Heat Exchanger Patch

Hi Trawler Forum,

I have a Sen-Dure heat exchanger for a FL120 that was purchased as a backup. The heat exchanger on the motor is damaged and I want to replace it with the backup. The backup is pressure tested and looks good. I will replace the anode with a new Zinc and plug. I should have left well enough alone but I backed out one of the two drain plugs on the unit. I am pretty sure it is the raw water side but have not confirmed that. The drain plug did NOT come out clean.

So my question is: Can I use JB weld or Marine Tex to seal it back in? Or should I tap out the drain plug areas and install a new plug(s) - I don't know if the thread is NPT or a straight thread and I do not know how to tap an NPT I think the material is bronze but I am not 100% sure about that... It could equally be made out of brass, I don't know. Also, I could have the radiator shop that pressure tested it weld them shut and mark with a permanent marker "Do Not Remove" --

What would you all do? Outside of getting a new one. This one looks good, it tested properly, and I already own it.

Thank you for your help!

What do you mean by, did not come out clean? Damaged threads on the heat exchanger?

There's nothing special about NPT threads. They can be chased or tapped like any other. Just need the right taps.

If it is beyond chasing, any reason not to drill it larger and tap it for one size larger plug? Unless it mechanically barely a fit as is, should be room. Doubt it is too cramped, unless others have been here before.

Thank you gkesden,

Yes, that is what happened, the male plug end threads sheared off inside the tapped hole. I thought as much that it might be okay to tap the hole and install another plug. As long as I am doing one I should do both so the drain outlets can be used for their intended purpose. There is certainly enough room to tap it out. The only question is what metal the existing plugs are made from. Thanks for your response.

I had the same problem 2 years ago. The threads on the pencil zinc were slightly shredded. I bought a thread "chaser" tap and with extreme caution, "cleaned up" the exchanger body threaded hole. I used some thread cutting oil, turning the tap a 1/4" at a time while trying to ensure nothing dropped in to the hole. When done I replaced the zinc with thread sealant. So far, 2 years later, it has worked for me.

While NPT threads aren't 'special', they are unique in that it's a tapered thread. Pipe taps are made to create this taper, how far you drive it in determines the size of the thread, and how far the mating part installs, so watch your depth. You may find yourself tapping, installing, removing, tapping more, etc... Not rocket science, just annoying. Go slow and Good luck!

If the problem is that some of the male threads are broken off and stuck, rather than female threads being broken, you might try to "chase" the threads. And, by that I mean slowly and carefully tapping them the same size to clean them out and clean them up, rather than cutting new ones. If it doesn't work, you can drill, go up a size, and cut new ones.

What is the heat exchanger made of? Unless it is something weird, being on the raw water side, I'd expect the plugs to be bronze. It could be cupronickel, but even on exchangers made of cupronickel, I think they often opt for softer bronze plugs. And, well, since they broke...

You can probably just call Sen-Dure and ask what the plugs are made of. And, if you can just chase the threads, they can probably sell you new ones.

Silver solder a ball valve for drainage.

Santana said:

Thank you gkesden,



Yes, that is what happened, the male plug end threads sheared off inside the tapped hole. I thought as much that it might be okay to tap the hole and install another plug. As long as I am doing one I should do both so the drain outlets can be used for their intended purpose. There is certainly enough room to tap it out. The only question is what metal the existing plugs are made from. Thanks for your response.

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I would expect a good outcome from chasing the threads with an NPT tap. The taper should help getting the tap started. Be sure to get the tap engaged properly in the old female threads so you cut out the remnants of the plug, not the body of the heat exchanger. Work the tap back and forth once it is well engaged in the female threads, the old plug thread material will probably come out in chunks and you want to let them clear.

Sounds like part of the drain plug is still in the exchanger? If so, will it leak if left as is? You could use some epoxy putty to seal it off. No big deal not having a drain plug, just remove the cap on the end of the heat exchanger and it will drain.

Can you pick out the male thread pieces with a dental pick? I have found them useful for similar work.

Heat exchangers and coolers are typically made of copper but many are copper nickel for better corrosion resistance. Plugs are typically brass as are the fittings soldered to the main housing. As others have mentioned, recutting the threads is easiest approach. If you use the same dia tap the bottom threads may be useless. If you have 6 good threads left you may be OK. Just coat the drain plug with some sealant and don't over snug it.

My fittings have a tube right under the drain plug and I believe they are 5/16-28. I would need a bottom tap. Check yours with a thread gauge.

Note that drain plugs are merely a convenience. You can drain the exchangers by pulling the hoses off.

You should have no trouble chasing the threads with a NPT tap. Chase the threads, work the tap back & forth, your objective is to sort of polish the thread surfaces. Blow out the debris with compressed air, or use a wire brush to remove any leftover burrs. Clean the mating surfaces with brake cleaner, then re-install the drain plugs. I'd use LeakLock or other dope on the threads, you can use a good grade of heavy teflon tape and dope on top of that if the fit is a bit loose. The tapered thread can be chased pretty deep and still seal up OK.

