30-04-17, 09:30 AM
The internal volume of the adapter, hose and anything else ahead of a non-return valve will reduce the compression reading. That includes any portion of the plug hole that the spark plug would normally fill that the tester adapter does not, i.e. if the plugs have a long threaded portion and the tester adapter does not you will get a low reading.
The volume of the cylinder with the piston at TDC is only about 20cc, if the tester adds even 10cc that’ll reduce the reading by a third.