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Old 13-10-2008, 10:24   #1
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Rebuilding heat exchanger

I have a Bowman heat exchanger housing dating from 1978 on a Thronycroft 4 cylinder diesel engine. It the type where an Aluminum housing is a fresh water reservoir with a tube bank running through it that carries raw sea water, the water enters and exits from big brass endcaps.

When I got the boat the housing and endcaps where so corroded they had detached form each other and where hanging loose. The engine itself had been pickled.

After much cleaning (there were some beatiful emerald colored crystals and the housing had about an inch growth of white powder), the endcaps seemed to have suffered no damage, and all the corrosion seemed to have been in the housing lips that engage the brass endcaps. Its amazing how they got chewed up. A new Bowman heat enchanger costs about 1/2 of what the boat cost me, so I want to avoid that.

The problem is that the housing lips will no longer hold the large o-rings in place and in some spots provide no backing for it.

Given that this is a low pressure application.. couldnt I just build up the material lost with bondo, or epoxy or fiberglass? I worry that welding an aluminum bead might defore the housing lips and then I would have to get the ends remilled (dont want to do that).

Any suggestions?

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Old 13-10-2008, 11:24   #2
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You can probably use epoxy to do the job.

What you will want to do is wire brush (preferably with a stainless brush) until all the aluminum oxide is gone. (The white flaky stuff.) Then take a nice heavy grit sandpaper 50 or less, and focus on the pitted areas to give something for the epoxy to key into.

Then take your epoxy product of choice, there are a few that work nicely... JB Weld, marinetex... to fill in the low and corroded spots. It will take half of what you think it'll need to do the job, but mix up enough. I'd suggest slow or normal curing speed and not the fast stuff, so you have enough time to work with it. The more prep work and time spent getting things flat will make the following steps easier, and increase your chances of success.

Once you have put a dollop of mixed epoxy on the low spots, take a piece of sheet metal that has a square edge long enough to span across the face of the aluminum housing, and use it as a screed to flatten the epoxy. Hold it at around 80 degrees and pull towards you. Go back and with a putty knife work a glaze of epoxy into small pits, nicks, or machining scars. Keep the epoxy away from the bolt holes!

Wait for the epoxy to cure.

While it is curing, take your bronze end caps and lay them on a precision surface. Use some prussian blue, or color the part with a sharpie to see how flat it is. and lay it on a piece of 600 grit paper. Work it around in different directions without applying any pressure to one particular spot. Count your strokes one direction, rotate a quarter turn... keep on going. Look at your work, you want more than 80-90% of the surface to be flat.

A hunk of granite is best, but any machined cast iron table will work. Think table saws, or any piece of equipment with the marks of a massive fly cutter. Glass will work, but you need a flat surface under it or it will flex slightly.

If your epoxy is dry... borrow the longest widest single cut mill file you can find. With no pressure on the file make a light pass across the face of the aluminum and epoxy. Do not cant, angle, or dig in, let it do the work. The shiny spots and smoth spots will appear... Only take off the high spots!

Now take some prussian blue, coat the bronze end cap and slap it up against the aluminum housing. You want coverage in even band of contact all around where the seal will sit.

Repeat your light filing, of only the high spots (high spots are where blue has transfered) on the aluminum until the surfaces are flat to each other.

Your file is wide enough and flat enough to span the gap of the oring. If you have contact on both sides, assume that all is well in the world between the two stripes of blue.

Now, install your new seal. Check to see what torque specs are on the bolts, failing that... use a torque wrench and torque the bolts in a star pattern to whatever your elbow says is good and tight. (You can also look up the bolt size in a machinist handbook and see what the torque spec is on the bolt... and cross your fingers that the guy designing the part did the same thing. )

Hope this saves you some money!


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Old 13-10-2008, 12:25   #3
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Tried an epoxy fix once on an old Yanmar Hx, think I used Marine Tex or JB weld, cant remember, probably JB. Did a great job with it out of the boat etc. It lasted about 5 minutes when warm.....
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Old 13-10-2008, 13:12   #4
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Don't put yourself through the aggravation. Get another heat exchanger. If it bolts separately to the engine you can probably find a refurbished unit that you can make work.
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Old 13-10-2008, 14:13   #5
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Thanks for all the replies. The problem is that I dont even know if the engine will run or what caused water to go in the engine in the first place that forced (two owners ago and over three years ago) the engine to be pickled, so I dont want to spend too much money on just a heat exchanger right now.

I checked the JB Weld website and it says its good up to 500 farenheit so it will probably be Ok in the short run. I'll try the JB weld method and report back in a month or so.

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Old 13-10-2008, 14:19   #6
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My son had a V8 5.0 in his 20 ft chris craft. There were some problems with it. I told him to replace it with a 5.7 litre V8 and new outdrive, and sell the old system on to recoup expenses. He knew better, and has discovered that the overall cost has been twice what it would have been if he had followed my recommend - and his boat is also slower than it would have been

I may let him forget this - in about 20 years time

The re-sale price is also less!

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