Hi.
Here is a problem that I have seen in many Retro Discovery engines. Usually the cylinders are of very good quality but very often the piston is not round, it has several "angles".
Because the correct fit is determined by how deep up the piston goes in the sleeve with a known force, the engine with this problem feels very good when new. Problems come usually after about 200 flights, first I notice difficulties in start, and very soon after that a loss of power and then the engine jams completely.
But in the beginning, engines with this problem give a wonderfull, strong 4-stroke run. I assume that's because of detonation caused by hotter-than-normal piston. Engines with this problem seem to run best with all-castor oil fuel, many synthetic oils cause unbearable detonation.
I think that the reason of this problem is bad quality control, it's never an issue with engines tested by the maker or with the engines that come with ready-to-go models. I guess that the deformation originates from some tool vibration from raw-machining or grinding. There is simply not enough material left to lap the piston round and smooth.
But to the original subject: This piston cannot be rectified, I would just end up with a loose fit. For this engine, I fitted a new piston, starting from an oversized one.
There is also another thing visible in the picture, I find it in well fitted engines too, and it should be taken care of after running-in. In both ends of piston "wall", there are marks of high friction/surface temperature. In a good piston the primary sealing area (near the top edge of piston) should have a constant width, but as you can see, the sealing area is wider near the piston wall ends. When the engine runs, the wall is hotter than the rest of the piston, and it expands and compresses against cylinder walls. I think it's very important to correct this deformation after the initial running-in, about 30 flights. It's easily done with #1500 sandpaper and some oil. I also relief the ends of piston wall by about 1/100mm.
Sometimes similar "shiny spots" can be found also elsewhere in the piston surface, usually near the wristpin ends. They should also be taken care of.
Also, in the RD.60 engines, the cylinder/case fit is sometimes too tight. I allways make sure that the sleeve has a smooth sliding fit to case. I rectify it by lapping the outside surface of the sleeve. If it binds when pushed in the case, it won't be round and you end up having high friction/temperature spots in the piston/sleeve interface. not good.
All these things cause bad and uneven thermal conduction and a normal user only may notice the detonation. And usually he tries to fix it by lowering the compression which may help a little, but actually it just messes up the thermodynamic balance of the engine.
There is also several small things to do to improve the RD.60 bottom end (with bearings etc.) if someone is interested.
But please, don't do anything unless you really know what you are doing. Most engines are just fine.
Lauri