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Single cylinder balancing

Firstly, any material that you can remove to reduce the reciprocating mass (piston/wristpin/ring/top half of rod) will make an improvement in engine performance, maybe not noticeable at lower rpm's but certainly at high rpm's plus it reduces stress on the engine parts. This will affect the balance of the engine but not greatly. However, it is a fact is that it is IMPOSSIBLE to balance a single-cylinder engine. The best that can be done is a compromise. The crankshaft counterweights will partly balance the reciprocating mass (piston/ring/wristpin/top half of rod). but this will introduce an out of balance at 90 degrees to the line of the cylinder but we have to put up with that. 
   Use a good quality scales, digital if possible. Take apart the crank assembly, weigh the bottom half of the rod and then the top half of the rod. Then weigh the piston/ring/clips/wristpin as one assembly. Add the weight of the top half of the rod to the weight of piston/ring/clips/wristpin. Now you need to take a percentage of this weight of between 50 to 60%. Lets say we use 55%. 
Add this 55% to the weight of the bottom of the rod and machine a piece of brass or steel of this weight to fit over the crankpin. Reassemble the crank with this weight in place and balance the crank on the ballrace surfaces on 2 knife edges. If the crank rotates because its heavier one side than the other( which it will do) you must grind metal off the heavy part of the crank-webs around the crankpin area until it does balance. At this point you will have balanced the engine to a balance factor of 55%. This balance factor will vary from engine to engine and is rpm dependent to a certain extent but at 15,000 rpm a factor of 55% is not too far off.

N.B.  Bear in mind that it is not possible to balance a model engine with a single sided crankshaft to anywhere near the optimum balance factor.  All that can be done to improve the situation is to remove as much material as possible from around the crankpin area and if possible add weight to the counterbalance by installing tungsten or copper inserts such as used sometimes by OS, CMB and Novarossi.