Cylinder & Piston Rework
Cylinder Port Retiming – Some nostalgia engines and conversions of R/C engines for high rpm use can benefit from retiming of the cylinder ports. For example the OS III series of engines has very conservative exhaust port timing in the range of 125 to 135 degrees. Most of the engines with this timing were designed for use with large diameter relatively high pitch props in control line events. Other engines, especially those designed for racing control line events, have exhaust durations closer to 140 to 145 degrees which is more in-line with operation at higher rpm in free flight. The easiest way to correct for conservative timing is to shim the cylinder or liner to obtain the desired exhaust duration. This also alters the transfer port timing generally putting it in a more desirable range of the mid 120’s. If the transfer port(s) timing falls too far from this goal it may be necessary to machine the port(s) to alter the duration.
Shims can be placed under the lip of the liner or under the cylinder for engines such as the Green Head Torpedo and the OS Max. Shims under the lip of the liner are turned from aluminum bar stock while those under the cylinder are made from brass shim stock. Whenever an OS Max or Green Head Torpedo is disassembled the soft, probably asbestos gasket between the case and cylinder, is replaced with brass shims and sealed with Red RTV Silicone sealant. This provides a consistent stack height ensuring repeatable port timing and deck height/compression ratio.
Cylinder Taper – Cylinders should be slightly tapered for best performance. The taper minimizes the drag of the sliding piston with most of the taper occurring below the ports. Taper above the ports also allows some growth of the cylinder to accommodate the thermal growth of the bore as the engine heats-up. Ideally the bore should be a few tenth’s of a thousandths more at the ports than at TDC. From the ports to the bottom of the cylinder the taper should increase one or two thousandths. In some engines the tightening of the cylinder or head bolts can distort the lower portion of the cylinder placing additional drag on the piston. This condition, most often be noticed on the Green Head Torpedo and a few OS III engines, can be alleviated to some degree by adding some cylinder taper below the exhaust port and careful control of the head bolt torque.
Piston Shape - Pistons should be slightly barrel shaped with the largest diameter slightly below the top lip of the piston and additional taper on the lower portion of the piston. Adding taper to the piston or cylinder can easily result in loss of the compression seal unless extreme care is taken. Piston and cylinder diameters are measured with external and internal micrometers capable of resolution in the range of 0.0001 inch.