Originally Posted by MassiveMods

You can interchange the sleeves between the Pro engines in .21 class.

you cannot interchange them between Pro and Sport class engines.

you cannot change between Sport .25 and .28 and Pro .25 and .28 .. Crank case changes will hamper the gas flow between the RTR crank case and Pro crank case .. So it wont really give you the max change .
Hampering the gas flow can cause over heating problems.

Re Special RC, Please be aware that the Go Factory does not modify the engines , they are stock gen 4 and 5 engines. Note he does not have gen 5.5
which are being distributed to Authorised dealers only. He is stating that they are modified so he can sell into other peoples areas. but be aware that you do not get any warranty with the engine from Ebay . only an Authorised dealer can factory warrant an engine.

MM