Ok so lots of useful ground has been covered in this thread which brings me to the last decision topic.
The following picture shows the new battery tray i position:
6 screws attach the tray to the airframe...in this picture, 2 columns of 3.
3 of the Van's pre-drilled holes in the tray match the plate nuts installed in the airframe - the two middle holes and the lower right. I've got screws in those positions in the picture.
The builder did not use the Van's pre-drilled holes towards the top of the picture to site the plate nuts up there. The holes are about 3/8" and 1/2" down from the pre-drilled holes as you see them. I spoke with Van's about that and there is no problem having the screws in those positions. So for the top two screws I can use the plate nuts as positioned. I'll use the old tray as a template, and drill new holes in the tray to match the locations of the top two plate nuts..
The partly blocked hole in the upper left is not a plate nut but just a hole drilled by the builder that presently serves no purpose.
The problematic hole is the one on the lower left. The plate nut hole is about half-a-hole off from the Van's pre-drilled hole. Drilling the hole in that location to match the plate nut location in the airframe will result in a "snowman".
So I outlined my plan to Vans:
I'm going to make a "doubler" that will fit either under the tray between the tray and the airframe, or a doubler that fits right in where the battery sits - the battery will be sitting on top of the doubler.
In either case, Van's suggested 3 rows of flush rivets to fasten the tray to the doubler. One in between the two columns of screw holes and one out outside each of the columns of screw holes. Same rivet size as used to fasten the side brackets to the tray - 9 rivets per row.
Method 1:
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Here is a picture of a test doubler I made out of thin stuff just to get a feel for how it would work. You are looking at the underside of the tray, so this is the one that would fit between the tray and the airframe:
Those tabs sticking out to the sides will allow me to include the battery hold down bolts as part of the system that connects the tray to the doubler.
Pros:
The battery hold downs contribute to fastening the doubler to the tray.
Cons:
1) While the doubler helps to provide bearing surface for the countersink on the problematic hole, it doesn't really provide the ideal countersink as described in Section 5. i.e. the countersink is not entirely in the upper plate.
Method 2
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The other doubler method is to fit a rectangular piece inside the tray which the battery will sit on. This piece can be thick enough to handle the entire countersink of the fastening screws. However the tray section will still partly miss the grip of the screw.
A piece of aluminum under the battery will raise the battery somewhat, however, the side brackets I riveted on the tray are tall enough such that more bearing surface will exist against the battery than with the Van's supplied brackets.
Here is a picture of the new tray with the taller side brackets I installed, and the Van's supplied bracket next to it:
So even with the doubler raising the battery a little there is still sufficient height to the bracket to hang on to the battery.
Pros:
Countersinks are correct for all screw holes.
Cons:
I give up having the side tabs of the doubler ue the hold down bolts. Although, I could dismantle the side brackets, have the doubler extend to the ends of the tray, rivet the brackets back on only now the rivets go through the side bracket, the doubler, and then the tray. This would be even stronger.