[Fadal Error]

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?

[DMF_TomB]

for sealing threads pipe sealant helps with imperfections in threads BUT

from an Engineering view an O-ring or a copper washer is standard seal method with straight threads and many hydraulic fittings.

once had a gearbox leaking oil. Turns out the 2 mechanics assigned at first thought tighten plug tighter. still leaking. then older mechanic makes younger mechanic drain bucket of oil into another bucket. Yep they dropped copper washer into the drained oil. once copper washer put back on no leaks.

HF store has O-ring kits as well as copper washer kits (assortment of sizes)

[RomanLini]

Threadlocker is used to stop a thread unscrewing from vibration etc. You don't have that problem.

What you need is a sealant or liquid gasket material rated for use in automotive coolant systems. ie; rated for that temperature AND those particular chemicals etc. Ask the guy at any large auto accessories store, they usually have a whole shelf full of sealant goops.

I expect most of the gasket silicones suitable for head gaskets would be ok, as head gaskets usually have coolant channels so they need to be suitable to seal that.

If you are set on Permatex I remember they have a liquid gasket product that can be used on all engine surfaces including head gaskets.

[Fadal Error]

I need threadlocker to ensure that it never comes apart.

The guys on Practical Machinist recommended an o-ring boss. I checked out a stock assembly and now I am integrating it into my parts. It seems like a good way to get a solid seal. I would still like to have a thread sealant for redundancy if I can find a combination threadlocker/sealant.

I really just want to bond the parts together so they never come apart. I didn't want to trust a press fit, so I threaded them. Once they are assembled, they never need to come apart again. I tried some Permatex high-temp sleeve retainer that I have been using for some press-fit aluminum parts, but it never cured on the thread.

[plasmator]

Can you spot the joint with a TIG welder?

[Fadal Error]

Can you spot the joint with a TIG welder?

No

[neutronics]

Can you spot the joint with a TIG welder?

If you don't want to seriously consider spot or plug welding, the only way I can see making the joint permanent is with epoxy. Epoxy has the advantage of virtually no setup, but it can be costly, messy and if you don't use up the batch within its pot life you have to trash the leftovers. The parts also have to be very clean, or at least modified with a knurl or groove on the O.D. of the male part and I.D. of the female part.

For welding, all you'd have to do is put a hole in the female part and plug weld it with a MIG or TIG.

[RomanLini]

Epoxy is not suitable for the higher temperatures associated with engine blocks.

Fadal-error, I'm guessing you don't work on engines much? This is a pretty typical application. A gasket type sealant will both secure it and seal it. There are a number of screw-in sensors and fittings into engine coolant systems and trust me they DON'T unscrew easily, especially the large diameter fine thread ones.

[DMF_TomB]

loctite pipe thread sealants
http://www.sealanddesign.com/categor...adsealing.aspx

chemicals and temperature and metal type sealed are factors. many sealants do not like high temperatures

[Fadal Error]

I talked to tech support at Permatex and they recommended high-temp thread sealant preceded by their primer. It is a mild threadlocker.

Welding is really not going to work here. My main problem has been with the large fitting on the right in this pic. The two small fittings are also screwed in.

[RomanLini]

I talked to tech support at Permatex and they recommended high-temp thread sealant preceded by their primer. It is a mild threadlocker.
...

Sounds perfect. There's no way that large fitting is going to move with 0.350" thread depth and a fine thread glued in with goop.

Nice product by the way, it looks very nice even if a little over-engineered.

[Fadal Error]

I did a pressure test with the o-rings tightened down by hand and they didn't leak at all.

Nice product by the way, it looks very nice even if a little over-engineered.

Thanks. It is reverse engineered from the original part that went NLA. Every single feature has a purpose.