Thanks again fir your input. Just for the sake of reference, i actually managed to get 700x32c Conti Ultrasport IIIs into 1995 bianchi alfana for the rear wheel. For the front, the vertical clearance on the stock unicrown fork was only 2 mm with 32s, which worked but was too little for me.

But this was easily remedied with a nicer looking fork with a real crown, which i used to replace the stock unicrown. I thought about filing the stock fork, but that would have meant taking material off the front of the crown, where the brake force is downwards. So mechanically not ideal .

I think it is really variable, no rule of thumb. But one thing that might be important is that 700c rims had a hook form earlier than 27s, i think, so the tires could be thinner and higher pressure than their 27 inch contemporaries. So the 700c Italian bianchi frames might have tighter clearances than the 27 inch japanese made ones.

The other thing is that i can live with 3-4mm chain stay clearance, which is what i have with the 32s on the alfana. Obviously not up to safety norms, but i can live with it.

One thing i will he interested to see is if i were to pop in a spare set of 700s into the Special, could i get even more than 32? This will have to do with the chainstay crimping, for sure. It could be that going down to 700 actually reduces possible tire width because it moves the tire from the optimal position in the crimp to a round part of the stay.

Let's see. Exciting as these things go