Here's how I think it works:
Compression stroke up, both valves remain closed and compression builds.
TDC: both valves closed, spark plug fires.
Power stroke down, both valves closed.
BDC: both valves closed.
Exhaust stroke up: the exhaust valve opens after BDC and then closes before TDC.
TDC: both valves closed.
Intake stroke down: intake valve opens after TDC and then closes before BDC.
BDC: both valves closed again.
To find TDC, remove the spark plugs after cleaning away debris around them.
Put a finger over the first spark plug hole after the flywheel (cylinder #1).
Crank the shaft counterclockwise until feeling a puff on the finger, signifying start of the compression stroke (Image 1 shows where the crank pins are when I do that).
Gently crank-check-crank counterclockwise until the crank pin points to the mark on the flywheel cover (Image 3).
The first piston should be at or near its TDC. But, is it near enough, judging by eye? How precise does one have to be in finding TDC for:
1. Replacing a distributor that was removed for cleaning, when the crankshaft was later turned for other reasons?
2. Setting the valve tappet clearance gaps?
Compression stroke up, both valves remain closed and compression builds.
TDC: both valves closed, spark plug fires.
Power stroke down, both valves closed.
BDC: both valves closed.
Exhaust stroke up: the exhaust valve opens after BDC and then closes before TDC.
TDC: both valves closed.
Intake stroke down: intake valve opens after TDC and then closes before BDC.
BDC: both valves closed again.
To find TDC, remove the spark plugs after cleaning away debris around them.
Put a finger over the first spark plug hole after the flywheel (cylinder #1).
Crank the shaft counterclockwise until feeling a puff on the finger, signifying start of the compression stroke (Image 1 shows where the crank pins are when I do that).
Gently crank-check-crank counterclockwise until the crank pin points to the mark on the flywheel cover (Image 3).
The first piston should be at or near its TDC. But, is it near enough, judging by eye? How precise does one have to be in finding TDC for:
1. Replacing a distributor that was removed for cleaning, when the crankshaft was later turned for other reasons?
2. Setting the valve tappet clearance gaps?
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