ORIGINAL: Sport_Pilot
...I have had many engines that would not idle smooth during break in and would stop with the piston near TDC when idled low. They will also shake, but this goes away when broken in.
Sport_Pilot,
Tapered-bore engine are not supposed to idle at all, during the break-in.
You are supposed to start at about 1/4 throttle and advance it to full, as soon as the engine starts. You are also supposed to lean it ASAP, to 400-500 RPM rich of max with the recommended 25% oil (half of which is degummed Castor).
The engine must be hot enough during the break-in, for the piston not to catch at TDC. That is the point of this technique.
Please read [link=http://www.rcuniverse.com/forum/Tapered%2DBore_Engine_Break%2Din_%2D_Upgraded/m_1850473/tm.htm]the tapered-bore break-in thread[/link] again.
And you are right about the piston catching at TDC, during starting attempts.
This makes the first few starts a bit difficult to go through (more difficult for some tighter engines), but it goes away as soon as you advance the throttle and the engine's temperature rises.
It never seizes while going full bore, so the added vibration at start-up is a short term issue. This is another reason for doing the break-in on the test-stand; not on your precious, sensitive model.
Since this does not happen on the model, it becomes a non-issue.