This helps mileage, also makes engine more responsive.
Air enters the engine through the crank and is diverted toward the ports through the holes.
The crank is first drilled out to lighten the crank, then filled with a special silicone.
Novarossi engines that use the "special" bearing have a thin inner race. They are expensive to replace, I mill the block to accept a std. Nova size bearing.
There are many engine modifiers that simply just make cuts with a Dremel to the sleeve and crank.
I have found the biggest difference you can make to a Nitro engine is optimizing the crank timing and port timing. Crank timing is the opening and closing of the crank at the block under the carb.
Team Driver Kendall Welch