For years, both with aircraft and with automotive engines, the leak down
(or differential compression test) has been performed with the piston at Top
Dead Center. The primary reason is that the top of the cylinder is where the
most heat and loss of lube (hence the most wear) is. See the following link.

Some (very few) engines valve overlap design might even prevent a reliable
BDC check, but I can see how it would be easy for a one man show.

Dave

Shortcut to: http://www.w-p-c.com/buggyleakdown.htm

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