From Bernie's site:

I always found that these worked really well for giving good flight performance yet were tough enough to withstand MOST crashes.
http://coxengines.ca/product.php?pro...5&cat=9&page=2

For training kids to fly a control line model these can "take a licking and keep on ticking".
http://coxengines.ca/product.php?pro...9&cat=9&page=2