This is the most critical sequence of the engine construction.
The stage begins with a fully machined and case hardened steel cylinder, a machined and case hardened steel piston, a machined and case hardened contra-piston, a case hardened and ground alloy steel gudgeon pin and a fully machined aluminium connecting rod.
The chosen materials combination is certainly not the easiest to machine to this stage, nor to finish and fit, but the final properties compensate for this difficulty. A hardened steel cylinder has an extremely long operating life. A steel piston can be thinner and have less mass than an equivalent one made from cast iron. A hardened steel piston will expand in operation, at much the same rate as a steel cylinder. It can therefore be fitted without interference, or 'pinch' at top dead centre of the engine stroke. Then there is always a lubrication film between cylinder and piston, there is less tension on the connecting rod when the engine is new, there is little break-in of the engine needed and there is minimal wear of the piston and cylinder bore. My prototype engines show almost no wear after countless hours of operation. Of course, once hardened, these two components must be machined by more costly processes and their accuracy must be of a far higher standard to seal against compression.
The S12L14 steel cylinder is carbonitrided and hardened to 62 RC. This process causes the dimensions to 'grow' and distort slightly. It must be resized by an abrasive grinding process. I firstly grind the outside diameter to fit the crankcase bore and at the same operation, grind the locating flange square to this diameter. I use an Overbeck Zetto 20 precision grinding machine that I bought used and then reconditioned. I was familiar with this highly respected production machine, as we used a slightly different model at the Taipan factory. The cylinder is held while grinding, by expanding an O ring inside the cylinder to hold it on a mandrel, firmly against the locating flange. This clamps without distortion and dampens any possibility of flutter while grinding to size. Next the bore is internally ground to size in another operation on the same machine. The grinding machine is now set for semi-automatic operation and reciprocates and infeeds mechanically while the 6mm diameter abrasive wheel sizes the bore at 65,000 RPM. The machine accelerates and reverses instantly at each stroke end, to minimise overcut; clever! The cylinder is nested on the previously ground outer diameter and clamped only on the flange so as to avoid distortion. The flange is at the top-dead-centre height to give stability at this point of maximum compression. The cylinder will later be retained in the engine by clamping only on this flange. This prevents any distortion that would occur if it was threaded or clamped under compression. I grind the bore to a slight taper and measure the size using a hard-chromed plug gauge from the lower end. I next diamond hone the bore on a Sunnen honing machine. I bought this machine well used, reconditioned it and then bought all new honing mandrels. It appeals that it bears a plaque stating that it once was Machine 71621, Property of North American Aviation, Los Angeles, California. Maybe it was used to make larger aeroplane engines? I hold the cylinder freely in a lightweight plastics fixture that I reciprocate manually. I hone the bore, above the ports, to almost parallel, checking the size with a plug gauge. At the Taipan factory we finished many tens of thousands of cylinders of different materials, tough, hard and hard-chromed; ground and honed by both automatic, mechanical and manual processes. The best were of hardened steel, internally ground and then finished honed manually by a highly skilled operator, -- my Father.The S12L14 steel piston has been carbonitrided and hardened and then tempered to 50 RC. Two sliding surfaces will not gall if one differs more than 7 points of hardness from the other. I firstly diamond hone the 1.6 mm diameter gudgeon pin bore. This bore was honed before heat treatment so now needs only a fine resizing for clearance on the gudgeon pin. I then grind the outer piston diameter, parallel and to size, on the Overbeck, clamping the piston on a mandrel by pulling back from the gudgeon hole and expanding an internal O ring at the skirt, to prevent any flutter. The contra-piston is ground to size on the Overbeck, using a similar set-up to the piston. The ground diameter has a slight taper, larger at the top. When coupled with a high length/diameter and thin wall, this gives a slight 'spring' interference to the fit. It too, is hardened to 50 RC and will adjust smoothly without sticking or backing-off. The gudgeon pin is a standard, hardened alloy steel dowel pin.The 2 mm diameter connecting rod big-end is honed to a closely measured clearance size on the crankpin. A delicate operation on a part this small! The honed cross-hatch finish allows a good oil-film between the 'rod and crankpin.I join the piston, gudgeon pin and 'rod. The pin is a tight fit in the small end of the 'rod and oscillates within the piston. I assemble the three so as to eliminate any possibility of distortion, by using a press fixture that applies force only at the end of the pin and at the position where the 'rod contacts the lower edge of the piston hole., The precise arbor press I use was once used in the Taipan factory to insert ball races in crankcases. Once assembled, there is little sideways float of the 'rod within the piston. The gudgeon pin ends cannot contact the cylinder wall. This restriction also prevents the connecting rod big-end from floating off the crankpin and contacting the backplate surface while running.All parts are now ultra-sonically cleaned in a solvent bath. The cylinders, pistons and contra-pistons are remeasured and graded, lubricated and assembled in matched units. The piston operates within a trumpet shaped bore, very slightly tapered below the ports and almost parallel at top-dead-centre. It has little clearance below the ports and an infinitesimal amount above. This gives good combustion and crankcase seal, but requires that all components be accurately aligned. The contra-piston is within a parallel section of the bore. All this takes much longer than it appears to, when only written down in this abbreviated description.
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Diamond Honing Cylinder to Gauge
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