do not over prop - do not over heat - do not overheat and do not overheat also vary engine speed and do not overheat - that is a good break in sequence - use plenty of good oil 20% min on ANY glow engine
also do not allow to over heat .
we ran .049 engines in --1949 or 1950 I forget - but it was the same then . do not overheat .
the worst tolerances in an engine, which also needed refitting very often was- FOX - some of the engines we had to refit twice to get proper running clearances. Early Enyas were a close second fit far too tight in many instances. The Chinese early engines were simply almost unusable- horrible fits.
Cox did the best job of temperature controlled machining - and could hold better tolerances than anyone .
Any of the current engines I have run - typically just needed a few trial run ups to see how they handled heat - NO extended running -I can tell in ten seconds if the fit is OK to fly- Then we fly varying th rpm and if I did not mention it - making sure to not overheat it.