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The PDS500x main compression adaptor is used when the spark plug thread is M12x1.25mm. Integrated is a 17mm cutout for spanner/wrench size of 17mm.
For the other spark plug threads, the main compression adaptor must be used with the appropriate additional adapter to perform engine compression tests with the PDS500x.
The adaptor's extremely small internal volume helps maintain the volume internal of the combustion chamber during the compression stroke. In this way, the total error of the measured pressure at top dead center is significantly reduced.
Also, this bi-metal compression adaptor is designed to have the minimum possible deflection and damping effects when pressure changes in a wide range.