I am trying to reproduce an Oil tank cap for a chainsaw. The thread is an unusual pitch and the original is no longer available. I have an original to cast from.  I am new to resin casting and I'm not sure if resin is strong enough for the threads. They are only tightened to tight finger tight and have an O ring seal. Would I need a special resin ?   Also re making the mould.  I think I would need a split mould. I'm not sure I can keep the cap hollow as it adds more difficulty in releasing. So I would fill with plasticine before moulding . I need to split the mould lengthways down the threads but fill the mould from the end. I'm not sure how to achieve this. Any help please .