These Tank Idler wheels are made to go with the Tank Track 2in and Idler arms in my things. They will work with other tracks on Thingiverse also. These are the main Idler rollers that hold the Tank Track in place and support the weight of tank. The stubs space them out to the right distance from tank side rail. These are made to use 1/4" x 3/8" x 1/8" sealed ball bearings and 1/4" x 1 1/2" shoulder bolts (stripper bolts). They place idlers at 1 1/2" center from the side rail, perfect for mounting track. Use (4) #212 National o-rings on idlers to make them run smother and quieter on track. See the worm gear case and idler arms for drill guide PDF download of frame drilling.Update October 2016: Uploaded a few more idler wheels to choose from. O-Rings are kind of expensive when you need a lot of them. There is a 2 O-Ring Idler roller and a Blank one with no O-Ring grooves. Print with raft and support. It is tricky but take a 3/8" drill bit and ream out the bearing holes by hand. Go in slowly until it hits the bearing stop and pull out. Press the bearings in holes. Bearings are 1/4" shaft x 3/8" OD x 1/8" wide. Can find them on ebay. Use 1/4" x 1 1/2" shoulder bolts and 10-24 nuts. You can file them down where they roll in the track if hitting the track links. Will work with tracks that have a .350" inside between track links.