The firewall is clamped in place. As you can see, I used quite a bit of cleco crocks to make sure everything is in the right positions and won't move.

I had to remove it a couple of time to tweak the longerons a little more so the outside of the longeron mated well with the steel welds for the motor mount.

Also had to cut back little more on the longerons length as I could not get the required spacing between front of side skin and firewall web.

Clamped the longeron to the WD welds and inserted some rivets to keep the holes aligned as per the manual.

Then, I measured multiple time to ensure I had the required 5/8" distance between firewal face and side of the F770 skin.

Measured this with the back side of the caliper but found out that bending of the flanges of the firewall is not equal at all locations (steel remember...)

Anyway, after carefull measurement and further tweaking, achieved overall 5/8" distance.

Verified again that the holder rivets were still in place and the longerons were clamped to the WD-602 brackets.

Since Vans tells you to take a break for a minute and return later to recheck, that's what I did. I guess this is a quite important step so I was a bit shaky to start drilling.

On the top side of the firewall to side skin, you can also notice that the flange moves in a bit. No way that last rivet will have enough edge distance but there's nothing you can do about that. The distance is set correct at 5/8 and it's pre-cut like that at the Vans facilities.

same think other side. You can also notice that 5/8" from F770 to firewall face makes the F770 bit come longer than the flange of the firewall. Worried at first about edge distance but made temporary markings on the firewall flange with a marker and measured and there is sufficient edge distance left.

Clamped even better in both directions..

And then started drilling away... This is steel, so remember, lot's of Boelube and slow drilling to keep your drills from wearing out.

When the right side was finished, started the left side of the fuselage.

Then completed the remaining 2 lines of holes through the bottom steel flange and aft aluminum stiffener of the firewall.

The firewall is drilled to the fuselage... what a day...