Yanmar 2GM20F mixing elbow issue

[bugbitten]

I've had the cooling system on my 2GM20F torn down, initially just to open ports to the FWP for a new water heater, but eventually I decided to replace all hoses and gaskets, and had the heat exchanger serviced. It's now all back together, but there's an ongoing issue with the mixing elbow.

I've had this boat four years and this was the first time since I've owned it that all this was done. I'm guessing it'd been years before my purchase that any of the hoses or gaskets were done. The mixing elbow inlet from the heat exchanger was nearly blocked. I decided I might as well put in a new one, along with a new union and exhaust elbow, since I couldn't separate the old ones.

All the parts on this job have been straight Yanmar, except for the mixing elbow. I had an aluminum one from ebay I tried first. The union would not pull very far into it, but I tried it anyway. Has some leakage but was able to run the engine long enough to make sure everything else was sealing properly and the water heater was working. I figured I'd take the mixing elbow to a machine shop and have the threads cleaned up.

Being an impatient sort, I ordered a stainless steel mixing elbow from an ebay seller, which also came with a new union. This seller recommended using marine anti-seize compound on the threads (any thoughts on that as a sealant?), which I duly ordered and applied. Same problem as before. Even when putting the mixing elbow in a vise and really leaning into it with a pipe wrench (which I didn't want to do with the aluminum one), I could only get it to thread as shown in the picture below.

So, my question is, should I just bite the bullet and buy a Yanmar mixing elbow, or get a machinist to clean up the threads (which look fine) on the aluminum and stainless units, or what? I realize that pipe threads taper down and that it will be increasingly difficult to tighten, the farther in it gets, but it sure seems I should be getting more purchase than is shown here.


John

 

[Charles Erwin]

I gather the order of assembly is the Yanmar exhaust manifold, your new 'union', then the new exhaust elbow. If the parts are different or the order of assembly is different - please tell us.

A bit of guesswork here but from what you are saying your new 'union' and new elbow are likely threaded National Pipe Taper. The Yanmar manifold is certainly threaded British Standard Pipe Taper.

These different threads - National Pipe (NPT) and British Pipe (BSPT - sometimes called metric) do not joint well because the thread count per inch and thread angle are different.

Use thread pitch gauge to find out and check with your E bay seller to discuss.

Charles

 

[woodster]

Charles is right John ...you need to qualify the threads are either metric(BPS) or npt and that what they mate with is also the same ....from the looks of you pic there is definitely not enough thread going in to the a joining parts two threads is just not enough ...you may need to find a mechanical contractor and discuss it with him and seek his advise

 

[bugbitten]

The first time I put it all together, I assembled the three pieces (exhaust elbow, union, and mixing elbow) off the engine. The exhaust elbow and the first install's union are new, genuine Yanmar bits. Both mixing elbows are from different ebay suppliers and are specifically listed for the 2GM20F engine, for what that may be worth. The SS elbow came with a new union (the Yanmar parts book calls it a "joint," so I guess I'll use their nomenclature from now on), which I'm using, and which, one should be able to reasonably assume, should thread just fine into their own elbow. He's been selling these a while, so it's hard to believe they're not the same thread. The elbow that connects the hose from the heat exchanger threads into both mixing elbows easily.

When I took the first install apart, I simply unscrewed the joint (since the mixing elbow has left-hand threads, turning the joint will either simultaneously tighten or loosen the two elbows) and left the exhaust elbow in place. I hated to pull it off, since the new gasket to the cylinder head was sealing just fine.

So, the second install involved starting the joint ever so slightly into the exhaust elbow, then sitting the mixing elbow on top of that. I was able to turn the joint a few revolutions by hand, pulling the two elbows together and starting to seat the mixing elbow on the exhaust hose.

I guess I've no choice but to take all the pieces to my trusted machine shop and have them check the threads.

Oh well, the weather's not too bad, so maybe I'll work on the lifelines a bit.

Thanks.

John

 

[Charles Erwin]

John

Good decision to take it to your local machinist and obtain a definitive diagnosis.

There is only one way to know if the seller of the new joint got it right and that is measure the thread pitch.

After many of these - I have never had any success assembling exhaust parts on the boat. Instead assemble and align the exhaust casting/joint/and mixing riser in the vise and then install the whole assembly on the motor.

Charles

ps Please do let us know the final result

 

[Delezynski]

Charles,

Not to hi jack the thread....

Are you on the Nor'Sea news group? Did not see your name there.
http://groups.yahoo.com/neo/groups/NorSea27/info

Greg

 

[bugbitten]

Charles,

Since you've done a bunch of these, what's your sealant of choice for the threads?

John

 

[Charles Erwin]

I use anti seize - the aluminum version which is a silver color. There are also copper, nickel, and graphite versions.

Anti seize type products are somewhat a sealant but more important they help prevent the threads from galling at assembly time. Galling - think cold welding of threads together - is a particular problem is stainless.

Note the parts I see are typically cast iron or steel. Stainless threads might be better protected using a different formula - so ask your machinist for his vote.

Charles

 

[bugbitten]

Took both elbows to my machine shop. They said they had too little call for that size left-hand thread to be able to do much with it. They gave me a couple of other shops that might help, but I went home instead, chucked the aluminum mixing elbow in the bench vise, put a bunch of teflon tape on the joint (in addition to the teflon compound that was already on the threads), and wailed away. I'd had this one too long to send back, so I figured what the heck. Got a couple more turns out of it, put it all back together, ran the engine about ten minutes, and so far no leaks! I hope to get the boat out in the next few days to run the engine longer and harder, but I'm feeling pretty optimistic.

I'm still very pleased with the amount of sea water discharged from the exhaust, compared to the previous elbow.

I'll contact the SS mixing elbow seller about the tightening issue, since that was recent enough that I should have some recourse if it's defective.

Will report back after some more running time. I'll also post a pic of the current installation, showing the substantial amount of thread still exposed.

John

 

[Steverhnm]

I bought one of those SS mixing elbows for my 3GM30 (same part), I am pretty sure its the same guy. I would be interested in what you come up with as I have not installed mine, I just got it and my boat is in Mexico and I won't be able to work on it for a couple months.

I like the idea of the SS elbow and union, did you ever get them to mate up properly?

 

[bugbitten]

I went back to the aluminum one, figuring I'd had it longer, which somehow made it more expendable. I stuck it in a vise, and just went crazy on it until I could get it no tighter. I ran the engine once for about 30 minutes and it seemed to seal just fine, even though it did not seat nearly as deeply as the original. I have not retried the SS one.

 

[jibes138]

I went back to the aluminum one, figuring I'd had it longer, which somehow made it more expendable. I stuck it in a vise, and just went crazy on it until I could get it no tighter. I ran the engine once for about 30 minutes and it seemed to seal just fine, even though it did not seat nearly as deeply as the original. I have not retried the SS one.

How many full threads are engaged? A threaded part has full strength with about three full threads engaged. More threads might be better for less cantilever effect if there is a side load on the assembly but three threads will be enough to hold the joint together. Here's a formula you can use to calculate it if you wish. http://en.wikipedia.org/wiki/Bolted_joint#Thread_engagement The idea is the pipe will fail in tension before the threads strip out, this means the cross section area of the pipe from the ID to the root of the threads is smaller than twice the combined shear area of the threads engaged. Thread tolerances on taper threads can mean a big difference in axial position. Since the angle is so shallow a small change in diameter causes a big change in how much the thread will engage.