Diagnostics... What's wrong and how to fix it?

Hey Everybody, I want to start this thread for diagnosing your MB. Please kindly attach audio/ video/ pics on how to determine what's wrong with an MB, and what you did to fix it.

Let me start with the dreaded air leak. I burned my first engine with this defect.
I'll speak in terms of my experience but feel free to comment.

First the Source of the Air leak:
1) Gaskets on the intake manifold.
2) Oil seals on the cranks.
3) Gaskets on the Cylinder.
4) Loose Spark Plug.

First Signs of an Airleak:
1) No Idle
2) The engine does not stop at full choke at optimized temperatures.
3) Seems to run at higher RPMs but no torque.

So Where is the Airleak?
For Gasket leaks, you might hear a very loud, very high pitched sound when you crank it.
For Oil seals, you will feel it's easier to crank, but difficult to start, and your clutch would slip, since your clutch would already be bathing in oil.
For the gaskets on the cylinder, you would see oily residues between the gaskets.
For the loose spark plug, you would notice a discoloration that looses the shiny color on the exposed metal.

So How to fix?
Replace gaskets with good quality ones. Replace intake manifold gaskets even before you start it for the first time.
Replace oil seals.
Always make sure your spark plug is tightened before each run.
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Here are some questions, I'm hoping to find answers in this thread:
So how do you know if your needle bearings, crank bearings, piston, piston rings are still Ok?
How does a healthy engine sounds like?
Chain Alignment? etc...

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