Firewall assembly questions

In fabricating the AUX Fuel Pump doubler, I notice on the plans it says to "trim or SCALLOP" the edge of the doubler plate to accomodate hole "C," which I believe is a mixture cable. I can see why,the hole is right on the edge of the doubler plate. But I've never seen the term "scallop" before. What does it mean in this instance." I assume it involves filing a hole large enough to compensate for the cable. But I don't know what the cable size is here so I'm not sure how much of a trim to make.

Also, where the recess is concerned, I guess you have the option of installing it with the flange on the forward side, or "hidden" under the stiffeners on the aft side. Wondering if there's a conventional wisdom for this that I should be considering. It doesn't seem to make any difference.

Bob,

The hole diameter is right there with the X,Y coordinates. In this case, it's a 5/8" hole. I'd suggest drilling that hole in the firewall alone first. Then cleco on the doubler plate. Then you'll see exactly what needs to be done (and that's what they call scallop). Just trace it out with a sharpie and file away. Basically, it's a semi-circular "cut out" on the edge.

Hi Bob,

If you are using Van's FW penetrations and method of sealing the cables and are going to use a carbureted engine, make the mixture pentration before you rivet the doubler in place. Then you will see that the doubler covers part of the hole up. Radius the scallop in the doubler so it is about 3/16 to 1/4" away from the penetration. Then your snapbushing cable seal will sit flush on the FW. Hope this makes sense. If you put the doubler in now and then drill your penetrations later, the doubler will be very hard to "scallop" in place.

Roberta

D'oh. Of course I shou'dve just read across to see the size of the hole.

I haven't picked an engine yet, so I don't know whether the mix will go there. I think I'll just diagram the hole and then cut the doubler plate back accodingly in anticipation that I might.

I don't want to start making a bunch of holes in the firewall that I may not need.

Say, that brings up a quesiton. Is there a point in the fuselage construction where I MUST have my mind made up on an engine? I'm sure that's never been asked before.

bc

I don't think there is a point where you can't decide on your engine, but the earlier the better so you can do prep work before you start closing thinks up. Once areas are finished and closed, it just gets harder to go back and make holes and fit things to accommodate your engine choice.

JMHO

Roberta

Before beginning the riveting of the firewall today, I drew some of the holes in the fuselage based on the X,Y coordinates on the plans.

The coordinate for the mixture cable is almos dead on the edge of fuel auxiliary pump plate. No problem scarfing out to a 5/8" hole, but if I widen it out to 1/4" beyond that (I'm not sure what is used here, a bushing perhaps), it's not only going to violate the edge distance of the rivet n the plate, it's likely to remove the hole altogether.

One other thing for those yet to begin on the fuselage. I was amazed how much the 1/8" dimple die expands the #30 hole. Using the C-frame, I gave it only a single whack, and -- voila -- a copper cleco won't hold anything. I was reading Dave Parson's site the other day and I think he noted that the rivet callouts seemed too small. I can see why.

Building the firewall is fun stuff! Especially after spending months of staring at wing ribs.

If I remember right, I did bump up to the next size rivet in several places on the firewall. The main concern with using a longer rivet is the possiblity of cleating the shop head. However, the longer rivets set fine. The only tricky part is right in the vertex of this angle:


Dave

Yeah, when I laid out those holes I realized they were on the inside radius (almost) and going to be tough to drive. Then I read your page and had that confirmed (and also gave me hope that I was doing it "right"). I might try to backrivet at that location instead of the handsqueezer and see what happens.