The only thing limiting the 48 IDAs to being bored out 51mm or 52mm or 85mm for that matter is this little pinhole, the working end of the mixture screw. Since building the IDA is a machining process, the location of this hole can vary from carb to carb--the farther back in the body, the better--but 99 percent of all IDAs can be bored out to 51.5mm with no problem. This one will stay at 51mm due to the size of the engine, a mere 2500cc.