Looking at the front of the engine, facing to the rear of the bike. There is a hose connected that goes through the tunnel in the sump. Just above the hose connection is a socket-head bolt. Mine appears to leak water. The Haynes manual says the drain plug is underneath the pump, so what's this?  
The bike is a 1985 K100. 
Thanks

Picture?

Could it be leaking from the thermostat housing? Is it that you are referencing?

Sounds like the lub oil pressure control slide bolt, but that shouldn't leak water...

Paul - k1100, k100 or k75. Most people put their bike and the frame number in their signature, helps enormously figuring out what you're looking at. There's a sticky on posting pics in the forum area. Post one up and you'll get better quality answers. Right now we're only guessing.

The rusty socket head plug just above the coolant inlet in this pic I found??
A quick parts check and internet search shows it uses a relatively common gasket ring used in oil pumps and is also used in Karcher pressure washers. A20X24-CU (BMW part #07119963342)


But maybe its the radiator hose that's leaking and not the plug.

That is the oil pressure reliefe valve there. If it were leaking coolant then you have bigger problems than a  leaking seal. As the Kaptain has said it is most likely coming from the radiator hose just below or dripping down from the top joint of the same hose.

Yes, that's the bolt. Oil pressure relief really shouldn't be leaking water. Eek! I guess that the water leak will be somewhere above it and the water is dripping onto the bolt head. Not another new radiator.... This will be my fourth.
Thanks.

it shouldn't be the radiator

more likely the seals in the water / oil pump have given up ....and someone has undone the cap bolt to the oil pressure valve ...and not replaced the fractured seal under the bolt head ...and possibly hasn't tensioned it up properly

there maybe other issues in the coolant system if oil / water has been transferring (highly likely )
possibly the radiator cap relief valves and seal has been affected possibly letting undo pressure to retain in the system and bleed across to the oil pump ..likely after stopping the engine ?!!

it might pay to remove the tank and inspect the radiator cap ...and the water reservoir for oil contamination  plus any sign of milkiness in the oil

just some thoughts ....

we don't - shouldn't go replacing stuff that is unlikely ...without proving the fault first

and I haven't seen any proving so far...
good luck

but get back to us after inspecting some more stuff

G'day Paul, If it appears wet and rusty then that may be a result of the rain its ridden through, or if its wet now then that may be an indication the radiator hose is leaking at that point where it attaches to the water pump spigot. You know you will loose the coolant from the motor when you undo that hose clamp and check to see how or if its been leaking, Either way to investigate its best to have a few items ready if all you have to do is replace that hose. New glycol coolant and hose clamp or hose to start the inspection.