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Cutting the notch/slot in the fuel tank attach bracket

RVFan671

Well Known Member
My 91 -4 had a bolt and nut with no notch in the attach bracket between fuel tank and fuselage. I can't find a service bulletin on making the update like the current plans call for which is to notch one of the brackets and use a plate nut. This allows for the wing and fuel tank to separate from the fuselage in a crash so the tank doesn't rupture.

What I am having trouble with is which do you notch and which gets the plate nut? I think you notch the mount on the fuse and put the plate nut on the tank which would be similar to this setup I found a picture for online of an RV-7. However, I also found this picture of an RV-8 tank that shows the tank bracket slotted.

The RV-4 electronic plans aren't much help, or at least not the pages I've found. It seems that both the fuse and tank might share the same part number (T-405). There is one picture on drawing 17A giving dimensions of it and this is the fuel tank drawing but no reference to a plate nut or notch. Then in the fuse section of the plans it talks about referring to drawing 39 which as the same part number referenced (T-405) and shows the slot.

Can someone confirm if I should be putting the notch/slot in the fuse side and plate nut in the tank, or vise versa (or something totally different)? If there was a service bulletin for this update, please reference it here, too.
 
Tabl attach

Mine is like the first photo.
Fuse bracket is steel and slotted.
Tank bracket is hole with nutplate.
 
Notch

My 1999 plans call for aluminum angle bracket on the fuse. Instead of a nutplate on the tank, I had the idea to use a castellated nut and key to set the grip(or lack of it) on the two mating bracket surfaces. I match drilled the tank and fuse brackets in assembly, then milled the slot in the fuse bracket.
 
If you are not going to remove the tanks, then the bracket bolted
to the fuselage would be the easiest to remove and slot.
tank-fuse-bracket.jpg
 
One of my good friends who is now hopefully in Valhalla crashed his first RV-4 and got drenched in fuel when the tank ripped open. Obviously Eir was looking after him as he didn't burn, and survived with a full recovery.

He told me he called Van to tell him what happened, and suggested this slot on the tank bracket in some sketches he drew. Very well could be that Van was already planning to add this modification - not sure. He put the slot in his second RV-4.

This is a very worthwhile modification to those of you that are flying -3s and -4s without it. I believe that the -6 had this mod in the plans, but can't hurt to check.
 
I already pulled my tanks and my fuse side bracket is riveted to the fuse so its probably easier to notch the tank side for me. I like the castle nut idea. Did you use large or standard washers?
 
This is a picture of my plans and the bracket on my -4.
 

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Hard to tell, but is that bolt safety wired?

No safety wire in this photo, it was an initial fitment and bolt was subsequently safety wired. I’m not completely sure safety wire is necessary in this application because the nutplate is self locking, the original configuration is a bolt with a self locking nut, regardless, I drilled a small hole in the bracket and safetied the bolt.
 
No safety wire in this photo, it was an initial fitment and bolt was subsequently safety wired. I’m not completely sure safety wire is necessary in this application because the nutplate is self locking, the original configuration is a bolt with a self locking nut, regardless, I drilled a small hole in the bracket and safetied the bolt.

If a bolt is torqued to anything less than spec, (even in a self locking nut), it should be safety wired.
 
So why do you think Van's called out for
a platenut when a locknut would work.
platenut.JPG
And then set it at a specific tension and
safety wired it..........:rolleyes:
 
If a bolt is torqued to anything less than spec, (even in a self locking nut), it should be safety wired.

This is the correct answer, I hadn’t considered the specified light torque in this application.
 
A nut plate was used instead of a nut, to help ensure that the bolt wouldn’t tilt to the side, and potentially jam the release of the clevis joint, if the fuel tank was displaced in a crash. That is the reason the nut plate is depicted in a specific orientation, because the attachment legs will work strongly against the lateral tilt of the bolt.
Mel is partially correct regarding the reason for the safety wire, because we are not fully torquing the bolt, but the other reason is this joint is actually subject to a small amount of rotation from wing flex at high g loads. This is the same reason that a castellated nut and cotter pin is used at the rear spar attach point, but for the forward attached point, the desire was to have something that would resist the tilting of the bolt, since it is just a single shear joint unlike the double shear clevis joint at the rear spar.
 
Anyone know how deep that slot is? They have a dimension on one axis but I don't see a dimension on the tank drawing or the fuse bracket drawing indicating how deep the slot should be. I'm right at the point where I am ready to cut the slot in the tank brackets.
 
Anyone know how deep that slot is? They have a dimension on one axis but I don't see a dimension on the tank drawing or the fuse bracket drawing indicating how deep the slot should be. I'm right at the point where I am ready to cut the slot in the tank brackets.

The part is slotted after it is drilled in position.
 
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