Italian to English Tech/Engineering Engineering (general) Heating installations

Does anyone have any ideas for how the distance "interasse" indicated by the letter "I" in the diagram you can see here: http://i579.photobucket.com/albums/ss234/lynkos/Varie/intera... could be defined. The context is the "listino" for a range of home heating radiators. It appears only in the heading of a list of specifications and in the following description: "E' disponibile una serie di misure progettate per consentire una rapida sostituzione dei vecchi radiatori. Gli **interassi** combaciano con quelli dei vecchi modelli per poter effettuare la sostituzione senza modificare le tubature in ingresso e in uscita". Thanks for your help.