You can damage the piston and cylinder bore!!!!!!! You have 2 different metals in close contact with each other i.e. steel inner sleeve and the aluminum piston! the 2 metals expand at different rates during warm up! so in other words if you hammer the motor right after start up, the aluminum piston will expand much quicker than the steel sleeve causing the pistion to scuff and gouge the cylinder wall! (not a good thing)
Proper warm up will cause the cylinder and steel inner sleeve to expand together so when its time for hammer time, the 2 different metals will be at the right clearances at normal operating temperature!