Engine mount thru firewall to fuselage

[DBeaulieu]

I've seen some with and without a penny washer (AN970) between the engine mount and firewall. I'm inclined to do so. Is there a specific yes/no/depends? Survey says...?

Thanks! Dave (from Maine)

engine mount penny washer.jpg

[svyolo]

I did engine mount to fuselage, and made the firewall holes just big enough. I am not sure if that was the best way to do it but it didn't seem like a bad idea.

[Paul Johnston]

Go ahead with the 970 - trap the firewall sheet so it doesn’t shake/ wear /rattle. The engine mount should be connected to the engine before it’s mounted to the fuselage.

[arborite]

Go ahead with the 970 - trap the firewall sheet so it doesn’t shake/ wear /rattle. The engine mount should be connected to the engine before it’s mounted to the fuselage.

Can you elaborate on that Paul? I’ve never heard that before. Is this a one time/first time setup only? Subsequent engine removals/mounting do not remove the mount from the plane on any plane I’ve ever worked on.

I currently have a scrap engine case mounted to the mount to allow mockup for the accessories and can easily mount it first before mounting the assembly to the firewall but never really thought about sequence
.
thanks.

[Paul Johnston]

My dynafocal mount wouldn’t accept the engine after it was bolted to the firewall. There’s no issue with conical engines or small Continentals.Also none of the dynafocal mounts on certified a/c have this issue,Just Bearhawks. One builder made his own mount that would be agreeable, replacing the one supplied.

[Bearhawk Aircraft]

Engine mount should make direct contact with the fuselage structure. Create a large enough hole in the firewall to allow this contact.

[DBeaulieu]

Bearhawk Aircraft Okay, I'm on it, gotta open up some holes on the firewall!

[DBeaulieu]

After discussing for a short time with others (Virgil, Jared, along with 3 other AP (two are current IAs) I went ahead and opened the holes in the firewall S/S to allow for direct contact of the engine mount and fuselage structure. Two reasons for this: (1) Avoids potential for firewall material trapped between the engine mount and fuselage attach points to wear away over time and subsequently the engine mount bolts to lose torque - the S/S is "softer" than the 4130 steel. (2) Enough other attach points for the firewall via the tabs on fuselage tubing, boot cowl etc. to secure the firewall. The AN970 washers when doing it this way serve no purpose, thus not used.

What I did learn is that a simple question is not so easy to answer sometimes! I think the confusion mainly occurs because of different engine mounts and fuselage attach design on various other aircraft and of course my limited experience in aircraft wrenching!

Firewall engine mount bolt holes.jpg

[Bcone1381]

) I went ahead and opened the holes in the firewall S/S to allow for direct contact of the engine mount and fuselage structure. Two reasons for this: (1) Avoids potential for firewall material trapped between the engine mount and fuselage attach points to wear away over time and subsequently the engine mount bolts to lose torque - the S/S is "softer" than the 4130 steel. (2) Enough other attach points for the firewall via the tabs on fuselage tubing, boot cowl etc. to secure the firewall. The AN970 washers when doing it this way serve no purpose, thus not used.

What I did learn is that a simple question is not so easy to answer sometimes! I think the confusion mainly occurs because of different engine mounts and fuselage attach design on various other aircraft and of course my limited experience in aircraft wrenching!

Thanks for sharing ‘why’ we do it this way. I had an IA tell me to install the washers once upon a time to avoid firewall cracking. Eliminating the SS sand which and the washers is industry best practice! Knowing why we do it this way helps everyone in the industry even That IA. I’ll check what I have in my installation.