They have different part numbers (377178 - 10hp, 375638 - 18hp). I don't know what the difference is. Mabey the hub?<br /><br />But, pre-58 10hp impellers are the same as those used on '53-'65 15-18hp engines. Mabey you have the wrong spare?
Found this. <br />There's a later post where he corrects the info:<br />#2002 : Sierra 18-3002 (Early 10s and 15/18hps)<br />#2003 : Sierra 18-3003 (Later 10s)
