OK Cub Mods
Modifications performed with a complete rework of a Cub are:
Better Plug/Chamber Design – A new head for the Nelson plug is installed. In developing the head it was found that installing the head with a small bead of red silicone sealant often provided a performance gain. This is due to elimination of some of the dead space from the threads and the thread relief at the top of the head by the gasket. Adding this small bead of sealant to a stock head can also improve performance although not as much as a Nelson head.
Do Not Use This Prop
OK Cubs are reworked essentially per the November 2000 article in the NFFS Digest plus the addition of a Nelson Head. Some very significant rpm gains have been achieved but flier beware that Cubs like to blow up if pushed too hard. Running much over 18K may constitute suicide. I would strongly recommend not more than 40% nitro fuel and use of a 6-2 or 6-3 APC prop as required to hold rpm within reason. Do not use a red Cub prop as they are not enough to load the engine, tend to flatten out at high rpm, and can come apart – all bad.
Lower Internal Pressure Loss results in the transfer of a larger charge into the combustion chamber.
-
•The 3 transfer ports in the cylinder are enlarged to the maximum allowed by the cylinder support ports. The
width of the 3 support posts varies by a large amount from engine to engine and the wider the posts the wider the
transfer ports can be made. This equals better performance.
• Crankshaft intake window is cleaned up and any sharp edges radiused.
• If the case has only one bypass cutout in the cylinder to case joint a second is added.
• Venturi is opened up and the diameter of the spray bar reduced.
Better Balance is achieved by reducing the weight of the reciprocating components.
• Crankshaft web is partially ground away.
• Wrist pin is drilled out and Teflon pads fitted to the ends of the pin.
-
•Lower portion of the piston is machined off which also provides some sub-piston induction.
Other Modifications
-
•A cylinder clamp ring is provided to eliminate the need to use pliers on the cylinder for maintenance. It is
recommended that the owner use this clamp ring if it is necessary to remove the head without disturbing the
cylinder/case joint. It is also recommended that the head and plug be double wrenched when changing a plug.
This eliminates any unnecessary torque on the cylinder/case joint. It has been found that excessive torque on this
joint can result in binding of the piston around BDC on some Cubs resulting in the loss of considerable
performance. This binding is minimized by taper lapping the cylinder below the ports.
-
•Brass rub washers are installed between the case and prop drive washer to eliminate galling of the case and the
prop drive washer when using an electric starter.
-
•A new spinner and socket head prop screw are provided which are designed to automatically center an APC Prop
without the need for a separate prop bushing.
