I have Shimano 600 calipers on my bike and when I got it I noticed the adjuster threads seemed stripped. They still worked okay but I decided I was going to replace them due to it. I took the front caliper off to measure it for a replacement and upon doing this I noticed that there never were threads int he caliper itself. Instead, it had a plastic insert when new but that was mostly missing. Is this a replaceable item or am I going to have to make something to take its place? If its to difficult (or expensive) I'll go with my original plan to replace the calipers.