Interesting question. If you're calling "participated" past tense in your original example, that may be part of your problem. It's really the past participle, and "had" is an auxilliary, or helping verb.
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AnonymousThat is the forth race I had participated in."This would work in certain contexts, but
AnonymousI think it is best to omit had and just say "I participated in"Yes, that's fine. You don't need 'had'. It's not wrong, but you don't need it.