SummaryI designed this Idler Pulley for a Solidoodle 2, but should work anywhere you need to replace a plain bearing Idler with a Ball Bearing version. The current one is designed for a 4mm shaft, and the bearing is 8mm OD, 4mm ID and 3mm wide . These bearings are available very cheaply on eBay (plaigbearings in my case)- search for "Precision Bearings - Proven Quality - High RPM Speeds" and select Model: 4x8x3mm Bearing, with Standard Rubber Seals. There is a small lip to hold the bearings in place - they occasionally case loose on my earlier versions