A long with what the others said.

I do not know how you get at the Ring Gear (on the Flywheel on Manual Transmission models and on the Flex Plate on Auto Trans) but if you inspect the Teeth you may find that there are burrs on the Teeth.

The result is that it can prevent the Starter Gear from engaging.

The burrs can be filed or ground off to return the Ring Gear to the proper width/Thickness. However, I do not know how you would gain enough room to do this as the other vehicles I have done this to and a removable pan under the Ring Gear.

Of coures if the tips of the Ring Gear Teeth are worn the Ring Gear needs to be replaced or the Flex Plate if that is what you have.

Sometimes the Over Running Clutch on the Starter Gear goes bad and the Starter Gear engages and the Starter Motor turns but the Gear does not due to an issue inside of the clutch.

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84 300D, 82 Volvo 244Gl Diesel