Oil filter bypass.
 

I have the old M-IV BBC. It has the bypass adaptor that uses 2 long Allen head screws then a cap screws on to it.

I want to get a 30lb bypass valve but not sure which one to get. Any help? What part # do I need and where to get it?
I also hear that you can just cap the hole and drive everything through the filter? Is there any drawback to this? I mean there is a bypass there for a reason even if it’s not a good one.
http://img688.imageshack.us/img688/9896/unled11a.jpg
http://img841.imageshack.us/img841/1440/unled10y.jpg
http://img39.imageshack.us/img39/967/unled9h.jpg

all the 30 psi valves discussed on here are for gen VI blocks..and are in the block, not the adapter...now it does look like the valve in your housing...not sure on your setup..

The reason it's there is to prevent oil starvation during extreme cold start ups or if the filter stops flowing for some reason. So you might say it's a safety device for engines run in cooler weather and people who don't change their oil and filter regularly. I eliminated mine beings I only run in the summer months and I change oil and filter often. I feel safer with out it knowing unfiltered oil is never going to run to the bearings.

If you have easy access to the filter if it were mounted on the block and no cooler then you may want to use one of these and eliminate all the remote mount stuff.
http://www.summitracing.com/parts/MOR-23775/?rtype=10

I plugged mine with a 3/8npt brass fitting if memory serves. Works great on a boat.

If you plug it (If it was mine I would) you need to make sure you have a high flowing oil filter like a extended length k/n or equivalent and let oil warm up (which you should be doing anyways) before getting on the throttle hard, Smitty