Today I worked on an OPTIONAL modification, which was to add access holes through the forward and aft sides of the F-404 bulkhead. Some builders have added holes as large as 2-inches in this area. I chose to go with a 1 3/4 inch hole, for no other reason that I didn’t want to go out and buy a 2 inch hole saw.
The image below shows these optional holes that I’ve added. In the post below, I’ll walk you through how I went about this mod.
Where to begin
The place to being on this mod (like every mod) is careful, very careful, planning. You need to be very careful here because you’re not only drilling holes in the forward and aft side of the F-404 bulkhead, but you are also drilling through the main wing spar.
Be sure to check the main spar very carefully so you don’t hit any of the upright spar spreader bars. Don’t forget, you also need to avoid the floorboard and rib supporting angles.
If you plan just right, there is BARELY enough room for the 2 inch diameter holes!
It is really hard to express just how much planning goes into the placement of the holes in the picture below. What I did was assemble the F-404 bulkhead, put the spars in place, put lots of bolts in to get the spar and bulkhead in as best alignment as I could.
From there I’m looking at all sides, measuring to locate the center of each hole that will not interfere with spar upright spacer blocks, angles and anything else that you don’t want to drill through that will compromise structural integrity.
Again, be very careful!
The following pictures are more of the layout details from various parts of the bulkhead parts.
When I was satisfied with my location, I drilled.
When drilling, I started with a small pilot hole to locate the hole saw. For the pilot hole, I drilled through the forward side of the bulkhead, and also drilled into the spar face to ensure my holes are consistently centered.
I also drilled into a scrap piece of MDF to help keep the hole saw from wandering. This worked really well. I also ran the hole saw backwards as this gives a slower cut and helps to maintain control while drilling. I find when cutting with the cutting teeth in the forward rotation, when cutting on aluminum it’s a bit too aggressive of a cut.
In the image below, you can see the pilot hole drilled into the spar face. You can also see how I’m clear of the upright spar spacer bars which are riveted on either side of the hole.
Eventually, you’ll have all the holes drilled and access through the spar and through the F-404 bulkhead.
Again, the reason for being very careful in your planning is because it is quite difficult to properly measure and plan the hole location. Don’t forget that the angles on the forward and aft side of the F-404 bulkhead are not in the same locations.
In the image below, you can see that I’m quite close to one of the rib angles, and this is with a 1 3/4 inch hole. Had I gone with the 2 inch hole, I would have been damn close to that angle and probably would have cut into it.
Here’s the other side of the aft face of the bulkhead. I have a bit more space here.
Here is the finished product – forward side of the F-404 bulkhead.
And here is the finished product of the aft side of the bulkhead.
That’s it. Not a difficult mod, but again I stress you need to be very careful in measuring the hole placement!
Please comment. I love to hear from my readers. Thanks again for coming along for this ride, you make my work worthwhile.