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Fuel line assembly through firewall

fabricflyer

Well Known Member
I was going to install fuel line AN fitting thru firewall, and got to looking at the plans, which I have inserted below.
They show putting the firewall doubler on motor side of firewall with the AN833 going through it, then the AN spacer washer and nut.
Wouldn't it be better to put the AN spacer washer on the back side of the firewall since the firewall is so thin?


fuelline1_zps104fd883.png
 
NO

I can only tell you what I did on my -8. Put the doubler on the aft side of the firewall, and the spacer washer on the firewall side.

Don
 
The doubler is defiantly on the cabin side of the firewall spacer and nut on the engine side.
 
I may be wrong but according to my -8 distructions on drawing op-34 only shows an 6d spacer and I have one on each side. I do see the doubler on op 32 for the -7
 
I don't known which ones are building an 8 or 7, but on a 7 the doubler is on the engine side. Two of the holes in the doubler are riveted to the vertical firewall brace as seen below. Therefore the doubler can't go behind the firewall.
fuelline3_zpsefcb805b.png
 
I followed the plans, but moved the location down slightly to clear the starter solenoid.

IMG_1959-M.jpg


Bulkhead fittings like this have more threads out towards the flared end and that is why you need the thick spacer to take up some of those threads. The doubler would interfere with the interior angle braces on the firewall, so it makes sense to put this on the engine side of the firewall and tie into it for strength.
 
Sorry now I am confused. I thought this doubler was installed during firewall construction on the aft side. Does this not penetrate the same doubler as the electric fuel pump?
 
This thread is about the fuel line for a fuel injected engine, the doubler you are referring to on the inside is only used a carburetor and associated pump which is quite a different setup. That doesn't get installed on an FI setup.

Sorry now I am confused. I thought this doubler was installed during firewall construction on the aft side. Does this not penetrate the same doubler as the electric fuel pump?
 
Mark,
That doubler is for mounting the electric fuel pump for carbureted engines. This doubler is for the AN 822-6D fuel line fitting for the injected engines.
All this still doesn't my question of wouldn't it make better sense to put the spacer WASHER on inside of the firewall.
 
Allen,

My $.02 is that the spacer washer should go under the bulkhead nut on the forward side of the firewall. This is similar to the reasoning why you typically put a washer under the nut on a bolt. The washer will prevent the bulkhead nut from rotating against the firewall/doubler during installation.

So the way it is shown makes sense to me.
 
I made this mistake

I made this mistake early on, from not studying the issue carefully enough. Here's my first stiffener, riveted on the inside of the firewall in the cabin. Note that it bumps up against the angle stiffener, but there's no way to rivet it TO the stiffener:

P1060188%20(Small).JPG


When I got to the firewall forward part of the project and started working on it, it bothered me once I discovered that it should have been on the forward side of the firewall, so that it can be riveted to the stiffener angle on the firewall, just like Bruce has said. I also noticed that without this stiffener, the fitting required two thick stainless washers to provide enough thickness for the nut. Clearly, it was made with the intention of having a stiffener on the forward side of the firewall. So I made another stiffener out of steel for the forward side of the firewall. I had lowered my fuel exit point, too, to allow room for the starter solenoid, so it had an odd shape. Here's the finished installation:

P1060191%20(Small).JPG


I wiggled the fitting firmly with my fingers when I had only the aft (inside) stiffener installed. It was pretty firm. But all I can tell you is, with the additional stiffener on the firewall forward side, it is MUCH stiffer and I feel much better about it now. You may also notice that I replaced the standard blue AN fitting with one made of steel. The forward stiffener is also made of steel. I don't want anything made of aluminum forward of the firewall, if I can help it, especially if it has anything to do with fuel. There's another thread on here discussing the issue of fire in the engine compartment. Dan Horton has done some tests that are definitely worth reading about.

I have more pictures and more details about my installation on my web site, here: Fuel Line Stiffener, Firewall Forward
 
Nice job Bruce, looks good. I understand you using steel AN fittings, but you look at a Cessna and they have those blue fittings coming through the firewall. I was also wondering about the galvanic reaction between the steel and aluminum parts. Just my thoughts.
 
Look at the fitting . . .

I did not see that you got your answer, but as John Thocker has kindly noticed I don't read posts throughly as i should sometimes. (just lazy)

The bulkhead fitting has a shoulder to align it with the hole. The threads start pretty quickly after. If the washer was on the cabin side, then the alignment would be done with the threads, not a good thing against thin materials.

