Compression test on a 1979 9.9?

I bought a boat last night with a 1979 9.9 Evinrude. While doing the test on the bottom cyl. my gauge would screw in all the way preventing me from doing the test. All the other engines I have tested the threads tightened 1/2 way up the threads. This one cyl bottomed out and I couldn't pull the handle. The gauge was resting on the piston.

Did I buy a headache. My first thought was maybe a heli coil was installed, but I could not see one.

Numbers were 60 on both cylinders. I backed the gauge out 1/2 way to measure the bottom.

Most screw in type compression testers contain two sets of threads, the second set being larger than the first. I assume that the spark plug hole has been drilled out to either accept a larger spark plug or, and most likely, some sort of heli coil which has either not yet been installed or has been removed. This would allow your compression tester to screw in too far.

I think you misunderstood. I used the same threads of the tester for testing both cylinders. The small one. I do have a larger threaded adapter for testing the larger spark plug holes. The threads seemed correct except the threads on my gauge went all the way in until the o-ring hit. Are the threads on the engines like pipe threads (tapered) or are they the same diameter the entire length/depth?

Spark plug threads are the same from start to finish..... no taper.

One of my compression testers incorporates both the smaller and larger threads on the gauge itself. This eliminates any thread adapter changes. I have no idea what type compression tester you have BUT I've never seen one that incorporates a O Ring. Perhaps the plug threads are stripped which would form a "V" type threaded area allowing your tester to enter too far?

My gauges...... One has a rubber beveled tip, normally used with an electric start engine. The other screws in and has a flat right angle edge that butts up against the head which prevents the tool from entering too far.

Ok Joe thanks. Mine is like your first type but the threaded section is very long on my tester. I have only checked a few engines, and they all stopped well short of the o-ring. Maybe most engines are not threaded all the way through to the piston, but someone chased the threads at one time on this one spark plug hole?
Who knows, I will find out next week when I pick it up.