With Pinched ABC /ABN /AAC engines, I agree with Ed Smith and Fuelman.
About 6 years ago, Fuelman came down to my area and the discussion of ABC break in came up and we thought he was off his rocker. He explaned why and the reasoning seemed logical so I started doing it that way. When we used to get about 3 gallons from a piston and sleeve set on my 3 sons rc cars, we were happy. Doing it his way and throwing the instruction manual back in the box, we're getting in excess of 10 gallons of fuel through a car engine before needing a new piston and sleeve. My kids race cars, I'm just a simple airplane and heli guy, and my ABC type engines which I have several of from the 25 size on up to an ASP 108 ABC, all have been broken in via Fuelman's advise (which I think is George Aldrich and Dave Geirke's advise as well) and have had no longevity problems at all and I fly very very often.
I do it Downunder's way with ringed engines and a couple of Fox lapped iron engines I have, and after years and lots of gallons, all my engines are like fine tuned, well oiled little machines.

Dr Nitro