BTW - thanks for the question, I am missing the washer and this has brought this to my attention. Again, I probably just did not read the drawing carefully :)
 
I take that back Bill, on DWG 36A, Section D-D, they show an AN fitting with an AN spacer washer on both sides of the skin.
Allen
 
Bruce, I agree, doubler on. 7 is on the firewall side... I really doubt if it makes any difference.. Except be careful as the starter relay might interfere and you might have to modify one of the mounting tabs

I have great pictures of this in my second builders log starting on picture 626 where you can see the doubler and the mounting of the relays


Smilin' Jack
 
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I take that back Bill, on DWG 36A, Section D-D, they show an AN fitting with an AN spacer washer on both sides of the skin.
Allen

Hmmmm - good point. Can't say I know all the logic there, unless the firewall location has a more serious result potential from a leak so it should be located more positively?
 
Starter Contactor Interference

I know this is an old thread but I seem to have stumbled onto a problem. I don't have the drawing in front of me at the moment so I'm afraid I can't refer to specifics.

I'm planning on an IO-360 so I installed my fuel line doulber while I was working on the firewall early on. That may have been a mistake. I recently tried to install my master and starter contactors on the firewall. I drilled the holes for the master contactor where I thought they should go. I have since double checked this position a couple of times. Then I built the contactor doubler. I have checked those hole locations as well. I then got ready to check the fit of the starter contactor and I realized the flange was overlapping my fuel line doubler plate.

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[/IMG]

As far as I can tell the master contactor is positioned correctly. The hole that is not through the firewall angle is right where I measured it should be. I was using a B & C supplied contactor, but I don't think that matters. You can see on my pic that I sketched the outline of the starter contactor flange. One other strange thing is the bus bar appears to need to be longer than what is shown in the drawing. I'm guessing this is because I used the B & C contactor but I'm not sure.

So this has had me really puzzled. Either I keep looking at it wrong or there is just an error in the drawings. At this point the holes for the master contactor are in the firewall so I just have to make it work, but still I can't let this go. My plan is to just move the starter contactor slightly closer to the master contactor so that the starter contactor flange will just clear the fuel line doubler. My only other option seems to be to remove the fuel line doubler and notch the corner to clear the starter contactor mounting flange.

Any ideas I what I could have done wrong? Or suggestions on how to fix it?
 
There seems to be some debate about that since on the back it wouldn't be able to rivet to the angle support on the cabin side.
 
Oh you mean the contactor doubler, F'7127D. I was referring to the fuel line doubler. Yes I know. I just positioned it on the forward side to decide where to drill the starter contactor hole.
 
Aha. Well, in that case, never mind :) My fuel line doubler is in a different location per the plans...F601Z, DWG 19. Totally different shape, mounts adjacent to the angle stiffener, not riveted to it. Maybe this is for a carbureted engine, for the Facet pump?

I just know that going strictly by the plans, I had no interference issues anywhere on the firewall...
 
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A couple of suggestions

After reviewing the plans I believe there is a problem with the plans layout.

Does the fuel line doubler have to be placed outside the firewall? A possible solution is to drill out the doubler and place it inside To fix the interference with the F-601N-R you could either bend a 90 degree flange in the doubler or rivet a piece of angle to it and then rivet it to the side of the F-601N-R stiffener instead of its face (as you are looking at it in your photo.)

This would eliminate the interference with the starter solenoid. Just one possible solution, I am sure there are several others.
 
After closely reviewing the drawings I see there is a conflict too.

I'm just surprised this hasn't come up more. I see that Bruceh put his fuel doubler in after installing the contractors and he adjusted the position. I think it was by a half an inch. This is probably the easiest fix but it requires a different assembly sequence or at least more forethought.

My plan is to:
1. Move the starter contactor closer to the master contactor so that it can clear the fuel doubler.
2. Make a new contactor doubler, F-7127D without starter contactor hole.
3. Move the other starter contactor hole to use fuel doubler rivet for the nut plate. This will position the starter contactor about a 1/4" closer to the master. You can see on my pic the adjusted position note. I think the starter contactor flange will the clear the fuel line doubler. Some minor grinding may be required, but at the moment I don't think so.
4. Figure out the exact location of the starter contactor hole on the F-7127D doubler by using the starter contactor to locate it. Mark and drill the F-7127D starter contactor hole.
5. Do any minor grinding of the flanges of the master and starter contactor where there interfere. I think this will be minimal. I went ahead and ordered a Vans master contactor to make grinding easier if needed since it doesn't have the plastic encapsulation of the mounting flanges.

I think that should take care of it.
 
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