Correct screws for 5.7 water pump

I need new socket head cap screws (3/8-24-1 1/4) to attach the P105 water pump to the hamonic balancer. I believe the original screws are black oxide coated and they have rusted terribly (inside the socket head, the threads are fine) The failure mode on these pumps is usually a leaky seal dripping salt water so I get a repeat performance every time.

I have the choice of alloy steel in black oxide, chrome plated, zinc plated or of 18-8 stainless. What is going to provide the best protection from salt water corrosion? Is there a compatibility reason I should not try stainless?

Whatever I buy, this time I plan to pack the socket with anti-corrosion marine grease which I suppose might make the problem go away.

Any thoughts?

Based on the choices you offered, I'd lean towards the zinc.....18-8 isn't 304 or 316, and if it's imported, I'd not even go there....

I've had good luck with nickel plating but they aren't "readily available"....and an anti-corrosion spray on may be more effective than the grease - and it can't shorten your belts' life either...

Cabo,

Would like to follow your progress. Actually, 18-8 stainless is Type 304 and has good corrosion resistance, however Type 316 has better resistance and is readily available. If strength is a concern, PH 17-4 (Type 630) is the way to go. Corrosion resistance is about the same as 304 but cost and availability are a factor. Every time I replaced an old corroded zinc-plated bolt I used a 316 replacement (like on raw water pump brackets) and never had a problem. Let us know what you decide – and in a few months you can share the rest of the story.

Thank you both for the guidance. Since all I knew about steel metallurgy I learned from "Forged in Fire" I did look up what all this stuff means. As said, 18-8 is a generic term (18% chromium, 8% nickel) and 304 is the same stuff except that 304 also specifies carbon, which I suppose would be important if heat treating is needed in the manufacture of the part. In any case, there are none of these bolts available in 304 although they are common in 18-8. 316 is the same as 304 except that chrome is down to 16% and there is 2% moly added to bring about even better resistance to chlorides (i.e. seawater). So 316 is the best stuff for boats.

I could not find any 316 socket head cap screws in my size. Grainger had the 1 inch version which would work (I have used that size before as there are no 1 1/4" in my part of Mexico). Still, I wanted to use the size that the engine used when new. Also Grainger here (everywhere?) is a pain to deal with. They don't like walk-in retail customers and in the past I had to beg to make a purchase.

Fastenere also has a local outlet. They sell the correct size in 18-8. They wanted more than twice as much as Grainger ($2.43 each), but it turns out Fastenere sells the same product on Amazon for $1.68 each. In the end I was tired of messing with it and did not want to make a long drive, so I just placed a "one-click" order for that.

I don't believe strength is an issue at all since the torque is taken up by some pins that go into the pump flange. The bolts just keep the pump seated. The best I could do on country of origin was "various countries" so I guess that is code for China. So let's hope the 18% chrome is really there. As I write this I think if I had it to do over, I would go to Grainger and try and get some in 316 (but a little short). But life is full of compromises and what's done is done.

I won't put these on until I return to the boat in Mexico in the fall when I replace a dripping pump. Then if everything works to the usual schedule the new pump will be dripping and ready for replacement sometime in the spring of 2020. We'll know how the 18-8 stands up at that point. And as Mark suggests, I will hit them with T-9 everytime I am doing something on the engine.

Cabo,
Agree with all. Another on-line source for fasteners which also provides good data is https://www.mcmaster.com/#. I have also noticed raw water pump fasteners and the raw water pump bracket are especially vulnerable to corrosion and need special attention to minimize rust damage. Definitely a good application for stainless bolts. I prefer to replace socket head bolts with hex heads in applications that are especially prone to rusting if the configuration allows. If a socket head fitting strips out and you cannot get a pipe wrench on the head could be a big problem. I also goop the threads with anti-seize. Thanks for sharing your homework !!

That McMaster-Carr link is a good one, very easy to find stuff and compare prices. But what prices! They do have the elusive 316 screw in 1 1/4" But they want $3.27 each. And when you look at the price for the more popular and more easily available 1" version they want $3.55 for it (about 3x Grainger). So these prices are clearly "retail" and most of their customers must get a wholesale discount. I did not bother to see what the upcharge would be for shipping, but on most of this stuff it is another 40%.

Interestingly enough, when you look at the less desirable and widely available version in 18-8, they ask $6.16 for the 1" version and $6.72 for the 1 1/4" Ouch!

Of course all of this stuff is cheap compared to most anything on a boat, but you know "A penny saved, is ....."