Macster

is 1st, then if the compression test numbers look poor (one or more cylinders reading lower (by some amount IIRC 15% or more) than the others or two adjacent cylinders reading low), then a leak down test can help pinpoint the reason for the poor compression test numbers.

Easy way to get a good leak down test number is to squirt some oil into each spark plug hole, crank the engine a moment. The oil gets spread around not only on the rings/cylinders between the piston and cylinder wall but on the valve faces/seats.

An older engine with rings that have higher tension -- at rest press against the cylinder wall with more force -- and thus seal better (but with a cost in increased friction) can look better, get better leak down compression numbers than a newer engine fitted with lower tension rings which have lower friction but can still seal well under real world operation conditions because the increasing pressure in the compression/combustion stroke force the rings out and thus create a tight seal when it is needed.

Sincerely,

Macster.