First of all, go to the web site and download the schematics of the model #. It will show all the parts and part #s. Secondly watch a u tube video of the actual strip down process of the professional 600 series stand mixer. With this knowledge, do one of two processes 1: strip down the machine, remove the gears and check for defects- order parts. Or 2: replace all the parts( little money) and have a totally refurbished , smooth running mixer. I kept the good used gears for spares. I replaced 90 % of the internals ( including motor) and have a smooth running machine. I initially burned out the mother 2 months after initial purchase, sent it back under warentee, and received it back . Wen I stripped it down 10 years later, there was very little grease in the gear housing. This is what probable ruined the gears. Use plenty of grease in the housing and gears. The grease is one of the most expensive items, but you can't find it anywhere, so order it, along with everything else, from Parts Select.
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