THIS IS THE NEW EFRA RULE CONCERNING .12 ENGINES
5.1 Appendix 7
The engine may have a total capacity of not more than 2.11 cc.
They shall be air-cooled, with front rotary valve, two-stroke induction.
The engines may have e maximum of four (4) ports in the liner, including the exhaust port, seen with the piston at lowest position.
No form of forced induction is allowed.
No form of variable port timing.
Only glow plug ignition is allowed.
One additional gap in the bottom (skirt) of the piston is allowed.
Additional slits or holes in the liner for cooling purposes are allowed as long as they do not reach the top of the piston at lowest position.
Standard and conical glow plugs allowed.
Where ever we say hole in this rule we mean a hole that is surrounded completely by material.

and i also know that because of the protest aver the Orion engines at the Worlds, IFMAR also ruled that slits or holes in the liner, below the piston at BDC are legal

AFM