I think you are correct that it needs to be grounded to the block.  I had
the struts deterioriate on an drive leg of an inboard/outboard even though
the zinc was present - turned out the zinc wasn't grounded to the struts.
However, marine engineers on the list could offer otherwise.

>From: Stuart MacMillan <stuart@cobaltgroup.com>
>To: Gregg Howald <ghowald@HOTMAIL.COM>
>CC: vanagon@GERRY.VANAGON.COM
>Subject: Re: Electrolysis and prevention of corrosion:
>Date: Thu, 17 Feb 2000 14:42:00 -0800
>
>I am not enough of an expert on electrolysis to know, but it probably
>will work.  I'm just thinking about how the current might flow in the
>damaging electroylsis, and having the zinc electrically attached to the
>engine seems like it would make sure that the zinc goes first and not
>the case.  Pure conjecture on my part, and probably bogus.   I think the
>best solution is to stop the electrolysis by good grounding and keeping
>the highly conductive acids from forming in the first place.
>
>We need a marine engineer who specializes in this on the list!
>
>Gregg Howald wrote:
> >
> > >The zinc should be grounded to the case, and that is how the $50/pair
> > >Z-Sticks from FGA work.  They are supposed to replace the water jacket
> > >drain plugs on each side of the case. Kinda pricey, but probably worth
> > >it.
> >
> > So, are you saying that simply adding zinc to the coolant reservoir is
> > inadequate?
> >
> > >I may experiment with attaching a water heater sacrificial anode to one
> > >of the plugs in a core I have. It would be another nice "trick" to
>throw
> > >at the problem along with using Dexcool.
> >
> > Why wouldn't the zinc alone do the trick?  Seems to work in the marine
> > engine environment.
> >
> > G.
> >
> > >
> > >--
> > >Stuart MacMillan
> > >Manager, Case Program
> > >800-909-8244 ext. 8208
> > >
> > >Getting your share of the Net yet?
> > >http://cobaltgroup.com
> > >http://UsedEquipNet.com
> >
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>
>--
>Stuart MacMillan
>Manager, Case Program
>800-909-8244 ext. 8208
>
>Getting your share of the Net yet?
>http://cobaltgroup.com
>http://UsedEquipNet.com

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