I don't know whether it is safe to use but the thing is that my replacement caliper seems to have a 54 mm piston diameter. And the one on my car I'm replacing is 51mm.

Strangely, if you google the honda part number for my caliper most are 54 mm and these are considerably cheaper than their 51 mm version.

Is it safe to match a 54 mm piston caliper (left front) with a 51 mm on the right front?

 

the dealer said a difference in braking pressure is not safe. The shop selling me the revised caliper (I think the owner is a fair person) says the difference is too insignificant to really matter.

Nevertheless, I'm going to play it safe and pay an extra $60 for a 51 mm diameter caliper.


edit: buying two 54mm calipers would also solve the problem, obviously)