I fitted a Double winder recently.
In order to maintain headroom on the existing stair below, it needed two steps before the winder meaning that the newel was in mid air.
By sizing the tread width to the existing stair the newel was extended down at manufacture aligned with and supported on...
Copper washers were supplied for nipples with parallel threads sometimes.
Boulter Boilers especially so. I never had one fail though. They were meant for single use, but if annealed would last forever.
Is the gritty substance magnetically attracted.?
Try dipping the suction hose into a container of oil. Do you have Pressure and vacuum gauges
Have you fitted a new drive dog. Are oil lines reversed.
It can only be something simple.