium pin rather than the piston.
I initially lapped the piston to give the same fit as I would use for a diesel, but this turned out to be a little tight for running on petrol, and the engine slowed considerably as it heated up. A whisker more was removed, after which it maintained a steady speed as it warmed up. Subsequent to this, John Maddaford, who has made many Atom Minors and numerous other spark ignition engines, told me that the ideal fit for spark ignition engines is when, with the piston and cylinder clean and dry, the piston just drops down the bore from top to bottom under its own weight.