What threadlock/sealant do you recommend?

I have two 6061 aluminum parts that I am making with 1.375" - 32 thread. I know it's a fine pitch. I could go a little coarser, like 28 or 24 tpi. I have an insert that screws into a block. I need it to seal and hold 15-20 psi of automotive coolant system pressure.

The thread is very shallow. It's only .350" deep due to limited space.

I bought some threadlock/sealant that was junk. It sealed most of the time, but I got too many parts back leaking (~5%). I would like to go with a name-brand company like Permatex this time.

Any recommendations?

We tried everything we could think of trying to seal cheap chinese 316 SS pipe fittings with 200-300 psi saltwater flowing through them. Even one drip per day from them would corrode other parts of the equipment. We eventually called loctite and they recommended 565 thread sealant with 6749 primer. It works extremely well. I see they make 567 which is a high temp version of 565. Our saltwater application was at room temperature.

If you never want to take it apart loctite 272 will hold freon 12 with a good clean thread. 515 and similar are excellent flange and gasket sealers. As he said call loctite....

Good point. I did call Loctite in the beginning and left a message, but I never heard back from them. In the meantime I found another product that looked right for the job. It didn't work out so well.

KD5CRW said:

We tried everything we could think of trying to seal cheap chinese 316 SS pipe fittings with 200-300 psi saltwater flowing through them. Even one drip per day from them would corrode other parts of the equipment. We eventually called loctite and they recommended 565 thread sealant with 6749 primer. It works extremely well. I see they make 567 which is a high temp version of 565. Our saltwater application was at room temperature.

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A concern that I have with the 567 sealant is that the thread is straight, not tapered pipe thread, so I really need a threadlock as well. There really isn't room for pipe thread in this application.

I guess I really need to call Loctite.

reengineer it to be a ORB and then use any of the loctite products.
ORB uses a jam nut behind a O-ring seal.

The ORB (o-ring boss) is a good idea. The proper name is SAE J-514. In a plug type application there isn't even a jam nut. The jam nut is only used when the index is critical. A similar size to what you are using is the -16 size, which uses a 1 5/16-12 thread. There are proper cutters that form the complete o-ring pocket as well as serve as the tap drill. You could shorten one of the cutters to work with your .350 depth.

When we can't get something to seal, we will do one of two things and one almost always works. Gasoila softset (do NOT I repeat NOT use rectorseal its marketed as a competitor but it can't hold a candle to Gasoila) wrapped in nickel reinforced teflon tape. Then more gasoila on top of that. This works especially well on stainless and large diameter piping. For smaller stuff pull a vacuum on the whole assemble and use loctite wicking grade. If there is a leak the loctite will be pulled into the leak and seal it.

As a Chemical Engineer, I'm delighted to know folks put so much faith in chemistry that they'd risk their reputation that a small bit of sealant will hold and seal straight threads in an automotive (vibration, stress, heat, agressive chemicals and moisture) environment.

That's said, if my reputation depended on it, I'd use an o-ring, or (if you really want to get fancy, like BMW does with their steering fluid lines) raised rings on the face of both nut and body, and an aluminum crush gasket. Similar to a VCR fitting. VCR Catalog Cut

Don't mean to sound smart-ass, but sometimes we engineers don't design stuff proper-like. And straight threads for sealing is not proper-like.

Good luck!

J

bosleyjr said:

As a Chemical Engineer, I'm delighted to know folks put so much faith in chemistry that they'd risk their reputation that a small bit of sealant will hold and seal straight threads in an automotive (vibration, stress, heat, agressive chemicals and moisture) environment.

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So, what you're saying is that a product designed for a specific purpose is not to be trusted for its specified purpose. Therefore it is worthless and the engineer who designed it is also worthless. Do you know what machinists think of engineers?
I have a 15 psi system that I need to seal. I had a 95% success rate with the product I used that was supposed to be good for 100+ psi and 2100 degrees. I figured it was overkill, but I was wrong.

Fadal, you missed Bosley's point. Joint sealing compounds are tested and developed for sealing tapered joints. You are not using the product for its designed purpose. If you want a reliable seal for a straight thread, you need a gasket of some type.

The tapered pipe joint design tightens up with metal on metal along the length of the joint, with very small, non-continuous voids. The sealing compounds act largely as a joint lubricant to allow the joint to be tightened. In almost all joints the seal is entirely metal on metal. The solids in the sealer are for the voids in the few joints that don't seal metal on metal.

You are sealing a straight thread joint. When tightened, you have a much larger leak path which is continuous along the non-contact flank of the thread, including both the minor and major thread roots. This gap may well exceed .010. There are some Locktite products designed for gaps up to .015, but IIRC they are rated for retention rather than sealing.

Fadal Error said:

So, what you're saying is that a product designed for a specific purpose is not to be trusted for its specified purpose. Therefore it is worthless and the engineer who designed it is also worthless. Do you know what machinists think of engineers?
I have a 15 psi system that I need to seal. I had a 95% success rate with the product I used that was supposed to be good for 100+ psi and 2100 degrees. I figured it was overkill, but I was wrong.

