Johnson Raw Water pump: Brass or S/S screws best?

[biscuit]

Last season the screw at the top of my Johnson raw water pump failed due to dezincification, resulting in the cam plate becoming detatched and wrecking the impeller.
This was unfixable at sea, as the remaining part of the screw remained solidly jammed in the cam plate, and later needed to be drilled out. (a case for carrying a spare cam plate?)
The screw is identical in thread, diameter and length to the impeller cover screws. As I had some spare s/s ones, I am using one of these as a replacement to secure the cam plate, with a smear of loctite. I see that these screws are supplied as both s/s and "brass" from most of the spares suppliers (VPenta). Can any engineers (or metallurgists) out there enlighten me on which metal should be used here? The word "brass" never inspires confidence!

 

[Silvercloud]

Stick with stainless steel and you will negate the problem of dezincification

 

[vyv_cox]

Dezincification occurs in seawater. The screws should never see seawater if the gasket/O-ring is good. Mine are 20 years old brass ones, perfectly OK.

 

[RichardS]

I would also go for stainless ..... and use copper grease.

Richard

 

[RichardS]

Dezincification occurs in seawater. The screws should never see seawater if the gasket/O-ring is good. Mine are 20 years old brass ones, perfectly OK.

Could there not be some seawater penetration, perhaps through capillary action, under the plate and onto the bolt threads?

Richard

 

[vyv_cox]

Could there not be some seawater penetration, perhaps through capillary action, under the plate and onto the bolt threads?

Richard

Anything is possible! But a dab of grease should take care of that.

 

[VicS]

Dezincification occurs in seawater. The screws should never see seawater if the gasket/O-ring is good. Mine are 20 years old brass ones, perfectly OK.

Cam plate fixing screw , Vyv, not the cover screws

 

[RichardS]

Anything is possible! But a dab of grease should take care of that.

Indeed. It was the reference to an O-ring/gasket which threw me.

Richard

 

[vyv_cox]

Ah, my apologies. Should have read the OP more carefully. I have a very old pump that I have used for demonstrations. Its cam plate is attached with a brass screw, which seems to be in good condition. There is a small risk of galvanic corrosion with a stainless one, so in the end it probably makes little difference if regular inspection is carried out.

 

[biscuit]

Thanks, all, very reassuring!