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Mobydisc said...
can't help you answer your question but
Nice one... always need to say something I guess

The thread should be metric, some fin head do have imperial thread but it usually is Tectonic fins (but call request metric thread).
I have had C3, Select, Deb, Drake, Tectonics and they all are metrics and use the same thread. You may have cross threaded the insert.
Does it do the same thing with both inserts?