Reply to "351C with 35,000 miles - Need to rebuild?"

Note there are various ways of doing a leak-down test and interpreting same: mine is to use a dual-gauge system starting with 100psi in the chamber. Using 100psi to start gives you % leakage directly. Run the test cylinder's piston to TDC, put the car in 1st gear and check the gauge reading after exactly 120 seconds (2 min). Note-100 psi in a cylinder may cause an unrestrained engine to move the crank slightly, spoiling the test.

Rule of Thumb for my personal leak-down tests: 2-5%: darn good engine (there are no zero-leak engines, even newly broken in with gapless rings);

5-10%: well broken in, typical street engine;

10-15%: worn but usable for more miles if you're careful with rpms;

Over 15%: there is metal missing somewhere in the combustion chamber; a general overhaul will soon be needed.

Repeat for all cylinders and also check for big variations between cylinders as with a compression test.

Some shops specify only a hot engine, some take a gauge reading right away for % leakakage or after some other duration, and some always wear their 'lucky' socks. You can do a quick diagnosis on potential trouble spots by listening for air flow in carb (intake valve seal), air flow in tailpipe (exhaust valve seal) or air or bubbles showing in rad tank or in valve cover breather cap (head gasket problems).
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