Drilling the Hole
Put the plug wires and other engine-top stuff back that I removed to install the fuel lines.
Thought a bit about where to go through the firewall, and made some measurements. There is a heater control cable penetration on the firewall that can serve as a reference for measurements on both sides of the firewall. I removed my glove box (it was in sad shape) and took a look at the inside firewall layout in that region. There is the heater box with two defroster outlets on top, and the wiper linkage and pivots above that. The flat center of the firewall is obstructed by the heater, and you wouldn't want to put the hole too high and maybe run afoul of the moving wiper linkage.
Here you see me drilling the hole with my very large step bit. Before this, I marked and center-punched where I want the hole, and drilled a 5/16"-ish pilot hole. Laying on top of the engine, seeing the steps on the drill was obscured, so once I was close I went a step at a time and measured the hole until I reached the diameter I wanted of 1.75".
J20DrillTheFirewall.jpg
The large step bit is pretty expensive, and a hole saw would work instead. The step bit can grab and put a lot of torque on the drill; a firm grip is needed. It is nice for sheet metal though, since it leaves a clean hole and does not distort the panel around the hole.
https://www.amazon.com/gp/product/B0C23FBR9D
This is the location of the hole from the front and from inside:
J20FirewallPenOutside.jpg
J20FirewallPenInside.jpg
The heater control cable penetration is seen in the engine compartment view above, located below the vacuum reservoir. It's 8.25" down from the lip on the outside, and 5.5" down from the bottom of the shelf under the dash. I wanted 2.25" below the shelf on the inside, which gave me 5.5" down from the lip under the hood.
Plenty of cable left on the inside.
J20ExtraCableInside.jpg
The pink and black unsheathed wires are for the "Service Engine Soon" light (Howell parlance, I'd call it the check engine light CEL). There is an ADL connector, a fuse panel with a Weather Pack connector for an ignition hot wire, and the two ECU connectors with blue clips. The extra wire is my antenna wire.
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