This could be a dumb question but how do you guys plug off the hole in tail pipes to pressure test them??
My headers are cmi's
Thanks in advance

I found a ball I could stuff into the collector to seal off the water outlet in my Lightnings.

Not likely to leak in the tails anyway. Just pressure test the headers and inspect the tails.,

I bought a 4" plumbing plug that has the rubber stopper and the wing nut. You have to crank it down pretty hard but it stays in there. Then take a flat piece of 3/8" flat stock and drill it with a header gasket, add a schraeder valve, and air it up.

Hook up water hose to bottom water inlet. Fill with water until it is just about to come out of the top water outlet. cap the top water jumper outlet with 1" rubber cap and hoseclamp. Most homes have water pressure of 40-70psi which is higher than your boat will run. Turn on water and look for water coming back down the runners. This is the only failsafe way to test them and also the easiest...air pressure tests can be very difficult and inaccurate.

Why would an air pressure test be inaccurate? If it holds air for 24 hours I would think that it would hold water too. Also, water has surface tension, so it is possible that a very tiny crack might not leak until it is heated up. There is another thread on here that discusses using a solvent like acetone or lacquer thinner to test for leaks because it has a much lower surface tension. Not necessarily saying you are wrong, just looking for clarification.

its a lot simpler to use pressurized water for most people than to fabricate a steel plate and insure that it is sealed well, drill and tap for a shraeder valve. You can easily get a false reading that your headers are leaking when in reality you don't have them sealed well enough. How are you going to know the difference???
There is a reason that most top engine builders use water pressure to test them rather than air. I am not saying it can't be done, just a LOT more work and not foolproof.

my main concern was how you plug the hole in the tail pipe

The header flange jig with the flat stock was really not that hard to fabricate with a drill press. Now if you don't have access to one, then yes, I agree that it might be tough to do. I just cut some 1/4" rubber and put it between the flange and the jig. Used 3/8" bolts with nuts on the backs to attach the header flange, and also installed a schrader valve. I didn't have any air leaks, but if I did, a quick spray with some soapy water would tell me if it was leaking around the flange. However, I do agree that with a header or manifold that uses a jumper hose it may be easier to pressurize the water jacket side. I had Lightnings, so it was near impossible to pressurize the water jackets. Had to pressurize the exhaust side instead.

Zeke, like I said earlier, install one of these in the pipe, then pressurize the exhaust side.