In the event that a head is surfaced... necessitating the installation of thicker shims under the precombustion chambers... to keep them from contacting the pistons...
The Reamer is used to cut the hole to align with the glowplugs... ( in addition to the cleaning function already mentioned ).....

Any reamer or drill put into a situation like this should have heavy grease on the threads to try to capture as much of the cuttings as possible...
It should be done in stages renewing the grease and wiping off the old grease and crud.... and renewing the grease for next step....