Outer - plastic
Inner - stainless steel
When you r fully disassemble of your model - put in all new bearings. Old bearings you should inspect carefully, unless you use some cheap ones(or RTR) which is throw away items. Before you inspect - do thorough cleaning as follows.
Remove side covers from bearings. Drop bearings into stainless inner mesh. Inner mesh goes into plastic outer container. Add cleaning solution (motor cleaner for example) to cover all bearings completely. Leave it to soak up for a day. Do some shaking during this period to allow to dissolve old grease as much as possible. Start to shake to rinse whats left after a day. There you go - completely clean and ready for inspection bearings. Now you check for any sort of wobbling and noisy sound during manual turning of inner racer. The ones which is in good condition to be packed with grease(don't put too much) and closed with corresponding seals.
Amazing product Simply the best for cleaning small parts especially bearings. Simply put the parts in pour in the cleaning liquid I use white spirits then shake the container. Repeat if the parts are extremely dirty if not remove them and dry them
Excellent piece of accessories from Yeah Racing, simple yet effectively remove residue and dirt on bearing. Just put suitable solution and the beating the the bottle, shake it and you got yourself a clean bearing again..