Opinions requested on engine work

[higgs]

I am replacing the mixing elbow on my Yanmar 3QM. There is a flange that is bolted to the exhaust manifold and they used stainless bolts, two of which suffered deterioration. One was corroded along a thread half way through and the other broke off in the manifold (another story). I do not have local access to metric stainless hex head bolts so am planning on using just normal bolts to attach the flange. I cannot think of any reason that this is a bad idea, but I may be overlooking something.

 

[jviss]

normal bolts

Really? Why not just find the correct bolt. To use and SAE bolt you'll have to go bigger, drill the manifold and tap it. Metric bolts are not har to find online.

BTW, I don't think stainless is a good idea for that application. I'd use "normal," mild steel.

 

[kloudie1]

I agree, carbon steel bolts , as were the original ones from Yanmar. Use an anti seize goop on the threads when reassembling. Nickel or copper based is fine.
hot salty water is not good for stainless.

 

[dLj]

Some of us would like to see high resolution photos of the bolts that broke. I'd be guessing, but you may have had a gasket leak. Fix that and your stainless bolts won't deteriorate as quickly or nearly not at all. Don't fix it and your non-stainless bolts will not last long at all.

Alloy choice depends upon how often you'd like to change these out.

dj

 

[higgs]

If there was a leak a new flange, gasket, and mixing elbow should fix that.

 

[higgs]

Here are requested photos. One is of the mixing elbow showing where it rusted through. The other is the compromised bolt. The second bolt broke off in the manifold. Being stainless, it was a real challenge getting it out. I think that there must have been a leak at the flange. All the old bolts showed some corrosion. It is, as far as I know, a fresh water boat.

 

[higgs]

The broken bolt posed a real challenge as it did not break off cleanly. I needed to drill it out for an easy-out, but there was no flat surface where I could start without the drill bit sliding around and damaging the threads in the manifold. Thank God for YouTube where I got the idea how to get around that problem. I tapped out the one hole in the old flange to 5/16 and hollowed out the center of a 5/16 hex bolt to act as a guide for the drill bit. (see photo). I put the old flange back on and ran the hex bolt so it was as flush to the hole in the manifold as possible. It allowed me to get a hole started, but it was slow going drilling out the stainless bit even using titanium drill bits.

 

[dLj]

, but it was slow going drilling out the stainless bit even using titanium drill bits.

Wrong drill choice - should use 135 degree split point cobalt drill bits for stainless.

dj

 

[jviss]

One piece of advice drilling stainless is to never let up on the pressure. If you do, the bit will heat the stainless and harden it, and you will have a terrible time trying to drill it.

 

[dLj]

the bit will heat the stainless and harden it, and you will have a terrible time trying to drill it.

While the advice is sound, the reason is thecnically incorrect. Austenitic stainless steels, what we typically find on our boats, do not harden through heating. They do harden through cold working (strain hardening), and if the proper feed is not maintained to produce strictly cutting, you can introduce strain hardening and indeed these alloys can get quite hard. I point this out only because I've seen people provide adequate cooling to avoid over heating but that's not the culprit...

dj

 

[Allan12210]

I agree, carbon steel bolts , as were the original ones from Yanmar. Use an anti seize goop on the threads when reassembling. Nickel or copper based is fine.
hot salty water is not good for stainless.

When I replaced my mixing elbow I replaced two of the bolts with studs. That way I had something to hang the new gasket and mixing elbow on. Trying to reach over the engine and keep the gasket, elbow flange and a bolt lined up without being able to see the hole was a PITA. The studs solved that problem. My local Ace Hardware had the metric bolts, nuts and split ring washers I needed. As Kloudie suggested, I too used anti-sieze compound.

 

[higgs]

When I replaced my mixing elbow I replaced two of the bolts with studs. That way I had something to hang the new gasket and mixing elbow on. Trying to reach over the engine and keep the gasket, elbow flange and a bolt lined up without being able to see the hole was a PITA. The studs solved that problem. My local Ace Hardware had the metric bolts, nuts and split ring washers I needed. As Kloudie suggested, I too used anti-sieze compound.

Smart

 

[garymalmgren]

You beat me to it Allan (12210).
Steel studs with bronze or brass nuts.

 

[RoyS]

Consider purchasing an HDI Marine stainless steel mixing elbow for a replacement. This small company specializes in manufacturing stainless steel replacements for the original cast iron mixing elbows.

 

[jviss]

While the advice is sound, the reason is thecnically incorrect. Austenitic stainless steels, what we typically find on our boats, do not harden through heating. They do harden through cold working (strain hardening), and if the proper feed is not maintained to produce strictly cutting, you can introduce strain hardening and indeed these alloys can get quite hard. I point this out only because I've seen people provide adequate cooling to avoid over heating but that's not the culprit...

dj

I didn't know that!

 

[higgs]

Thanks for the responses. Job is done and I used standard bolts I got at the hardware. Ran the engine for a while and came back the next day to retighten the bolts since things change when metal heats up and cools down. I will run the engine a few more times in and out of the harbor and recheck the bolts again. It was mentioned to consider HDI Marine. I am not sure if the elbow I got was theirs, but my replacement was after market stainless. It was $200. During my search I came across a used Yanmar elbow for over $300. I am sure a Yanmar replacement is a lot more. Their parts are exorbitant.

 

[CaptainStan]

If you use stainless steel bolts then use carbon steel nuts. My mixing elbow rusted out and I took it to a shop and they welded a patch on it. That was 4 years ago and it is still fine.

 

[JamesG161]

Here are requested photos. One is of the mixing elbow showing where it rusted through

I see something else on your first picture and has nothing to do with BOLTS.

The Exhaust Tube looks Over Heated.

That suggests not enough Quenching Water or poor quenching.

Very hot conditions will separate your bolting flange, cause a leak, and corrode you bolts.

I think that there must have been a leak at the flange.

That elbow did not RUST through.
Jim...