I got one one those impellers and sanded it down but decided not to use it. Those raised portions are meant to make a seal against the cover and inside the cartage so I feel the thickness does matter. I think I got the thickness OK. The problem I have with this modification is you end up removing all the rubber from the brass center. Now the brass is in contact with the aluminum, no mater way which you face it. I couldn’t come up with a way to remove some of the brass from the center, so I sent a couple of bad impellers to Brian. I take them out while changing the gear lube when I’m done running one of these motors for the season. They’re really easy to get at on these.
A "Boathouse Repair" is one thats done without having tools or the skills to do it properly.