Without seeing the engine, we'd not know what your difficulty may be. With R/C car engines, the most common problem is a connecting rod that seized to the crankshaft and then twisted off. This is caused by a lack of lubrication at the lower-end of the rod.
As far as "proper" breakin is concerned, there are so many different tall-tales about how to properly break in an R/C car engine, that we'd not be sure of that.
Also, please see the FAQ on temperatures. They are not reliable until you've found out what temperature your engine should run at, and you find that out while you're running the engine, not from someone else who tells you what temp you should use.
http://www.osengines.com/faq/product-faq.html#q14
You may wish to send the engine to the service center to be checked.