How do I determin if my local court has jurisdiction over state lines?

I don't see the tort, all I see is a breach of contract.  Nevertheless, I think it likely that the California court would have jurisdiction over the defendant, as he negotiated the contract over the phone (I assume) with someone he knew to be in California, discussed the problem with you in Californa, and more important, twice shipped the engine to California.  However, you won't know for sure until you file suit in California and the defendant, if he chooses, moves to dismiss the case for lack of jurisdiction. One caveat.  While I'm pretty confident that it would be constitutionally permissible for the California courts to exercise personal jurisdiction over the defendant, this case sounds like it may be litigated in small claims court.  Many small claims courts have limited jurisdiction, much less than is constitutionally allowed.  If you're going to file in small claims court, you should ask the clerk of the court if you can sue a resident of another state in that court.