[AT] Off Topic 2001 Ford F250 6.8L engine. Oil pressure gage port size.

[tmehrkam at sbcglobal.net]

I am having problems with the oil pressure switch on my F250.  Yesterday the gauge "Foo Gage" stayed at zero when the engine was started cold.  I shut it off, checked the oil, started the engine which sounded good and the gauge finally came up.
I changed the oil and Filter Sunday.  Motorcraft oil and Filter So it could be a number of issues. 

1. Bad oil pressure switch.2. Bad check valve in the filter.3. Worn bearings or oil pump problem.
I am getting a new filter and would like to install a mechanical gauge.  Does any one have experience with this engine or the 5.4 Liter engine of the same vintage. I am going for parts at Noon and would like to know the port size.  My best guess is 3/8 NPT.
PS: Drove it home and then back to work with no indications of low oil pressure and the gauge worked fine.  But others have reported oil pump problems on this engine.   I only have 240,000 miles on this thing and do not want to mess it up.