I have stainless marine exhaust on my boat and i have a questionn. The chrome elbow comes off the manifold and up then the pipe runes straight out of the boat...my question is at the top of the pipe out the boat is a plug screwed into the top like a bung....What is it and why is it there and what can i do with it

O2 Sensor bung?

A picture would be worth a 1000 words. Post one and somebody will be able to tell you. It depends on the size and location of the bunge as to what its for.

What can you do with it??? If its not broke, don't try and fix it, you might break something.

im not able to post pictures or else i woiuld

Post it on flicker, photobucket, or a similar site and then post the link on here.

if it goes all the way through the inner and outer pipes its for a 02 sensor,if it just goes into the outer pipe then it for relieving pressure at the tip..you would pit a barbed 90* fitting in the hole and use a through transom fitting to plumb the water out of the boat..mainly for reversion purposes..