Bearings are what enable the gears to rotate smoothly. They are usually produced away from bronze or brass, and they in shape into races (or grooves) in the gearbox housing. On the other hand, inner filters are more expensive than externals and it is much harder to switch inner filters, https://filtergearbox92233.anchor-blog.com/14407980/5-easy-facts-about-filter-gearbox-described