Heat exchanger Zn

[Randygh]

Several weeks ago I drained my exhaust manifolds and heat exchanger. The unit is mounted vertically and the Zn anode plug is at the bottom. I put in a new anode (1/2" dia x 2" long) plug in the fall of '03. I was amazed by how much of the anode was eaten away. It looked like a wooden pencil chewed on by a first grader. My boat has been salt water for an approximate total of about 1 month and fresh water 3 weeks in the last 2 years. If the boat was in a salt water slip, I'd need to change that Zn every couple months. Is this normal?

The Zns on the outdrive and stern are alittle moth eaten by nothing like the exhanger Zn.

[Tedster]

Those pencil zincs do go fast, but unless you drain your system each cruise no matter what water you are in they will be eaten at a much faster rate than the other zincs the I/O has. The good news is they are cheap, buy them in bulk. :D

[Randygh]

I picked up a couple of them from Fisheries. I'll change it each fall when I drain everything.

[Numbknots]

Randy,

I have two of them on each heat exchanger on my Detroit 8.2 litre diesels and I check/replace them every 90 days. One reason they are consumed faster is they are submersed in "warm salt water" (standing water) after the engines are shut down and the water in the manifolds/exchangers assume ambient "engine/room temp". It is thought of by many "experts" that zinc's become less effective if they are submersed in fresh water and then brought out/dried and then dunked again into salt water? (I have been told that the zincs will glaze over and be insulated by the algae--etc.??)

Another thing to note: not all zincs are of equal quality as to content, (ask you local boaters and they will likely have a better handle on which ones to use for best results.

Also another no-no----Don't use teflon tape to seal the brass pencil zinc adaptor to the manifold or heat exchanger as it is imperative that a good bond exists between the Zinc holder and the attachment metal.

http://www.boatus.com/boattech/casey/23.htm

for some more good reading (porcelin library material is this article:
http://www.kastenmarine.com//mbqCref.pdf

Tim

[Randygh]

Would it be ok to use a copper base antiseize paste on the plug threads? I would think the compound wouldn't interfere with the flow of electrons between the anode and the exchanger. Would the compound benefit the flow of electrical current?

[bradvo]

Why would you need to add anything? It should seal fine without a paste? have yuou tried?
Brad

[Randygh]

The plug will seal without antiseize, but what about the plug seizing to the exchanger? It probably won't be a problem because I'll change it annually.

[Numbknots]

Randy,

Most of the Pencil Zinc's I am familiar with are threaded into a Brass NPT adaptor and should in itself provide an "antisieze component" between the two differnet metals? (The NPT is a tapered thread and you should not have to tighten it more then "firmly snug" or equivelent torque for the size of the thread.

Tim

[Randygh]

Tim--Thanks for the info. :D :D

[Go Aweigh2452]

I pulled mine after ss showed me where it was. I have had it there for 4 years, took a 5/8 wrench and it came right out and replaced same in 30 secs. No seizing or anything noted. Brass plug looked great even w/o the zinc which was long gone...

[Numbknots]

I used to have a 94-2452 with a 350/alpha11 with fresh water cooling/heat exchanger. I learned there are a couple critical areas to watch out for:

the Exhaust manifolds are cooled above the elbow by "raw water". the only seperator between the "coolant" and the raw water is an aproximately .020 stainless steel plate that seperates and connects the exhaust elbow and the exhaust manifold.

The Exhaust manifold is Cast Iron and is very suseptable to rust! It also will heat-up and and crack almost imediately if the engine is ever started wthout cooling water in the Exhaust system. The Bravo11 has a pump in the lower unit as well as the belt driven pump. It is critical to keep "both" serviced and working properly to get adequate cooling.

the short version is I had what I thought was a bad starter/battery one day while fishing on the Columbia river. The engine would not turn over, then after a couple of tries it would and all was good (so I thought) Well I was really puzzled and decided to put the lower unit in a drum and started the moter at home. almost instantly I noticed a slight white smoke and the smell of antifreeze? I hadn't noticed any significant drop in coolant from the overflow and in the water I couldn't smell the antifreeze and when the engine warmed up (and underway) I couldn't see any smoke?

I I pulled the plugs and found one on the right side was looking considerably "whiter".

I put a radiator pressure tester on on the heat exchanger and found that sure enough there was a leak that could be heard in the exhaust (seperated the bellows) and as well it could be heard/felt in the plugh hole that was showing "white". (as long as the exhaust valve was open)

I pulled the exhaust manifold/elbow and found the crack under the stainless gasket (corroded badly) and the manifold had a hairline crack between the water jacket and the exhaust riser.

Soooo what was happening was (only if the engine stopped with that particular cylinder with the exhaust valve open) water/ antifreeze would drip into the cyclinder and would 'hydrolock' the engine. If I could bump it over with a strong battery then the rising piston would force the water out and the engine would start an run (seemingly normal) Luckily I found the problem, and secondly the engine didn't start with water in the cyclinder and bend a rod!!! $300 for a new manifold, Merc's improved gasket/spacer and 3 days of knuckle busting and the problem solved and a lot learned as to what to keep maintained and look out for in the future.

Hope my little lesson can be of some value to others. especially if you do (as I did) and buy a used boat where it was mostly in fresh water but the PO had last used it in salt water, brought it home, never flushed the heat exchanger or the exhaust manifolds and then let it sit in its own brine for the winter months.

Tim

[Go Aweigh2452]

Tim, good advice for sure. I flush my engine after every salt dip she takes. Run her for at least 10 but normally 20 mins with the muffs on. I even go so far as to pull the bildge drain plug and reverse fill the engine compartment until I get the rear bildge pump to run and flush the salt water out of that. (I got the dog once when he was standing too close to the pump when it lit off... didn't see him for about an hour...) ;)