New motor was running a little wimpy changed out plugs and wires and just noticed a pretty big exhaust leak on the flang of the headers I felt it hitting my hand and it was like it was the tail pipe so it's a big leak.. What can this cause and how can I fix it?

 

Flange at the head, or flange at the collector of the headers.
I hate the 3bolt collector flange. They bend so easy when you tighten them.
I prefer people cut them off and slip a 3" piece over the collector and build their exhaust from there. Or if they insist on having them, I weld the back 100% and grind the inner lip off, so that a gasket actually fits and isn't just hitting on a tiny piece of the collector.
To fix it, if its at the collector, you'll need to take it apart, flatten the flange somehow, then reinstall but not over tighten. You can get copper gaskets which are ok, or thick aluminum soft gaskets. I like he alum ones if I had to choose.
But, if it were mine, and I had the room before a bend, I'd cut the 3 bolt flanges off and just use a band clamp to connect the two pipes. I do this on most all my exhausts I build. Slip fit with band clamp, or just butt the two pipes together and use a band clamp. They stretch and form to the pipe and seal very nicely.

 

Cause being to my understanding, the headers get real hot and when not built correctly will crack from change in temperature.

 

When the engine is running correctly, the headers, and manifolds, only get to "hot". Not super hot, red hot, or yellow hot. Normal running wont cause a crack in the header. Rapid heating and cooling would both headers and manifolds, and just high temp heat cycling from towing can cause issues.
We really need to know where it's leaking, at the head or underneath at the collector. From there we can better determine the best way to solve the problem.

 

Sorry for the late reply fellas been workin, it's below the collector

 

I stand corrected, although that's why I said "poorly made headers". What would be the reason for manifold and header cracks. I have an exhaust leak in my manifold on my 351W.

 

And it seems to be only from the right collector the left one feels
fine

 

It's really not being "poorly made" that causes cracks. Heat cycling causes a lot of stress and along with vibration cracks form.
Even a nice set of stainless headers crack. The expansion rate where the tube is stretched vs where it isn't, around a bend, is of concern always with stainless.

Your manifold could be cracked or your studs where the flange bolts on could have rusted and let he flange loose some.

Leaking from one side and not the other is only a case of one being warped and the other one not. Either "fix" both sides or only do the one that's actually leaking. Up to you.

 

And to add a little more info these are the shorty headers summit sells there pacesetters.. And I'm gonna go with the weld idea and try and diych the flanges

 

Aaah. Ok.
I was thinking longtubes.
Hmmmm. You're being difficult now.
If its leaking at the ball collector of the header then I'd say either there's something in there before you tightened them, the headpipe flange was a little oval when you installed them, or they need to be tighter. On manifolds I usually use impact wrench and gun it super tight. Headers I'd probably use ratchet but still darn tight.

 

Well sorry for being difficult haha but like I said it's a pretty heavy leak so would tht cause it to run bad? And would I. Be better off with just welding it

 

More than likely the flare on headpipes is oval.
Try taking a hammer and hit the flange all around the flare to try and form it to your header ball collector and try to close the gap. Then tighten the nuts a lot.