Bill Rouse sent me a personal email (he does not have a lot of internet access currently) telling me that the Onan uses the Vetus Waterlock type LP50. However he also pointed out that the Onan has a temperature sensor and automatic cutoff - so maybe a spare waterlock for the Onan is not as necessary as for the Yanmar.
I think I will just get a spare waterlock for the Yanmar.
Eric, getting a Yanmar waterlock constructed from metal sounds intriguing. What did you use - stainless steel? Any chance that your source would make more of them? Given how often impellers fail, I imagine it would be cost effective. Even the Vetus plastic ones are not cheap.
