I have increased the compression on my OS FS-26S today.

Stock compression: 7.8:1, now +/- 13.5:1 (haven't measured after the mod, will do tomorrow).
Took the liner to the machinist and he skimmed off 1mm the top lip (2mm thick).

First problem: he skimmed just a tad too much (0.10mm) and the piston was that much above it (no problem with the head), so I got a fine sandpaper and passed it on the piston.
Second problem: the intake valve strikes the piston on intake stroke, so i had to increase the rocker clearance for the intake valve.
After final assembly, I put it on my test bench and tryed with the only fuel I have for now, Model Technics Formula Irvine 5% nitro (18% oil, with 4% being castor and 14% klotz). The engine was extremely hard to start, unlike before! I had to resort to my trusty electric starter.
Observations: won't idle as low as before (over 3200rpm or quits), HS needle valve opened a quarter turn. Max rpm with a Aviomodelli/Graupner 10x4: 10300rpm and making some bubbles from the head gasket (needs a new one) due to high pressure. Couldn't hear any knocking.

Disassembled it and every part looks perfect, no signs of undue contact between parts, everything well lubed up!
Will make some 0 nitro fuel and test it again as soon as I can. In the worst case, I just have to buy a new liner and reset the valves.

See ya