Today I disovered a leak coming from my oil filter adapter. I just recently put this motor in my 98 yukon. I was setting the timing and started it up and once I shut the truck off I found the leak. Is there a gasket or something that is suppose to go on it because I don't remember putting one on.

There is a gasket between the bypass housing and the engine block.

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If you didn't have to remove and reinstall the bypass, the gasket is likely old and is in need of replacement. Even if you did replace the gasket during engine installation, it may have shifted during assembly or might not have gotten torqued correctly.

Are you certain the leak isn't from the oil pan seam or the rear main seal?
Both of these are very near the adapter.

When I got under to look at the leak, the sepage really seemed to be coming from the bypass. Don't figure it will hurt if I get someone else to take a look and get their opinion.

Well bought another gasket and o-ring that was a little thicker than the ones I was sent with the engine and it seemed to do the trick.