Question Piston engine compression checks - 14mm vs 18mm testing?

[Neal]

My CubCrafters cc363i (Lycoming IO-360) has automotive spark plugs using a 18mm to 14mm adapter. I've been doing my compression checks and got 80/80 and then this year decided to get a borescope and let my maintenance shop do compression testing. I was surprised at first that they removed the adapter which I had planned to never remove, not a big deal, but they got 76/80. This makes me wonder if compression checks are different at 14mm vs 18mm? I also realize compression testing is not a science, the accuracy seems suspect at times.

 

[Cactus Charlie]

Removing the adapter gives more freedom to move the borescope around. It is also possible the shop did not have a hose adapter to fit the 14 mm thread. I made mine.

edit - removed comments based on typo in original message

 

[Neal]

Typo. Fixed.

 

[Cactus Charlie]

The difference between 80/80 and 76/80 could easily be caused by differences in tester calibration and/or difference in technique.

If all their reading are a bit lower than yours then I see no cause for concern. If three are the same and one is lower then monitor that cylinder for changes.

 

[Neal]

Their technique was a little different too. I moved my prop to max compression at TDC. Not concerned. Just curious of 14mm vs 18mm testing. Guess I could take a reading at 14mm and then do 18mm for an experiment.

 

[Cactus Charlie]

The size of the spark plug hole can't possibly make any difference. The reference orifice of 0.040 inch diameter. Any variation between 14 mm and 18 mm will likely be leakage at the hose adapter.