TOPIC: Rude impeller keyway

Changing the impeller on a '78 70hp Evinrude model 70873C. All spec tables say the impeller part # is 0382547. I ordered an online part from a reputable marina and they sent the correct part. Today I removed the LU to change the impeller and surprised to see that the keyway is much different. The impeller that was in there had what looks like a nylon half crescent moon and the impeller itself has a graduated cutout to accept this key. I've never seen one quite like this. I went to my local OMC parts dealer and all he had was the normal impeller with the straight keyway. The driveshaft is normal with a flat surface where the normal key would be. I recall one other impeller that had a normal keyway but the key itself was stainless that would fit the flat shaft and a normal keyway on the impeller. Any suggestions on either where I can get the correct impeller, or the key that would fit a correct part number?

Found what I was looking for, called a wedge type impeller. Wonder why it's not listed as an OMC part on most charts.
Johnson / Evinrude 40-75 Hp Wedge Type Impeller OEM #: 0396725,

Looks like the split was 1979, so maybe your '78 was a late production run with the "upgraded" shaft and impeller. Here's a '79 at marineengine.com which shows the wedge-type impeller being used (#58 on the diagram):

www.marineengine.com/parts/johnson-evinrude-parts.php?year=1979&hp=70&model=70973R&manufacturer=Evinrude§ion=Gearcase-20+Transom

www.marineengine.com/newparts/part_details.php?pnum=OMC0396725

Probably right. I bought the other impeller on line and I've contacted them. Hope to return the one I have and maybe they have the other. Just have to pay shipping both ways.
Guess that's the price I pay for ordering the part before I removed the LU. Too soon old and too late schmardt.

A couple of other scenarios for you to consider. The gearcase could have been replaced on the motor & therefore picked up the newer design? You're right it can be confusing if you don't look before you order. The other thing is by looking at your shaft I think the motor originally had the interim impeller/key setup before they went to the wedge. The flat shaft design came about to stop shaft breakage. The first design had a round roller with a corresponding round slot cut in the impeller. The roller would move back & forth across the flat on the shaft. Therefore no wear to speak of. If you look at your shaft you can see this pattern. The later wedge type key & impeller will not leave this pattern & is cheaper still to produce. The latest design works well in salt water as well as fresh & is the accepted way of driving an impeller. Just thought you might like to know how it became this way?

Dan in TN

FYI, there were 2 different plastic keys. One was white & one black. I believe the size was slightly different.

I've changed several of those with the roller pins, I've also encountered a flat shaft with a SS key that fit the flat surface and a bump that fit the impeller. This is a first for me. The key on this one is white nylon. Funny thing is that when I first opened it up the white piece of plastic fell out and I thought something was sucked up. Wasn't 'till I looked at the setup I realized what I had. I ordered another impeller that comes with a new key from marineengine yesterday. I'll keep the other impeller 'cuz I have another motor that it will work on...….I think