Thank you, everyone, All great answers, and suggestions. I will let you know how I get on. It is a fine thread 32 thread pitch or a metric 1 fine thread pitch. I might have that terminology off but hopefully, my explanation is enough to explain the approximate size. So to use the correct tap, this is info I need. I put a call in and I hope to get an answer. The dental pick is a good idea to potentially clear it out... it is soft metal. I am going to hit it hard tomorrow and hopefully be done quickly and accurately with no leaks!!

Thanks!!!

Santana said:

Thank you, everyone, All great answers, and suggestions. I will let you know how I get on. It is a fine thread 32 thread pitch or a metric 1 fine thread pitch. I might have that terminology off but hopefully, my explanation is enough to explain the approximate size. So to use the correct tap, this is info I need. I put a call in and I hope to get an answer. The dental pick is a good idea to potentially clear it out... it is soft metal. I am going to hit it hard tomorrow and hopefully be done quickly and accurately with no leaks!!

Thanks!!!

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You may have fixed it by now, but those numbers don't sound right. 32 threads per inch is pretty fine, NPT threads are much coarser. Also, 1mm would be 25 threads per inch. I don't know metric pipe fittings, but here's a quick chart of NPT (inch) sizes. Note- Pipe threads are specified by the ID of the pipe, and the actual diameter of the fitting is much larger.
One trick to determine the size is to go through your drill bits and find the largest one that will fit through the tapped hole; this will be close to the 'tap drill' listed.

Kirwan said:

You may have fixed it by now, but those numbers don't sound right. 32 threads per inch is pretty fine, NPT threads are much coarser. Also, 1mm would be 25 threads per inch. I don't know metric pipe fittings, but here's a quick chart of NPT (inch) sizes. Note- Pipe threads are specified by the ID of the pipe, and the actual diameter of the fitting is much larger.
One trick to determine the size is to go through your drill bits and find the largest one that will fit through the tapped hole; this will be close to the 'tap drill' listed.

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Thank you! Yes fixed installing today - have two new gaskets, o-rings and anode pencil. I hope it all goes okay.

And yes the chart is correct it is NPT 1/8" with a 27 thread. Took me a little time to figure all that out.

Greetings,
Don't you all agree that if the US subcribed to SI measurements (metric), life would be a LOT simpler? 27 TPI. What engineer came up with THAT?

This past summer after changing the anodes in my Sp135's, I decided to remove the drain plug on the oil cooler (not sure why I decided that). As soon as I turned it, the boss the drain plug is tapped into just crumbled. I wasn't in port so I plugged it up with some epoxy putty and cruised that way the rest of the season. Yesterday i replaced the cooler with a new one from American Diesel. It was the starboard engine so that cooler is on the outboard side of the engine. I was really dreading the job but was pleasantly supersized it went so smooth. Had it changed in about 30 minutes.

RT Firefly said:

Greetings,
Don't you all agree that if the US subcribed to SI measurements (metric), life would be a LOT simpler? 27 TPI. What engineer came up with THAT?

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a full description of any NPT thread will fill this page! A lot of ROW doesn't know how to properly tighten an NPT fitting. With good reason, there is no torque spec.

RT Firefly said:

Greetings,
Don't you all agree that if the US subcribed to SI measurements (metric), life would be a LOT simpler? 27 TPI. What engineer came up with THAT?

Click to expand...

Personally I think if Europe would switch to our measurements that would ALSO work. Personally I think its stupid that 24mm is not exactly an inch. Who came up with that weird measurement??? The math to deal with partial mm takes much longer.

jimisbell said:

Personally I think if Europe would switch to our measurements that would ALSO work. Personally I think its stupid that 24mm is not exactly an inch. Who came up with that weird measurement??? The math to deal with partial mm takes much longer.

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I grew up with imperial and had to learn metric.
That said math is based on 10, metric is based on 10, so who decided to make imperial based on 12?

Greetings,
Mr. j. Re: Post #19: Well, since the rest of the world has gone metric (SI),save 2 other countries I really don't think there is much chance of Europe going back to an archaic system where measurement units are based on the length of some Sovereigns appendage although the US is in good company. Myanmar (Burma) and Liberia are also users of the Imperial measurement system. I was schooled in the Imperial system and I remember well having to memorize 5,280 ft. and1,760 yds/mile, 16 oz/pound and 40 oz/quart. WHAA???? There are only 32 ounces in a quart...See the problem?



During my working career, the vast majority of ANY measurements used were metric. Temperature, mass, volume, pressure etc. so I'm quite comfortable using the metric system.


I suspect it would cost many, many millions of $$ to convert American manufacturing to a fully metric system BUT it IS coming and at THAT point I won't ever have to try to find an 19/32 wrench again.



So, 5 1/2 yards in a rod but how many rods in a mile and a half? (Hint: 480). Personally, I think an inch is a stupid unit.

You have to watch the use of thread sealant or Teflon tape, paste on zinc plugs,,,it should be metal to metal contact and the zinc is sacrificial..it may not leak, but wont be metal to metal,,,u can check continuity if sealant is on there, if not I would take it off, before the cooler gets ate up inside,

RT Firefly said:

...


I suspect it would cost many, many millions of $$ to convert American manufacturing to a fully metric system BUT it IS coming and at THAT point I won't ever have to try to find an 19/32 wrench again.

...

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I suspect not a high percentage of fasteners are US made. Maybe now is the best time to convert.