How do I seal this compressor?

Nutfarmer said:
How did the elbow seal? The straight pipe threads I have seen in hydraulic lines use an o ring and what looks like a half nut to seal the threads. The Asian straight pipe threads in metric sizes have to be measured to figure out what they are. Japan’s are different from China’s. It can be very confusing.
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I am not sure. There is a tiny, smooth area at the base of the male thread the same diameter as the thread itself. It does seem like it seats into the female port, but I am quite skeptical this could provide a reliable seal. Vibration would tend to break this sealing surface in a hurry. What's more, there is no way to adjust the angle of the elbow, and in fact when tightened, it juts away at a crazy angle nearly 180 degrees from the way it needs to point. It's a little hard to see the surface, but here is a photo.Compressor Elbow.JPG
 
aliva said:
The fittings are also referred to as Boss O Ring. The design allows you to rotate an elbow in many directions before tightening the nut to seal. The o ring has to have a machined flat surface to seal against.
View attachment 412611
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Right, and that is the problem. The mating surface is a rough, uneven casting. Nor was any sort of sealing feature provided with the pump.
 
Ulma Doctor said:
if you were so inclined, you could tap the bores to the NPT equivalent and swap from metric fittings
the work may be worth the reward
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I thought of that, too. I fear the junction would fail. The 27mm diameter of the male thread is nearly exactly the same diameter as a 3/4" pipe, but the pitch is way off. Tapping with a 3/4" pipe tap would probably remove more than half the metal, greatly weakening the port. That makes me very, very nervous. I don't think I could successfully hand ream the port to 1".
 
I like the copper washer idea mentioned above. You can control the position by the thickness of the washer. Once you have it where you want it, stake it in several places with a center punch or chisel. Mike
 
The actual size of these threads is 26.8mm x 1.5mm, and I can't find any standard pipe threads of this dimension. That, in and of itself is not a big deal. I can take a 3/4" schedule 80 pipe and match those threads on the lathe pretty easily. If I chase the threads with a 1.5mm tap, rather than single-point threading, I can taper the pipe, rather than going with straight threads (my lathe does not have taper capability). I am not sure how well that would work. On the other hand, I am not sure I could hand file the port flat enough for a metal to metal seal. Maybe a fiber washer? PTFE? Here is a shot of the female port.
Compressor Port.JPG
 
I would consider brazing up an interface
Consisting of a bushing /tube interface brazed in place after being installed into the head port.
If space is abundant, coil the interface line to the tank for cooling and vibration absorption
 
@lesrhorer

I think there are some misconceptions being put forward above.

Standard O-ring boss ports include a conical surface which is the sealing surface on the housing side. O-ring face seal designs also exist but they are different than O-ring boss seal designs.

www.cpvmfg.com

O-Ring Straight Thread Boss SAE J1926/1 - CPV Manufacturing

Used in Cat. 64T and Cat. H864T female straight thread boss fittings
www.cpvmfg.com www.cpvmfg.com

If you attempt a face seal by flattening the surface on your compressor head, you will need a method of preventing the O-ring from being crushed or extruded. In other words, the installed O-ring needs to be fully contained within surfaces smooth enough to seal and with clearances small enough to prevent extrusion of the seal.

Regarding epoxy and heat, I'm not sure the right epoxy wouldn't work if the joint is well designed. By 'well designed' I mean both parts have very close CTEs, there is a lot of faying surface, and you do not use an epoxy that cures glass hard. 3M Scotch-Weld 2216 is not glass hard (brittle) when fully cured. Regarding the effect of elevated temperature, the epoxy will be weakened, but I doubt that the seal would fail in a well designed joint. I would use a Jam nut or broad shoulder against the housing for additional mechanical security.
 
lesrhorer said:
The mating surface is anything but flat or smooth. I thought about possibly taking a file to it to try and get it flat and smooth enough to seal and using a metal o-ring, but I am not sure I can pull that off.

If, as reported, this thing gets hot enough to soften Lead solder, normal Epoxy resins won't work. Maybe there are some resins with high enough temperature resistance? I don't know of any.
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60/40 solder begins to soften at around 320ºF and becomes fully liquid at 370ºF. Permatex makes a high temperature epoxy for applications to 500ºF. Furthermore, that is the point where it is entering the plastic region. Even if soft, it will not flow so its ability to act as a sealant should still be good.

Finally, there is not much lost to give it a try. The epoxy is available on Amazon for $7.
 
I have some Permatex Ultra Red RTV Silicone. It is supposed to be good to 343C (650F). I think I would prefer to try that, unless the outlet temp exceeds 330C.

I have filed the outlet port by hand until it is as smooth and flat as I can make it by hand. Exactly how perpendicular it might be is another matter. I also ordered a 1" copper crush washer and a jamb nut. I can easily machine a groove into the nut to secure the washer. Hopefully one or the other of the RTV silicone or the copper washer will work. The 3/4" Schedule 80 nipple I ordered is stainless steel, so it should have no problem whatsoever with the heat. I have a 3" x 2.5" piece of Aluminum hex bar I am going to tap with 3/4" pipe threads and into which I will machine some fins for extra cooling.
 
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