Does anyone know what these plates are for on my PA-12 firewall?

[Erud]

Hi all,
The old firewall on my PA-12 has 2 plates on the upper corners of the forward side, and have not been able to find any info what they are for. This firewall and boot cowl are being replaced, and the brand new firewall from Univair does not have these. No one that I've shown them to has had any guesses, they seem to have been added by someone at some point. Does anyone here happen to know?

Thanks,
Erik

 

[skywagon8a]

They are to reinforce the firewall to prevent cracking from the cowl stiffener attachments. The stiffener attach screws lock the firewall hard to the fuselage. Then the flexing of the firewall will crack the firewall without these reinforcements plates. Four locations.

See p/n 10792-3 on this PA-18 drawing. https://www.supercubproject.com/drawings/pdfs/A3310052.pdf

 

[Erud]

They are to reinforce the firewall to prevent cracking from the cowl stiffener attachments. The stiffener attach screws lock the firewall hard to the fuselage. Then the flexing of the firewall will crack the firewall without these reinforcements plates. Four locations.

That makes sense, but makes me wonder why the new firewall doesn't have them. Did PA-12's come this way from the factory, or were these added later? If it's a modification, is it a common one? This particular plane has been around a bit, and I do not know if this firewall is original or not.

Thanks for the help!

 

[DENNY]

Additional support for Cowling door channel? Patches for holes? What does the backside look like?
DENNY

 

[skywagon8a]

It was included on the original PA-18 drawing dated 5-19-49. I don't have access to PA-12 drawings, It is possible they were an afterthought on the PA-12 since the -12 was produced prior to the -18.

 

[skywagon8a]

Additional support for Cowling door channel? Patches for holes? What does the backside look like?
DENNY

Stiffeners for the thin firewall to prevent cracks.

 

[Philly5G]

My PA-12 doesn’t have those plates, but as others have said, they look to be about where the top cowl and side cowl door mounting rails are. Mine are bolted through the side of the fuselage and not bolted to the firewall though

 

[Steve Pierce]

Add them, I have had to repair two PA12 boot cowls in the field from the cowl support channels vibrating and cracking the boot cowl. Piper fixed the issues on the PA18.

 

[Erud]

Thanks for the info, all. Maybe we should take the plates off the old one and add them to the new one?

 

[Steve Pierce]

Here is a thread I started about this issue and the repair I made.

PA12 Boot Cowl Cracking

I have found the boot cowl cracked where the cowl rails angles attach on both this 150 hp PA12 and a 180 HP Crosswinds STOL PA12. The Super Cub has angles riveted into the boot cowl to reinforce this area. Doing it after the airplane is assembled isn't a terrible job but something to think about...

www.supercub.org

 

[Erud]

Thanks for that, Steve!

 

[Steve Pierce]

Here is a photo album I made of the other one I repaired. https://photos.app.goo.gl/h2ur2gajik5ZctXp6

 

[KJC]

Those—12 boot cowls were really all over the place right from the factory. Some had the lip of the firewall turned forward, some aft. The Cub Doctor sells these support brackets which I have used before. They go under the lip or between the skins of the boot cowl and firewall. Check for proper length of the support channels before riveting.

https://www.cubdoctor.com/stainless-firewalls/index.html

 

[Steve Pierce]

I would still suggest the angle brackets from the 3003 aluminum boot cowl and the firewall. You can see the result without in both Super Cruisers that I maintain.