changing bolts on heat exchangers; let's get down to brass tacks

Nov 24, 2014
130
Catalina 310 Staten Island
Last week, I was doing routine engine checks on the Universal M25xpb diesel on my 2003 Catalina 310. I noticed a slight drip from the port side end cap of the 18 year old heat exchanger. I tightened the 3/8 diameter brass bolt and it snapped right off. I removed the heat exchanger, and took it home in an attempt to drill out the bolt. I ended up replacing the unit, as it looked severely corroded on the inside. The aftermarket Seakamp unit, like the original is bronze, or some other copper-tin alloy. So are the bolts holding the end caps in place. I am contemplating replacing them with much stronger stainless steel. Would the benefits of having stronger bolts outweigh the disadvantages of galvanic corrosion between dissimilar metals? To reduce this effect, I am considering using an anti seize compound on the threadss I also though of draining the seawater via the petcock after each use. Any thoughts or feedback, please respond.
 
Jan 30, 2012
1,093
Nor'Sea 27 "Kiwanda" Portland/ Anacortes
My opinion of course -- the old bolt , the one you broke, might have been more brass than bronze. Brass contains high concentrations of zinc and the zinc is lost in dissimilar fastener assemblies. Probably you will have noticed a pink-red color in the broken bolt which means the zinc gave up so the bolt became useless.

As to the new HX - concerns over dissimilar metals are automatically eliminated by using the bolts the manufacture provides. As to anti seize - if you use these lubricants at all - do not use standard auto supply Antiseize. The off- the- shelf stuff is a sliver - gray color. It contains aluminum and aluminum and copper alloys together in fastener assemblies is a no-no. You can obtain Antiseize copper at Amazon. You could use a dab of grease for that matter.

Where assemblies are subject to vibration you would do well to use a softer alloy fastener. Also the alloy of stainless you get from the hardware store is quite unknown. Probably Seakamp provided the bolts they did because those are the best choice.

Maybe call Seakamp in Bellingham Wa and get their opinion?

Charles

p.s. Remember to change the zinc in the exchanger regularly.
 
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Jan 4, 2006
3,914
Hunter 310 West Vancouver, B.C.
I am contemplating replacing them with much stronger stainless steel. Would the benefits of having stronger bolts outweigh the disadvantages of galvanic corrosion between dissimilar metals?

You have two things going for you here:

1. The heat exchanger head being a mystery metal with a copper base is getting reasonably close in galvanic reactivity to the SS fasteners you are looking at installing.

2. The big thing in galvanic corrosion is surface area. If the anodic area (corroded part) is big and the (cathodic) area is much smaller, you're in much better shape than the reversal of areas. You've got small SS bolts (cathodic) and the big head (anodic) so you should be good.

If you're anything like me, you'll have it apart after the first year of service to see if there's any corrosion going on in there. I'd be surprised if there was.

I also though of draining the seawater via the petcock after each use.

That's a bit extreme even for me.

I am considering using an anti seize compound on the threadss

Just grab the closest grease at hand and that should be good enough. Maybe insert a lock washer at the nut as the grease may make things a little more suseptible to backing off due to vibration.


I ended up replacing the unit, as it looked severely corroded on the inside.

Be sure to follow @Charles Erwin 's recommendation :

p.s. Remember to change the zinc in the exchanger regularly.

and watch those internal anodes.
 
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Ward H

.
Nov 7, 2011
3,130
Catalina 30 Mk II Barnegat, NJ
What was the condition of the zinc on the old HX? On my M25-XP, there was a sticker on the HX saying to check the zinc every 3 months or something like that. (Sticker was lost when I had the HX reconditioned)
I agree draining after each sailing sounds extreme. I do remove my zinc, it's at the bottom, to drain the HX as part of winterization and install a new one in the spring.
 
Nov 24, 2014
130
Catalina 310 Staten Island
My opinion of course -- the old bolt , the one you broke, might have been more brass than bronze. Brass contains high concentrations of zinc and the zinc is lost in dissimilar fastener assemblies. Probably you will have noticed a pink-red color in the broken bolt which means the zinc gave up so the bolt became useless.

As to the new HX - concerns over dissimilar metals are automatically eliminated by using the bolts the manufacture provides. As to anti seize - if you use these lubricants at all - do not use standard auto supply Antiseize. The off- the- shelf stuff is a sliver - gray color. It contains aluminum and aluminum and copper alloys together in fastener assemblies is a no-no. You can obtain Antiseize copper at Amazon. You could use a dab of grease for that matter.

Where assemblies are subject to vibration you would do well to use a softer alloy fastener. Also the alloy of stainless you get from the hardware store is quite unknown. Probably Seakamp provided the bolts they did because those are the best choice.

Maybe call Seakamp in Bellingham Wa and get their opinion?

Charles

p.s. Remember to change the zinc in the exchanger regularly.
The bolt looked to be 100% brass. I did not notice any reddish discoloration on the broken bolt.
 
Jan 4, 2006
3,914
Hunter 310 West Vancouver, B.C.
COMPLAINT :yikes::mad::yikes::mad:

Why did you change your avitar ? ? ? I thought it was most interesting.

PS: go with the SS bolts.
 
Nov 24, 2014
130
Catalina 310 Staten Island
What was the condition of the zinc on the old HX? On my M25-XP, there was a sticker on the HX saying to check the zinc every 3 months or something like that. (Sticker was lost when I had the HX reconditioned)
I agree draining after each sailing sounds extreme. I do remove my zinc, it's at the bottom, to drain the HX as part of winterization and install a new one in the spring.
When I bought the boat 6 years ago, the condition of the zinc was non-existent. I am the fourth owner, and the vessel had very light use; only 350 hours on the engine over 12 years. It sat at the dock most of the time with stale sea water sitting inside. My guess was that the zincs were changed about as often as we change POTUS
 
Nov 24, 2014
130
Catalina 310 Staten Island
COMPLAINT :yikes::mad::yikes::mad:

Why did you change your avitar ? ? ? I thought it was most interesting.

PS: go with the SS bolts.
Probably take your advice, after calling Seakamp for their opinion. If you are curious about my avatar change, feel free to IM me. Some things are frowned upon in this forum