I beg for somebody's help

I have a 2004 Cav. I am putting a oil pressure gauge in and the oil pressure sending unit which is the unit which goes into your engine block is 1/8 male thread. The problem I'm having is knowing what the thread hole is on the motor so I can get an adapter to fit the 1/8 sending unit. I have no idea what size adapter I need or for that matter I can't find one. I was told by one person he thinks the hole size is 12mm x 1.75. I looked everywhere for that adapter and no luck. If someone has an adapter for this application I would be glad to purchase it off you. Or can someone please, please, please tell me where I can find one. Thank you very much