This is true but not quite for the reasons normally assumed. I'd never really believed that running a new ABC rich could cause any damage but with nothing to back up my suspicions I kept quiet about it. Until the day I managed to get a brand new ABC for a very good price (yes, ebay so I decided to run an experiment. I ran it stinking rich for 45 minutes then stripped it down. The rod was running in nicely but the piston looked like new. It wouldn't run in let alone be damaged which was fine because I wasn't trying to run it in, I just wanted to find out if there'd be any damage at all. I wrote up the results [link=http://www.rcuniverse.com/forum/m_2875125/mpage_1/key_/tm.htm]here[/link] in RCU with photos.

It took me some time to figure out why there was no damage to the piston until it occurred to me that all that extra fuel overcools the interior of the piston which naturally cools all the piston as well so it can't expand as much as the top of the liner does. This is similar to heating up the cylinder head to reduce the pinch, the head gets hot but the piston stays cool.