Warning: "High temperature" Red RTV not flame resistant

[Captain Avgas]

I've noticed in recent years that "high temperature" RTV gasket sealants have become almost the defacto standard for use on RV firewall transitions.

Typical products I now constantly see on RV firewall transitions include Permatex Red and Ultra Copper RTV gasket sealants as sold by auto stores.

However I would caution against using these products as firewall sealants because they are quite unsatisfactory in that application.

RV builders get confused because these products are marketed as "high temperature". However being temperature resistant and being flame proof are two quite different things.

The Red and Ultra Copper RTVs are actually formulated as gasket materials and are only good to temperatures of 700 degrees F (intermittent). They are certainly not designed to be exposed to naked flame.

In fact when exposed to any naked flame the Permatex Red and Ultra Copper sealants actually ignite almost instaneously. Once ignited they will continue to support combustion quite vigorously even once the initial source of ignition is removed (ie they become a combustion source themselves). And when they ignite they give off copious quantities of choking white smoke.

Needless to say you do not want either flames or choking smoke penetrating your firewall.

Builders contemplating using these products (or those who have already used them) might like to try burning a test sample for an enlightening (pun intended) demonstration.

For firewall sealing I recommend RV builders use a true "flame resistant" product. They could investigate Flamemaster CS 1900 sold by Vans or 3M FireBarrier 2000+ sold by ACS. Both of these products can cope with temperatures up to 2000 degree F (intermittent).

[RickWoodall]

I have wondered about this for awhile. Many builders just gob that rtv all over. For the pass throughs, they pop in two plastic snap bushings and a gob of rtv. Then they use the aluminum air box etc. I know of several airplane fires and finding a better way, even if for only a few seconds seems wise.

For cables I used the stainless eyeballs, and for the wires I used the stainless sleeve with firesleeve wrap that safeair uses. Not sure if that is the best, but worth the trouble and cost to me. There is very limited rtv on my firewall that I am banking to save my bacon in a fire. I did do a torch test on the clear high temp rtv and could not get it to ignite, it did smoke a bit and kind of crusted up. I was pleasantly surprised that it did not break down or ignite very quickly for me. YMMV

[DanH]

Bob is quite correct. Ordinary high-temperature RTV's are too often used for inappropriate applications.

A few words about 3M FireBarrier 2000+. It is also a silicone base sealant, specifically a special polysiloxane mixed with calcium carbonate, about 50-50 by weight. It will not itself burn or support combustion. However, when a thin layer is subjected to direct 2000F flame the silicone binder will burn away, leaving the calcium carbonate as a powder.





These properties do not make it a poor firewall sealant. The trick is how you use it. It is intended to fill gaps in between components. When tucked into a gap or corner or placed under a mechanical shield (like a standard firewall shield) the sealant remains unaffected. If placed in direct contact with flame it should have a minimum thickness of about 1/4", in which case the silicone binder will cook out of the exposed surface, yet the calcium carbonate will remain to protect deeper layers of sealant.

Bottom line? You can't run a cable through a rubber grommet, smear some FireBarrier 2000 on it, and call it protected. It still needs a standard two-part stainless steel firewall shield over the assembly. The FB 2000 just makes it airtight.

The stuff should be terrific inside a tubular firewall penetration like the ones Rick described.

FireBarrier 2000+ appears to be safe on the cabin side of a hot firewall, "safe" meaning it doesn't seem to generate serious smoke and it doesn't burst into flame. However, be aware it may lose its adhesive ability, another good reason to use it with mechanical fixation...ie between things.

[mburch]

This leaves me wondering what is the best product to seal the corners of the stainless firewall recess (F-601K). I sealed the ~quarter-inch holes in that thing with high-temp RTV, but every time I look at it I start wondering how well it would actually work if called upon to do its duty. It's hard to implement a "metal sandwich" in this location due to the geometry involved.

mcb

[Brian Vickers]

Thanks for bringing this to my attention. Very timely for my project.

In the good old days, when Tony Bingelis wrote the firewall forward book, his illustrations show asbestos washers below the firewall shield to protect the grommet. This seems like a good system but products with asbestos are a thing of the past. I don?t mind cutting my own gaskets from sheet material. Any ideas on a substitute material that is approximately 1/16? thick, or less?