Heat Exchanger zinc and threads

Kevin Henderson · August 04, 2014, 05:18:29 PM

Continue to have a nagging problem with my HX and zincs.
For a long time I have always felt that there was not enough room inside the HX for any zinc size that I bought. I had the HX cleaned a couple years back and since then I would regularly change zincs. Each time It felt like I was hitting something. This last weekend while trying to change it out I felt as though I may have "cross thread" the zinc nut. I have pulled the HX to clean and service and get a good look at both, the threads and the zinc length/fit.
As you can see in one phot the zinc you see is 1/2 thick and cut down to 1 inch and if screwed in completely would stop at the wall on the inside of the HX.
Secondly, I do think I may have cross thread and want to know if I can re-tap the threads in the HX.
Here's the details:
3" HX
11/16 zinc hex head bolt
3/8" zinc threads

Any suggestions or insights?

Ken Juul · August 05, 2014, 05:45:10 AM

Only way I know to clean up inside threads is with a tap. You can purchase one, or find a machine shop that will do it for you. Easy since you have it out. While it is apart, take some measurements so you know how long the zinc should be. We all have to cut them down.

Kevin Henderson · August 05, 2014, 07:54:25 AM

Quote from: Ken Juul on August 05, 2014, 05:45:10 AM
Only way I know to clean up inside threads is with a tap. You can purchase one, or find a machine shop that will do it for you. Easy since you have it out. While it is apart, take some measurements so you know how long the zinc should be. We all have to cut them down.

Thanks Ken... Thats the little jewel of info I have really been wanting to know. I'm guessing then that 1" or less is the length that all of us need to cut the zinc down to fit. Seems that everyone sells 2", 1 3/4' and 1 1/2".
Tapping out the HX tomorrow night with a buddy of mine that's a much more savy mechanic/machinist than I am.
Any idea on the size for the brass plug? seems to be about 1/2"

Stu Jackson · August 05, 2014, 08:18:26 AM

Quote from: Ken Juul on August 05, 2014, 05:45:10 AM
We all have to cut them down.

I don't.

I posted a picture a few years ago, sorry, haven't been able to find it yet.

The REAL answer is simple: check out YOUR HX with the zincs YOU buy. Take the end cap off and LOOK. I have consistently recommended that skippers remove their HX every two years or so to check for buildup in the ports (saltwater skippers).

Stu Jackson · August 05, 2014, 09:27:03 AM

Found it:

http://c34.org/bbs/index.php/topic,4279.msg24636.html#msg24636

Even a longer zinc would fit, and I have had different lengths (and widths) over the years.

That's why YOU need to check YOUR HX. Forget what any of us say. Your boat, your zinc.

And, Kevin, this link also answers your question about the threads.

Ken Juul · August 05, 2014, 11:57:03 AM

I have never seen a HX like yours Stu. That is too easy. All the ones I have seen look like Kevin's picture. I always have to cut mine.

Kevin it is in the link, but make sure your friend knows it is pipe thread.

lazybone · August 05, 2014, 12:57:33 PM

Kevin, if you are taking your HX to a radiator shop to be rodded ask them if they can braze on a new female pipe nipple.

That's if you find yours is knackered.

Ron Hill · August 05, 2014, 02:24:46 PM

Kevin : As Aldo mentioned go to a radiator shop and see what they can do.

I mentioned years ago that C34 owners should remove the old Zn and stick a #2 pencil in the hole to check the depth that the Zn need to be.

Also, go to Defender or Glen mar Marine to get new end caps and gaskets.

A few thoughts

Les Luzar · August 06, 2014, 11:50:16 AM

I have always had to cut my pencil zincs to the length of 1 inch above the threads, otherwise they seem to hit something in the HX. Consequently, I usually purchase four to six zincs at a time, and cut them all at home so when I need to change my zincs, they are pre-cut and ready to go. I also use the thin zincs, not the fat ones, and I change them every 4th month. This way there is still enough remaining to remove them and re-use the plug. The plug is re-useable, so there is no need to purchase a new plug every time you change the zinc. Also, keep in mind that since the threading is a pipe fitting, the plug will not screw in all the way. Once it is snug, it is tight enough. You can tell when you have it tight, and then stop. Do not over tighten it..... there will always be thread showing from the plug. This is normal.

mark_53 · August 06, 2014, 01:32:09 PM

QuoteI also use the thin zincs, not the fat ones, and I change them every 4th month

How many pencil zincs do you have? So far I have only found the the one on the HX.

Stu Jackson · August 06, 2014, 05:10:09 PM

Quote from: mark_53 on August 06, 2014, 01:32:09 PM
QuoteI also use the thin zincs, not the fat ones, and I change them every 4th month

How many pencil zincs do you have? So far I have only found the the one on the HX.

That is the ONLY one.

Kevin Henderson · August 06, 2014, 09:17:05 PM

Thanks everyone!

Got with my buddy tonight and we we chased the threads with a tap and now the brass plug screws into the HX correctly. As for length... Unlike Stu's Utopian HX design

Mine is limited to something just shy of an inch in length from the top of the threads if I use a 1/2" diameter zinc. If I use a 3/8" I may be able to squeeze about 1/1/4... But I think Ill stay with 1/2".

For anyone that has not taken out they're HX themselves, its a good drill to perform. I now know with certainty the length of zinc I need. My next step is to give it a good cleaning by soaking in some Lime Away. Replace the end caps and gaskets and re-mount.
Now.....

Where's that coolant burping thread?