Is the nub pin located across from the OT= Top Dead Center on the degree wheel?

Member stretch suggested another way of checking for TDC but it takes more courage. In your case you would get the Piston close to TDC say 5-10 degrees before top dead center (you inaccurate point ought accurate enough for that) on the compression stroke.

Then you remove the valve spring retainer and valve springs and let the valve go down till it contacts the top of the Piston Head then you set up your Dial indicator so the plunger is on top of the Valve Stem.
As you rotate the engine in the direction of rotation the Piston + Valve rises the indicator needle moves and right when the needle stops would be TDC.

Member Stretch posted on the above long ago if you can find the thread he posted in.

Lining up the camshaft timing marks also gives you a rough estimate of TDC.

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84 300D, 82 Volvo 244Gl Diesel