I'm not 100% sure I understand your post but if you just want to model a 2-1/2" BSPP thread then you could use the ISO Pipe Thread G75.184. I believe this is the same as a 2-1/2" BSPP. For some reason the Fusion 360 BSP thread series doesn't go above 2".
I hope this helps.
If this answers your question please mark the thread as solved as it can help others find solutions in the future.
Marcus Wakefield
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I've created a Windows application (and now Mac as well) for creating custom thread files for Fusion. You can find out about it here. Hope you find it useful.
If you need to know how to offset threads for 3D printing then I've created a guide here which you might find useful.
If you would like to send me a tip for any help I've provided or for any of my software applications you've found useful, you can do this via my Ko-Fi page here.
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