AFT tophat rollers

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has anyone replaced the larger rollers on the lifting mechanism, they look like tophat bushes? mine looks as if they have a flat on them, as they will turn freely with the weight off them, then will only turn so far as the roof lowers or raises, not sure if they are hard plastic or nylon.

i notice grease was used as lubricant, so probable picked up some grit to prevent rollers turning in the first place.