Futon Dowel Insert

So our futon uses a system where you pull the couch from the front and it opens up into a bed. There are four parts on the main frame (the part right under the mattress) where metal dowels coupled to plastic pieces protrude into grooves carved into the larger supporting frame. The plastic bits slide along the grooves and that's the whole mechanism for working the futon. Unfortunately, there are inserts on the main frame where the dowels are inserted, those inserts are cheap nylon and 2 of them h