That sounds like an isolated case, and I wonder how long ago it was and if the FAA will have softened its stance by now due to the fact that it's a well established path to the Super Eagle 470-50 by PPonk STC, which is what all of these cores become if following the STC instructions.
I hear what you're saying, though... The O470-U has a factory TBO of 2,000 (now 2,200 hours if newer than serial number 1006000). Some of the TSIO520's have TBO times of 1,400. That's a big difference for a commercial operator.
Interestingly, "CMI does not provide a TBO for engines that have been assembled with parts not provided by CMI."
List of TBO times and CMI's stance on TBO:
http://www.continentalmotors.aero/up...20SIL98-9C.pdf
Charlie Melot of Zephyr Aircraft Engines in FL, who really is an excellent engine guy, often said that the lower compression 470 engines were in his long experience more likely to make 2,000 hours than the higher compression 470-U engines. Charlie, by the way, just sold his shop last year to longtime employee L.J. Warren.
Charlie's and other's testimonies regarding longevity of lower compression engines are compelling enough that I am sticking to 7.5:1 c.r.

Originally Posted by
mike mcs repair
it was at an air taxi in Valdez, the FAA thinking was, the time steve put on his original 470 converted engine to get certified higher TBO was not on a converted 520....
Steve and his wife and his helper were living in a tent out in front of our hanger in Naknek, hauling fish to put time on original engine, then he had to tear it down in front of FAA to demonstrate for the longer TBO...
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