M130 head leakage
 

Noticed 1.5 in. drop in radiator water level and started looking for clues. I have owned this 250C for 25 years and it has been running fine, this is a new item. Found a very small trail of coolant from near #4 spark-plug and the the coolant hose on the left side of head. So I tighen the hose clamp and then cleaned the head and block down to the pan and ran the engine. There is a leak. And the leak is around the spark-plug hole threads. I have been shade treeing cars for 50 years and this new to me. Any ideas out there? ie: head gasket or cracked head or?
Car engine runs fine. No smoke of any color, steam or noises.