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 .

If you make it solid to keep the mould simple, then you can use our Xencast P2 Fast Cast Polyurethane Casting Resin for ease of use and speed of cure.  Add about 20% milled carbon fibre to it to add tensile strength, black pigmentation and reduced brittleness to the finished cast. If it doesn't turn nicely, you could add 5-10% graphite powder as that will help make the surface a little bit more slippery. 

I would be tempted to run the mould split line horizontally across the step between the lid and thread of the cap.  Silicone moulds should be flexible enough to still get the threaded bit out (or just unscrew it!!) and the flash line right on the cap tops base edge will trim off neatly and be almost invisible.

Warren Penalver
Easy Composites / Carbon Mods - Technical Support Assistant

That sounds like excellent advice. So you think I could unscrew the piece from the mould without damaging the silicone threads ?  I think the releaser is going to be important. Could I just use spray silicone as a releaser?  Any particular moulding silicone to hold the threads for multi use ?  Do I need a vacuum chamber or degasser ? BTW.. I have another project coming up where I will be trying to reproduce a silicone grommet. I understand I will need a strong flexible durable silicone for that.
Thanks for your help btw.