I have a 1.25"ID titanium tube with 24 threads per inch but I want 12 threads per inch. Can I just run a 12tpi tap down there and get the new thread count?

Wouldn't recommend it without going to a bigger size. Ordinarily I'd say the metal will be brittle and barely hanging on, but titanium is quite tough and soft... might turn out alright. If you can switch between SAE and Metric you might be able to get the equivalent of a quarter to half a size larger instead of a full size.

I would not do it. I am a machinist though so it may work but I know from the professional standpoint, that's not a good idea.

Don't do it. Either suck it up and use the 24tpi or buy another one that is coarse thread.

thanks guys well I have to cut the length down anyways too long, so the threads will be gone now. but now I might use a tube adapter in the end but can't weld titanium to chromoly. You know anything you machinists use that I can basically "glue it in? like jb weld. Or like a solder or braze? I'm also gonna drill a hole and put a bolt thr the rod and tube adapter since I can't weld it.

the fine threads, 24TPI and much shallower than the 12tpi. You need to get to correct drill bit for the 12tpi tap, drill the threads out, and re-tap. It may work just fine, but measure first. However, if the 24TPI the same bolt size as the 12? What thread and diameter sizes are you looking to use.

if you just jam the new tap in there you will most likely end up with a broken tap or useless threads.