Connecting new PVC tub drain into existing iron pipe

Hello,

I am replacing my tub drain with a new PVC set. The PVC drain needs to connect to an existing iron nipple. The iron nipple is male threaded. Right now I'm using a plastic slip nut and compression washer.

I've got it all hooked up and have been testing. The only time it leaks is when the tub is full and is draining. The leak point is at the connection of the PVC to the iron. It does not leak when water is flowing continuously and the tub is not full.

What can be done to fix this? Should I change how I try to connect? Is it just a matter of ensuring my current connection is aligned correctly?

 

Where's your trap?

You don't connect slip joint directly to threaded iron pipe, you use a threaded F/NPT x SJ (slip joint) adapter.

Example

 

The trap is below the iron nipple I'm trying to connect to.