Heli-Coil and anti-seize compound

Assume that you will be using SS bolts? You have a high potential for galvanic corrosion between SS insert and Al.

So, whatever you use, use it liberally. You'll need to keep the moisture out of the threads.

Intuition says either SS or Al filled anti-seize is good. No direct experience though.

Most of the stuff I worked on is in orbit, where it's dry.

Yes, my bolts will be stainless steel.

I don't have a clue if I should prefer stainless based anti-seize to match the bolt/insert, aluminum to match the base, or if I am "barking up the wrong tree". I don't know if the type of anti-seize might be for temperature, pressure, or other criteria and my idea of matching materials might be less significant.

I should have also added to my initial question. If I was not using "thread locking" inserts then I would have a conflict to deal with. Documentation instructs me to use anti-seize. Common sense would tell me to use a LocTite thread locker to keep the bolts in. For "normal" threaded inserts do you normally use anti-seize and therefore are forced to use mechanical lock washers or nylon insert nuts?

You'd use thread locker if your insert would not be of the locking type. And in this case, you'd not expect to remove it from time to time, but assembly it in place once for life. As you have thread locking ones, you're ok. Use anti-seize to avoid corrosion make the job of the thread locker, if I'm able to make myself clear...

I really think its a good idea that way you're doing the things. And anti-seize compounds are good to avoid corrosion locking of bolts or the tradicional aluminum thread comming-out-with-the-bolt-and-insert-oh-$#&&!.

Nickel and copper based ones are good for hot applications, I've seen it to be used in many places in aircraft turbines to avoid messing everything up during the severe heating and cooling and moisture condensation and etc.

You should be ok with a graphite one, specially because they're cheaper. Apply a thin layer to the mating parts and be happy forever.

I recommend silicon. It will last in the moist climate. Ever notice those desiccant bags that come with moisture-sensitive items? Most have silicon in them.

that's silica(desicant), not silicon

As Yosemit3 indicated, you need to pay much more attention to spelling. A sealant would probably be a silicone (there are many...).

The desiccant is commonly silica gel. it only works in sealed containers. Outdoors it will quickly absorb its alloted amount of moisture, and then serve no purpose whatsoever.

Silicon is element number 14, directly underneath carbon (number 6) in the periodic table. It is similar in many ways to carbon; like carbon, it has thousands of compounds. Pure silicon is hard and very brittle. It is mostly used in the manufacture of semiconductor devices. Virtually useless as a sealant!

Thank you to everyone that responded. It was very helpful.

Bruce

I'd go first to the Loctite people for a recommendation. They probably have the best application engineering of any of the companies that source this stuff.

Take note of the fact that the helicoil people make Nitronic 60 helicoils. About twice the price of regular stainless steel helicoils. Nitronic 60 is a highly gall resistant type of 18-8 stainless steel. May be overkill for you depending on how tight the bolts are going to be. If that teflon coating on the stainless steel bolts is too thick it may produce more grief going into the thread locking inserts than it's worth. Strongly recommend testing before you commit your bucks to buying parts.

Nitronic 60 helicoils are very popular in the semiconductor equipment industry where the use of stainless steel fasteners is almost universal for equipment going into the clean rooms of the chip manufacturers. (This was a kind of a been there done that experience for me).

Of note a bit of my own experience from 40 years ago when I lived in New Jersey (with its salted roads) as a very house poor young mechanical engineer who did all his own maintenance on the family vehicles. The people at Worthington Pump had just discovered the original Neversieze and I got hold of a can the stuff. Applied to the exhaust system fasteners and joints made the every other year exhaust system replacement easy. I never had to buy new u-bolts either. Great stuff.

Ed Weldon

note: you will have less than 0.10" walls on your tapped holes. I can't find the correct chart, but this sounds fragile, in aluminum.

That will result in a weak assembly.

Good point.

I've been involved in the marine industry for more years than I wish to admit. The product that I have found to be the best is something called Tef-Gel. I have removed SS to aluminum fasteners that have been installed for years and the stuff is amazing. It's been around for probably 15 or so years and I think was initially developed for fasteners in yachts, but I believe the USCG and Navy use it now. What's great is that it is teflon based and doesn't get all over everything you don't want it on and cleans up pretty easily.

It's a bit pricey, but one gets what one pays for. You can find it at boat supply stores.

Since what you're really after is a cushion layer to prevent galvanic corrosion, just use moly grease (molybdenum disulfide bearing grease); sticky, lasts forever, cheaper than Never-Seize. Never-Seize was and is primarily used to keep fasteners from seizing under heat, the different grades used for different temperature ranges.