If one "Locks" the Crankshaft against inadvertent movement.(@ TDC)
(Either at the Starter aperture,or the rubber plugged Oil Pan Opening)
AND then EXACTLY marks the point on the scale of the H.B. the Pointer
is set to...
When re-assembling (Unless New Timing chain components were installed)
re-aligning the pointer BACK at the exact same spot on the H.B. scale...
will yield exactly what you started with...

Howeveah,There is No guarantee that's ACTUALLY TDC since you're just
replacing everything AS YOU FOUND IT.

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'84 300SD sold
124.128