Long story short I have an old tt fork laying around and I’m doing a bike build. Any reason I can’t save some money and buy just a frame and use this fork?
It depends on the fork and frame in question. A lot of times TT forks will have less rake. That could cause the bike to be twitchy. You also have to take leg length into account. If the frame was designed for a certain length fork and the one you have is a different length that could cause more problems.
Granted, we are talking about very minor differences, as most bikes need to be built to within a certain spec to work with industry standard wheels, brakes, etc. So it may not be enough difference to notice. But it is something to keep in mind.
It seems more common to buy a frameset rather than a frame. If you are factoring in all the costs then it’s worth knowing if a frameset comes with a headset. The advantage of getting it all as one is that it should all work vs worrying about all the details when you get them from different places.
Assuming that the fork has the right stem size, brake type, tire clearance, etc then the bit you need to think about is the angles. Does the head tube angle of the frame and the rake of the fork put the front wheel in the right place for handling? One way to do that is get a frame that would normally come with (is designed for) a fork with the same dimensions as the fork you currently have.
ETA - chadtrent already said this, I hadn’t seen that reply for some reason