BDC is simply where the piston is at the bottom of it's travel. The easiest way I've found to put the piston there is to simply turn the flywheel. When it spins effortlessly the piston is clear of the pinch and can be left where it is. If you need a tool to turn it, you're nearing TDC and encountering the pinch in the sleeve.

As for what happens when you leave it where it stops...some engines don't care. My CV-RX didn't. Some engines do, however, and you can ruin the piston/sleeve in these engines. If yours is high enough compression to warrant loosening the plug I'd say it would prefer to be at BDC during cooldown.


Oh, and watch this video. It will show you what's going on inside the engine.

[youtube]http://www.youtube.com/watch?v=CscBSOoB5_A[/youtube]