From the EFRA rule 5.1
http://news.efra.ws/fileadmin/docume...andb10App7.pdf
The engine may have a total capacity of not more than 2.11cc. They shall be air cooled, with a front rotary valve, 2-stroke induction. The engine may have a maximum of 4 ports in the liner, includeng the exhaust, seen with the piston at the lowest position, No form of forced induction is allowed, no form of variable port timing.
Only glow ignition is allowed. One additional gap in the bottom (skirt) of the piston is allowed, no additional holes in 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.
The bold part could be a rule once brought in by the manufacturers but never notice taken by the the drivers. So yes, you may dril as many holes in the sleeve but the may not be seen above the piston in the lowest position an in the piston there my not be holes (boost ports).
If we did stick to the old rules we should have disqualified a lot of racers and nu manufacturer can be set responseable for local ruling.
We did take out this rule, in this way if not listed in the Dutch rulebook the EFRA rules must provide the answer (as we have set the tree of rules).