Patching up a Firewall

[yahugh]

I'm in the process of converting an RV-7 from a Subaru engine to an O-360. The firewall has a number of non-standard penetrations that are no longer used and which I need to cover up. I'm wondering how to do this in a manner that will retain the integrity of the firewall. There are two openings like the one shown below. I made some patches out of 24 ga stainless sheet metal which I planned to attach using the existing nut plates. But will this do the job? Should I use some sort of sealant, like MIL-S-38249 Firewall Sealant?

 

[wirejock]

My opinion, seal with 3M 2000+. Hopefully Dan will chime in. He knows.

 

[cajunwings]

Supposedly the firewall is not structural. Plate over it with stainless steel, use the existing nutplates and add at least 4 more fasteners around the perimeter. Screws and all metal locknuts will work fine here. Assemble with Fire Barrier 2000+ and put a bead around the perimeter.

Don Broussard
RV9 Rebuild in Progress
57 Pacer

 

[Ralph Inkster]

Another product you could use Flamemaster CS 1900 firewall sealant.
Fashion a cover plate of stainless steel of same thickness or thicker. Double or triple the number of fasteners around the perimeter of the hole, especially corners.

 

[yahugh]

Thanks all for your replies. Additional fasteners are incoming. As for sealants, the Flamemaster CS 1900 product claims a higher service temperature than the 3M Fire Barrier 2000+ (400°F vs 302°F) but what really gets my attention is the former's claim "Can withstand flash temperatures up to 2,000°F". Does this difference really matter or is it just lipstick on a pig?

 

[Ralph Inkster]

Either would do the purpose actually. Use the one you can source easiest.

 

[Mikeyb]

Thanks all for your replies. Additional fasteners are incoming. As for sealants, the Flamemaster CS 1900 product claims a higher service temperature than the 3M Fire Barrier 2000+ (400°F vs 302°F) but what really gets my attention is the former's claim "Can withstand flash temperatures up to 2,000°F". Does this difference really matter or is it just lipstick on a pig?

Here is the link to the DanH firewall sealant tests.

Firewall sealant and Fire Safety

Can anyone recommend a source for a firewall sealant that can easily be mixed/dispensed in small quantities? Each time I need a dab, I can use Pro Seal but the shelf life is short and I can't find a small quantity at a reasonable price. What else might be out there? The high temp RTV is only...

vansairforce.net

At post #42 he makes the comment:

CS1900 looks like an excellent sealant for use between fay surfaces and in thick sections with mechanical fixation. Good resistance to direct flame but with a lot of smoke and burning, so I'd be very careful about allowing any more than a trace to show on the cabin side of a firewall. Adhesion is poor. Mixing is a PITA.