Seems like a common problem, and I have read the forums for suggestions on what to test. I need to do a compression test for further information.

One problem I am having is that a universal compression tester does not fit in the spark plug port. The port is recessed.

I took a digital caliper and measured the spark plug threads which are 13.6 mm which matches the compression tester size. But the problem is that the area just behind the threads are larger than the corresponding area on the spark plug.

the spark plug behind the threads measures 15.2 mm
the compression tester behind the threads measures 16.7 mm

Where can I get a compression tester that fits? Or am I missing something stupid?

Harbor Freight's compression tester fits mine just fine. Maybe just add a thicker o-ring to the threads?