Heat Exchanger Zinc Size

[Mike McDonald]

I purchased a couple of magnesium anodes (zincs) for my heat exchanger.  I went by the specs and ordered E-1Z-M Engine Magnesium Anode from BoatZincs.com.  When I removed the old one, it was virtually gone.  Just a stub was left.  The thread diameter of the new one was too large for the fitting that it screws into. The specs on the new ones that I purchased are below.
MODEL    DIAMETER  LENGTH    WEIGHT (LBS)    UNC THREAD    
E-1Z-M       1/2"      2"               0.10               3/8-16    

Apparently I have a different heat exchanger.  I don't see any model info on it.  It is approx. 3" diameter and kind of a maroon color.  The zinc fitting is located on the aft port side of the body of the heat exchanger. I have attached a rather poor picture of it that I had on the computer.  Can anyone tell from the picture what type it is and if so, what size zinc I need to buy.  It appears to require a thread diameter of approx. 1/4" or 5/16".  Not sure of the thread size or the overall anode length.  I would appreciate any help you can provide.  I can try to get a better picture if that will help.

I have a 1987 C-34 with an M25XP engine.

Thanks.
Mike.....

[Stu Jackson]

Mike, that's a standard 3 inch HX for the M25XP.  Nothing special.  When we got our boat, I found it easier to purchase those kind of "I don't know what size these really are" things from a local chandlery, bring the old one in and match 'em up.  Then I wrote down where I got them and what size and model number they were.  For zincs, I bought in bulk, half dozen at a time.  IIRC the bronze head is 11/16" box end wrench size.

[Les Luzar]

I have been re-using my same bronze head nut since I bought my boat six years ago, so no need to purchase the bronze heads, simple re-use your current one. Also, I use the pencil zincs (I think it is a 1A), which you can get at West Marine or any chandlery. Measure the depth that the zinc can penetrate into the HX, because in my case I have to cut off about a quarter of an inch of the top of the zinc to insert it, otherwise it will hit the rods in the HX and won't fit. Consequently, I inspect and change my zinc every three months. I don't want to change it before I cannot remove it from the bronze head.

[Mike McDonald]

Thanks for the input.  I'll check with West Marine to see if they have them.  I'm not sure of the length, since the one I removed was only a stub.  The boat came from salt water and had zinc anodes, the boat now is in fresh water, so I'll be using Magnesium.  I guess I'll bring the bronze head to insure the proper thread size and diameter.  Thanks again.
Mike....

[Ron Hill]

Mike : This has bee written up by me many times:

The diameter could be 3/8" or 1/2" in the 3 inch HX on the M25XP engine.  It depends on the Semdure 3" HX.

On the length you'll have to measure it yourself.  Take a #2 pencil and stick it up into the hole after you remove the Zn HX nut.  Then compensate for the thread length and cut the new Zn to that length.  Maybe you won't have to cut at all, but you won't know that until you measure with the pencil.  It all depends on the position of the cooling tube array inside the HX.

A few thoughts