This really bugs me... Zincs

[drh1965]

Does this happen to anyone else? The pencil zinc in my heat exchanger always disintegrates right where it screws in to the brass fitting that screws into the HE. I end up replacing these things every few months with 90% of the zinc still intact. Basically, about 10% of the length of the 3/16" thick zinc shrinks down to a thread at the brass fitting every time. Are there better zincs out there where this doesn't happen?

 

[Zaphro]

I haven't found a better or for that matter a different zinc. My solution was the 'fish' zinc. I clip it to the engine and run the cable out over the side when in port. Seems to help the zincs attached to the shaft, prop support and engine last much longer.

 

[Ted]

Zincs

I don't have this problem on my engine. (Yanmar 2GM) I don't know if this has any bearing on your problem. My pencil zincs have a gasket between the zinc and the plug it is attached to. I wonder if this may prevent the premature deterioration of a zinc at the threads of the plug. Do you have a gasket on yours?

 

[NotCook]

Or, does this even prevent the zinc from functioning as a sacrificial zinc at all? Is there contact at all between the zinc and the HE? If not, there wouldn't be much point.

 

[PeteK]

Or, does this even prevent the zinc from functioning as a sacrificial zinc at all? Is there contact at all between the zinc and the HE? If not, there wouldn't be much point.

Wouldn't the threads still make the necessary contact??

Pete

 

[Paul Cossman]

checking pencil zinc continuity

Or, does this even prevent the zinc from functioning as a sacrificial zinc at all? Is there contact at all between the zinc and the HE? If not, there wouldn't be much point.

I wondered the same thing after I installed my zinc with teflon tape around the threads. I checked it with the continuity function on a digital multimeter and found there was solid continuity between the HE and the zinc bolt head.

 

[jibes138]

The purpose of the teflon tape is to fill in the gaps in the mating screw threads to prevent leaks, not to galvanically isolate the materials. Screw threads are intended to make contact at the pitch diameter and as they are torqued up the threads deflect a bit acting like a spring until the torque increases enough to shear the materials and result in a stripped thread. The major diameter of the external thread and the internal thread will have clearance built into the dimensions causing a very small gap all along the thread that will allow leakage. The teflon tape fills this gap and prevents the leaks. The tape is so soft that as the bolt is torqued it will smear out of the way and you will have metal to metal contact along the pitch diameter of the threads for electrical continuity.

 

[drh1965]

A gasket, eh? None on mine. I don't really understand where it would go either. Are you saying that where the zinc threads in to the brass fitting, there is a gasket of some sort?

I also don't use TFE tape. I wonder if water gets into the brass fitting and this accelerates the deterioration. I may try some tape as an experiment.