Ryobi P421 vs. Ryobi P423: Differences. While both are part of Ryobi’s 18V One+ tool line, the Ryobi P421 and the P423 also have a couple of notable differences, such as the following: Grinding Speed. While the Ryobi P421 has 6,500 RPM, the P423 has 10,400 RPM, which makes it faster and stronger. More @Wikipedia
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