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Fadal, didn't mean to imply that the design is cr*p, or that you engineer is an idiot or anything. And I sense from your reply that this may be a situation in which you have 5000 of these things that are already made and ready to go out the door as soon as you solve the sealing problem. I understand that my comment may have induced stress. Sorry, I was trying to be helpful.

Whatever anyone's sentiments, straight threads without gaskets on the face are not in general made to seal stuff. That you get 5% failure with a very high-performance thread sealer (2100°F! Wow - what sealant is this?) testifies to this fact. My note was to suggest that a minor design change will make your life easier. The shuttle had a problem with seals. No one would say that the shuttle is a pile of cra**y engineering. They redesigned the seals, and shuttle performance improved.

My suggestions were to help you. A five percent failure rate for any part of any manufactured product today is so far from acceptable it's not funny. If this product is "standard", for standard autos, and you sell 10,000, field servicing 500 units is gonna be unpleasant. And it it's high-end, and you sell 100, having 5 customers out there dissing your product in a small community is bad for business. And even if you get something that works in your shop, applying even the best threadlocker to an improper (straight thread) design could leave you up at night, worrying about a failure in use.

Back to your question. You want a way to ensure few leaks in an automotive environment. If this is for super high temperature and the user will need to disassemble and reassemble this occasionally, I'd figure out a way to use a crush gasket. That's probably significant design and engineering, but it's proven in automotive use. For more normal temperatures, the o-ring boss design seems to me a reliable, simpler, and cheaper way to go. It may mean an extra counterbore and grooving operation, or you might get away just by making your thread relief groove a bit deeper. Or you could silver-solder or braze a Swagelok fitting in the hole. Or you could change the threads to tapered pipe thread. Gbent's excellent description points out that this will give you a much more reliable mechanical seal. A taper with sealant should be a joint you never have to worry about. I personally don't like taper threads for stuff that is frequently reconfigured or disassembled, though.

I hope that this is taken in the spirit it was written. If one reads the responses on this forum you will see that if you ask "Should I use product A, or product B", your answers will range from "Use product C" to "That's not the way I would make it. I'd use a Qwernsdorfer Bifurcation Indexable Step-taper machine, with the Nermbly attachment. It's perfect for this operation". Or, someone will chime in with a story about their Uncle Al, or something. Usually, these comments are made to be helpful, I think.

Good luck,

J

Thank you for your help.

The product I was using originally claimed to be a combination of a threadlocker and thread sealant. That seemed to be the right fit for this application. I tried tapered thread, but it wasn't working well. Part of the problem is that the insert has a shoulder that needs to sit flush against the main block. It is nearly impossible to do that with a tapered thread, especially in production with minor variation in size of the two mating parts.

I am going to add an o-ring (or two) to the design. I think I will still look for a threadlocker that acts as a sealant because I would like a backup.

I got to take a look at an o-ring boss assembly and it looks like the way to go. Thanks for that recommendation.

One more thing. Once this is assembled, I don't want it to ever come apart. I could machine the whole part from one piece of billet, but it would get a lot more expensive. Is there a bonding agent that will be a permanent bond that anybody recommends? I have been using Permatex green high-temp sleeve retainer for some other parts that press together. Maybe it would work in this application.

Sorry for the curt reply, Bosley. I appreciate your help. This problem has indeed been causing me stress.

Thanks for your gracious note, Fadal. I'm glad to see that you have a path forward. Hope a good solution lowers your stress.

Good luck!

Well, I put a 60 degree chamfer on the female thread and installed an o-ring on the male thread where I relieved it at the base. I have a flange to push the o-ring down into the chamfer. I had to use channel locks to crank it down flush and that's with a 32 pitch thread, so it should be a pretty solid seal. I will pressure test it tomorrow.

The Permatex sleeve retainer that I used on one part last Friday was still wet today, so I don't think it is going to work as a threadlocker. I called Permatex tech support today and left a message, but I never heard from them. Maybe tomorrow.

I got through to Permatex tech support today and they recommended 592, but it is really just a thread sealant for tapered pipe thread. I don't know if I can trust it to retain the inserts. This part bolts directly to the engine head, so it is subject to constant vibration.

I am leaning toward just using a high-temp permanent threadlocker and trusting the o-rings to do all of the sealing.

Just a reminder if you are using Loctite type materials on Aluminum parts you need a catalist to help with the activation. Since Permatex bought them the distinction is not obvious. The old hard set Permatex is generally one of the best sealers to prevent leaks. An application with a porous weld on a Diesel tank was causing lots of difficulty. The Loctite sealant made for Hydraulic fittings fixed it.

Good point. That is something I learned along the way. I got some primer to prep the surface. Hopefully I can get to it today.

I did a pressure test with the o-ring assembly and it held 60 PSI with the inserts screwed in by hand. I'm sure it could hold more, but that's all I could get before my thumb blew off the open port. Good system.

Fadal Error said:

I did a pressure test with the o-ring assembly and it held 60 PSI with the inserts screwed in by hand. I'm sure it could hold more, but that's all I could get before my thumb blew off the open port. Good system.

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Compressed air and the body can be a fatal mixture, so do not do that again.

Nev