Full Ceramic (ZrO2) Bearings. (Set of 2) * Basically, all sizes are in stock and listed. When they sell out, it takes time to restock. (Purchase early...) ↓ Specifications below Standard Size: MR63 (also known as 630 size) Outer Diameter: 6mm Inner Diameter: 3mm Thickness: 2.5mm Examples of compatible models... 25 Alphas BF TW 20 Alphas AIR TW 18 Alphas AIR Stream Custom 16 Alphas AIR 22 Silver Creek AIR TW Stream Custom 23 SS AIR TW 21 Gekka Bijin AIR TW PE SPECIAL etc., but this information is for reference from web sources. (I'm sorry, but I have not confirmed installation on actual devices.) Therefore, please make your own judgment regarding compatibility. It rotates most smoothly when used dry, but when installed in the spool system of a bait reel, a high-pitched "shhh" sliding noise may occur during casting. If you are concerned about this, apply an appropriate amount of oil. (It should become a little quieter.) However, because it is full ceramic (ball retainer parts are PTFE), there should be no seizing like that of metal cage & metal ball bearings (even in a dry state) at the rotational speeds of a bait reel spool. In other words, (even when used in saltwater) it will not rust, and it is an ideal, maintenance-free bearing (at present) that does not require oiling, etc. Because it is a non-magnetic material, it is easy to install near the brake magnet. Please note that after installing it in a reel, you may not be able to receive manufacturer warranty, so please do so at your own risk.