Firewall Engine Mount Holes (clear/not to clear?)

[noema]

I got my firewall perfectly match-drilled to the engine mount bushings so it fits the AN6 bolts. Very happy with it. But then I read the old manual pdf (this is not mentioned in the current manual about the firewall) which recommends having the firewall drilled bigger so the engine mount is in direct contact with the fuselage frame. I don't see a problem with having the firewall in between especially when the holes are precise. I also prefer having the additional attachment point for the firewall.

Still I would like to hear other builders' opinion on this. To clear or not to clear the mount holes, that is the question?

[Nev]

I had the same question. I followed the old manual. Apparently there was an update, I couldn’t find it. So some of mine are enlarged so the firewall sits around the frame. But I believe the way they recommend now is to sandwich the firewall in between and I think that makes sense. I’m sure Mark will chime in to confirm.

[Mark Dickens]

I had the same question for Mark and the bottom line is it's optional. That's why page 39 is missing and the last few words on page 38 are crossed out. I would add that Nev is right, it's recommended to sandwich the firewall, but if you do it the other way, it's fine.

[jaredyates]

The most current version of the manual is on bearhawk.tips and the most current best practice is that the holes need not be cut to shape.

[Mark Goldberg]

No need to trim away the firewall material. Just extra work that is not needed to be done. Mark

[noema]

Sounds good. Thanks for the input.

[Utah-Jay]

Does the firewall come pre-drilled for pilot holes? Or do you have to line it up and drill yourself?

[noema]

It has pilot holes.

[Utah-Jay]

Thanks a million noema as some of this stuff petrifies me

[Pbruce]

On a similar note, are the factory holes in the firewall considered accurate for the purpose of locating exactly where the firewall sits? If so, that pretty great, because it hard to find other definite reference points.

[noema]

On a similar note, are the factory holes in the firewall considered accurate for the purpose of locating exactly where the firewall sits? If so, that pretty great, because it hard to find other definite reference points.

I double checked and considered them well placed. Left/right symmetry was good. Up/down seems to be a bit of a compromise between having lower overhang for the tunnel and keeping the bottom corners fairly close to the longerons. I probably will have to reduce the height of tunnel slightly and still need 1/4-1/2" spacers to reach the tabs. I believe I also read that some builders are not using the taps at all.

I will know for sure when the boot cowl is in place. I am still wrapping my head around all the details with that.

Screenshot 2021-08-27 124426.jpg

[BTAZ]

My understanding is CAD plated bolt(typical AN hardware) and Titanium don't play well together.

So if you make your firewall from Titanium it is better to clearance the firewall so there isn't a chance for the bolt to touch it.

[N942VT]

I don’t normally use those tabs. There’s plenty of strength there. They can be handy for mounting fuel lines etc.

[spinningwrench]

Check the location of your firewall holes!!! Mine were drilled 1/2" off in height on the quick build kit and I had to get a new firewall. Mark did provide that for the cost of shipping. The original instructions were for the engine mount to clear the firewall but now apparently it is OK to sandwich. Per a previous comment however the titanium firewall might prefer some clearance from the engine mount bolts for corrosion purposes.

[svyolo]

I have had a bit of a hard time getting around dissimilar metals in airplanes, but have tried to get my head around it from experience. Big anode, small cathode is OK. Big cathode, small anode, not so much. I have personally screwed this up almost 40 years ago. I installed wood panels into a steel van with aluminum screws and they all turned to white AL oxide dust in 6 months. But a BH or Cub has lots of steel tube and lots of AL and it seems like an even amount of the two are OK.

Small anode, big cathode are a castophe, that I created once. Steel fasteners in mostly aluminum are oK for 10 to 30 years, but you might have to chase threads and use never seize. Or end up drilling and using heli-coils to fix the threads. I am up to help-coils in a couple of engines.

My firewall is cheap CP titanium, and I clearance the holes just because that was BH convention a couple of years ago.

[noema]

I fell in a rabbit hole:

https://xhawk.substack.com/p/galvanic-corrosion-in-218-words

Interestingly, mild steel is more noble than aluminum alloys from a galvanic corrosion perspective. Cadmium-plated screws match aluminum alloys well but not necessarily stainless steal/titanium/carbon fiber.

[svyolo]

The firewall flange is AL and attached with AL rivets to the either SS or titanium FW. The flange is screwed or riveted to the boot cowl. That is a lot of AL to donate electrons to the firewall.