WB asked about the increased surface area of the piston on the cylinder wall, yes piston temps would be lower with the
increased surface area contact in the BB. The touching points do not remain the same in a BB vs the OEM bore since the contact for the increased piston surface area and larger dome would dissipate more energy to the cylinder.
Metal on metal will generate heat yes, but not to the extent you are thinking and most often tolerances in the BBs are as low as OEM or slightly higher, so friction loss would be negligable so long as there is sufficient lubrication and the piston design is transferring heat efficiently to the cylinder wall and maintaining the correct tolerance. In addition to a big bore with increased compression, race fuel would reduce CHT's in the combustion chamber, so heat is less of an issue with adding a BB as most of these fuels burn cooler than regular pump fuel. Not adjusting the timing correctly to compensate for the longer burn time of the fuel could result in more fuel burning in the pipe and generating heat this way as well.
Most often times the added heat in a BB is due to the tuner looking to go with leaner jetting (burndown) or running pipes etc, that the chassis does not have sufficient cooling for. Lots of the conservative BB setups last a long time but do not make the big power like the "on the edge" kits. A perfect example is my triple - My big triple does not like heat! Those three pipes spewing heat under the hood, with the regular hood on - not the mesh one, anything under 30 mph and it starts to load up and bog after some slow running, get it moving and get some cool air into it and cooled down, its a whole different animal
. Again, not what the chassis was designed for so it suffers a little bit at the slower speeds due to the added heat from the pipes.
There's a lot more engineering to it than just the moving parts of the BB kit itself.
NSC