Installing the Firewall

If you are following along for your own Bearhawk build, here are a few items I learned while installing the firewall.



Installing the firewall, or at least drilling the holes, should be one of the first assembly procedures you do after receiving your kit.  I realized that the rudder pedal assembly was in my way which made it difficult to drill my holes.  You should at least drill your holes, then set the firewall aside until later.  This suggestion is not offered in any of the Bearhawk builder manuals or plans.

The firewall is stainless steel and is very hard.  So you will need VERY SHARP drill bits.  Cheap drill bits will not work.  For the engine mount holes, I used a 3/8" pilot point bit which made the SS like drilling aluminum!

Keep in mind, the firewall is not sandwiched between the fuselage and the engine mount.  It is attached to 9 tabs on the fuselage with AN3-4 bolts.  The 5 engine mount holes should be drilled oversized so the engine mount and fuselage have a pure steel to steel connection.  I drilled the engine mount holes with a 3/8" bit, then opened them up further with a step drill.

The kit firewall comes with two pre-drilled 3/32" engine mount holes at the top of the firewall.  Drill one of the holes to 3/8".  Then attach the firewall to the fuselage with a bolt through that hole.  The firewall will now pivot.  Aligned it to the 2nd pre-drilled hole and use clamps to hold it in place, then drill the 2nd hole to 3/8".  The 2nd hole on my firewall was off by about 1/16", so this method made the two 3/8" holes engine mounts holes perfectly aligned.  Run a 2nd bolt through the 2nd hole that you just drilled to the fuselage.  Now the firewall is stabilized and you can go ahead and drill the remaining 3 engine mount holes (3/8") and the 9 tabs on the fuselage (3/16").  I inserted the AN3-4 bolts after each 3/16 hole I drilled, further stabilizing the firewall as I progressed.

 The engine mount receivers on the fuselage will need to be reamed to remove paint and welding slag.  I tried using the drill 3/8" bit which took some time, was getting hot, and kept binding.  Then I remembered that I had a 3/8" reamer.  That worked perfectly!

Firewall attached and fuselage back on the rotisserie.  I inserted 1 3/4" bushings on the arms of the front rotisserie mounts to keep it off the firewall and allow some space to do some work.  I'll be mounting a brake reservoir soon and needed the opening for lines, etc.