Ace Hobby, the North American distributor for Thunder Tiger, handles all Return to Manufacturer Authorizations (RMAs) for Thunder Tiger engine warranty issues:
http://rma.acehobby.com/RMA_Consumer_main.asp
Simply contact Ace for return instructions and they will get your engine serviced or replaced.
Ace Hobby also has a forums section on their website where they provide technical support for Thunder Tiger and Ace products:
http://www.acehobby.com/forum/forum_topics.asp?FID=8
If you have a problem with a Thunder Tiger engine, Ace Hobby has plenty of resources available to help you