With stock or the sonnax or any 4th servo when stacked you should have between none and about 1/16th inch of gap between the cover and center peice but the actual servo being in contact or within 1/16th of the pin . With the stck stock 4th servo ussually just flipping the servo will do this, However if needed you can add a washer of whatever thickness needed to get this and type will do they are 3/8 in size and various thickness can be had at the hardware store. Same is true of all other servos