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Message: <blockquote data-quote="LarryMc" data-source="post: 427182" data-attributes="member: 13895">You should check your impeller so that you will know for sure what you have. The aluminum 6 7/8 has the part number 10623 stamped on the lower end. The stainless has the part number 855708T60 cast on the inside of the upper end. Whoever converted the motor may have already swapped a stainless for the aluminum. You won't know until you look at it.I have tested all three types of impeller in my motor trying to get the best performance. In my boat running in still water, the aluminum and stainless 2S impellers both run 4900-5000 RPM with 23-24 MPH. The cutback stainless 4S impeller runs 5100-5200 RPM and 25-26 MPH. The 2S impellers gave a little better holeshot but less max speed. As you can see, for practical purposes, there is not a lot of difference. At least, as far as my boat and motor are concerned.Both the Stainless 2S & 4S impellers use the same part number, so you have to look at them to tell the difference.The first photo below is the 4 stroke impeller alongside the 2 stroke impeller (4S on the left). The second photo is a closeup of the 4 stroke impeller</blockquote>