How much leeway in locating throttle cable hole location on firewall?

When you first get your firewall, you're told to add doubler plate for a fuel pump. This assumes, however, that you're going to put a carbureted engine on the beast. But I decided, later, I'd have fuel injection. The little fuel pump isn't needed because a high pressure pump is installed upstream. But there you are with a doubler plate and two nutplates.

This weekend I located and drilled the holes for the doubler for the fuel line pass-through, moving it about 3/4 inboard because the plans call for part of it to be riveted to an angle that the starter contactor already has a nutplate mounted on.



This did not cause a problem until I realized that the distance between the doubler for the fuel line, and the doubler for the fuel pump I'm not using, leaves me no room to drill a hole for the throttle cable. (You can see on the fuel pump doubler that I "scalloped" out room for a hole for the throttle cable, but the hole for the spiffy SafeAir "eyeballs" needs to be 3/4" and there's a one inch nut that holds it in place. That isn't going to work here.

I could move the throttle cable hole further to the center, but now it'd be in danger of hitting the engine mount and I just saw images on Van's Air Force this week of what can happen over time doing that.

What I could do is slice an angle off the bottom of the fuel line doubler, ignore one of the rivet holes I've already drilled, and drill another.

The other option is just to locate the pass-through for the throttle cable through the doubler that was originally installed for the fuel pump.

But I'm not sure how much leeway I have in moving the location of this hole all over creation.

Larger images available on the blog.

I think you have lots of leeway

with the location of the throttle and mixture cables with a couple caveats. First you want to clear all the obstacles, and second, you don't want any sharp bends in the cable as installed and finally, if you move it too much, you'll need to buy a non-standard cable. Mine are nowhere close to the Van's locations for various reasons, so I ended up with custom length cables from Spruce$$.

In general, a too long cable (or moving the firewall penetration to a straighter shot from the handle to the servo) is not a big deal. Too short won't work at all. Looks to me like using the existing facet fuel pump doubler would probably be OK if there are no obstructions FWF. If not WORSTcase is you might need a cable that is a couple inches longer.

Good luck!

Wiggle room

Hi Bob,

Here's an idea of what you're facing:



The picture is a little deceiving in that there is plenty of clearance between the control cable and the nose gear strut. Also, the mixture mirrors the throttle pass-through for my application.

Some pics from the inside of the fuselage are here.

I ended up scalloping the fuel doubler to allow room to put the throttle cable in the location called out by Van's. I posted some pictures in an update on the blog. I don't have a return line from the fuel.

I did "test fit" the VA-128 (or 138, I forget which) hose from Van's. Ish. Seems too long. We'll see.

One question on the the swivel pass-through that you have installed (which I'm trying to install). I interpreted the instructions to say punch a 3/4" hole, so I bought a Greenlee punch. It appears to be too small. The unit looks like about 1". But I want to make sure I haven't missed something before I enlarge (guess I'll have use a Unibit). What size hole did you drill for yours?

The good news was my wife stopped by the hangar for a few minutes to hold a bucking bar as a backrivet plate. First rivets she's driven on the project.

Bob Collins said:

.........First rivets she's driven on the project.

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Happy Valentines to You.


Concerning you eyeball fittings. I'm not sure which fittings you have, so can't give you a exact size to go with. So take your fitting and put the calipers to it, add a .030 and there's your size.

I have the TTP-125 eyeball from Vans, I think I needed a hole a little over an inch.

In regards to the punch size, you may have seen someone state they used a 3/4" conduit punch. Probably borrowed it from an electrician. A 3/4" conduit punch will knock out a hole just over 1" dia.

Yes, I have the same passthrough. I'll order the 1 1/8" punch. $69. Added to the mistaken 3/4" punch $50. So basically I have a $120 hole. Have I mentioned how much I love airplane building?

Bob Collins said:

Yes, I have the same passthrough. I'll order the 1 1/8" punch. $69. Added to the mistaken 3/4" punch $50. So basically I have a $120 hole. Have I mentioned how much I love airplane building?

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$120 isn't too much to pay to have your wife buck her first rivets on Valentine's Day . There are worse milestones!

We went out for the 29th anniversary last night... dinner, then the movie "Crazy Heart" (great movie. go see it). I ALMOST -- almost -- said after the movie, "hey, want to stop by the hangar and buck those rivets?"

But I decided not to push it.

3/4" conduit punch

Hi Bob,

I just went out to the garage and verified that you need what is called a 3/4" conduit punch. It should be stamped on the side of punch. This will make a 1-1/8" hole in which the TTP-125 will fit through.

Good luck,

Bob, try this link. HF knockout punch $15.99, it'll work just fine

You've been a huge help, Bryan. Above and beyond! Thanks very much.

I used the fuel pump doubler for my fuel pass through. Instead of the 90 degree bulkhead fitting, I was able to use a 45 degree. My throttle cable is in the Vans location, but slightly modified for an ECI Horizontial intake. See pic's.

Maybe it's the camera angle on that last picture but is that fitting going to hit the mount when engine is running?

I SHOULD have gone through the old fuel pump doubler but -- as usual -- I was obsessing about sticking close to the plans.

Bob Collins said:

Maybe it's the camera angle on that last picture but is that fitting going to hit the mount when engine is running?

I SHOULD have gone through the old fuel pump doubler but -- as usual -- I was obsessing about sticking close to the plans.

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No problems with clearence (see second picture). The second picture has a temporary AN fitting screwed over the threads.

I've now got the fuel line attached to the mechanical fuel pump. I used the van's fuel line and it fit great.

Steel fitting

#N72CR said:

No problems with clearence (see second picture). The second picture has a temporary AN fitting screwed over the threads.

I've now got the fuel line attached to the mechanical fuel pump. I used the van's fuel line and it fit great.

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Remember that fuel fitting should be steel FWF!!!

RV7Guy said:

Remember that fuel fitting should be steel FWF!!!

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Hi Darwin,

I'm a little confused. Is this because it's a fuel line?

My manifold pressure kit from Vans was not supplied with a steel fitting, and I believe the brake reservoir is not steel either (as well as the rivets).

I can see your point, and the steel fittings are available in a 45 degree bulkhead from Spruce, just wonder if this is a fuel only issue, or should I replace the Vans supplied fittings as well?

Steel in case of fire. That been said, Id better go check on mine, but am almost sure Vans supplied an aluminium one.

Right. Van's plans call out an aluminum fitting.

Ugh. I swear I'm undoing everything I've been doing lately.

Bob Collins said:

Right. Van's plans call out an aluminum fitting.

Ugh. I swear I'm undoing everything I've been doing lately.

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I do believe there are numerous aluminum fittings firewall forward. Afterall, some of the fuel lines are aluminum too, as well as my gascolator. I do have a steel fitting that attaches to the carb for stress, rather than fire.

L.Adamson --- RV6A