I just recently purchased a THS racing pipe and put it in my stock engine for my Kyosho FW05T- Plus on my first run one of the screws disappeared when cleaning it after 1 tank of fuel.

purchased new set of screws and installed them with new nuts, after one tank and while cleaning it, i noticed the manifold was loose, so i tighten the screws again and the nuts.....

What could be the possible cause of this? should i buy new screws and nuts? should i buy a different manifold?

Thx in advance!