+1 stupid design but it's there!!

I've used Dr Bob's pressurize method to great success when trcking down the exact same leak. The bolt was leaking and the water pipe spigot to the heater valve was about a week away from catastrophic terminal failures.

 

Also, you may need an extra washer for this bolt to clamp down enough to seal. It's not that the bolt penetrates the water jacket, even though that's what I said earlier. It's that the bolt also secures the coolant block off plate under it and it can bottom out before it clamps the plate adequately to seal. Also, there is a gasket under the plate that needs to be renewed sometimes.

 

Thanks Bill. Both washers were there. In my case, the bolt was really loose. I put a dab of RTV sealant on the bolts and re-inserted. Hopefully, the gasket didn't get contaminated too much by the coolant. I'll keep an eye on it.