Hi Colin, sounds like you need a Cotter pin for that much torque? Grub screws are only for very low torque applications. On some models/applications, I replace them with "plumbing" olive or double cone compression fittings where I know there is some torque to be transmitted. e.g. a drive pulley, flywheel, etc.
Maybe not everyone appreciates how much torque oscillation occurs on reciprocation engines, so things like flywheels have to take the torque one way from the power stroke, then deliver it back to the shaft on the "between power" part of the cycle. So the compression fitting will see maximum torque applied and released every cycle. This oscillation will often exceed the durability of a grub-screw.... unless made into a shear pin of course, by some undercut.
Additionally, grub screws directly acting on a shaft have a point contact. Use of a brass insert or other makes a full surface contact which is better able to transmit torque without exceeding the coefficient of friction, due to the significantly larger area of contact pressure.
Output Pulleys, gears, etc. see the full power max. torque to "no power/torque" oscillations from the crank.
Cotter pins, shear pins, keys, splines, tapers, compression fittings, etc., are all designed to withstand more torque by higher friction of the connection, or metal in shear. Especially oscillating torque.
But a grub screw onto a flat on the shaft will probably operate an eccentric on many models as there is low friction, low acceleration (sinusoidal, not peaky) and low mass generating the oscillating torque, not the power of the engine. But you need to know exactly where to put the flat!
Hope this helps?