Nuts similar to Blackriver autoblocks. Fit every fingerboard trucks.
Inside the metal nut there is a polyamide ring which works in the same way as in skateboard trucks. It's quite hard in the first spin because polyamide fits perfectly in the axle on which is screwed. The less you twist these nuts the longer they will last. If you change wheels frequently the ring will loosen and you will have to use a glue.
Video guide for gluing nuts:
Price for the set of 4 nuts.