Hi,I used to have a Cervelo R2.5 and ran with the michelin PRO2. The clearance at the rear of the bottom bracket / chain stay section was so fine that if the tyre was dirty it would scratch the insides of the area. I posted a question to Cervelo who said it was to do with the stack height/ construction of the michelin tyre and perhaps run a lower pressure or change tyre. Not really an answer but a comprimise not worth its wait. Then surprise surprise surprise Cervelo recalled my R2.5 and gave me a new R3. I was told a run of R2.5's were recalled due to poor gluing results on that BB section. Later it dawned on me that I may have pulled the chain stay into the lugged section when the glue had failed and effectively pulled it tight/ friction jammmed it. The R3 despite having identical geometry and larger BB has a much larger clearance with the same tyre, and nearly the same with a Michelin Pro 25 mm tyre

My suggestion is given the C40 is a lugged frame, is to check the gluing of the frame at the wishbone and if possible accurately measure it against another C40

Unlike Cervelos, I seriously doubt that the construction quality of the Colnago is the problem.