When to use next size rivet?

I made a few mistakes riveting the attach bracket. On the left bracket (HS-908-L, HS-908-R) I had to drill out one of the rivets. It enlarged the hole slightly and now I'm wondering if I need to use the next larger rivet, which would be a AN426AD-5-9. The hole was enlarged to the point that a #28 drill fits very snuggley. If I was to put a AN426AD-5-9 in there I would have to drill it out using a #21 drill bit. So my question is should I drill it out so I can use the next larger size rivet, or should I just use another of the same rivet even though the hole is slightly enlarged?

Try pre-squeezing...

Hi Michael,

Even if you try to squeeze/shoot an AD4 in the sloppy hole, you may have trouble keeping it from bending over because of the slop. If the hole is still round (i.e. not a figure 8 or really oblong), what you may be able to do is pre-squeeze an AD4 rivet to swell it a little and take out the slop. Put it in your squeezer and just barely squeeze it, check the fit, rinse and repeat if required. Note that you may have to use a half length longer AD4 to do this and keep an acceptable shop head, since the rivet will shorten slightly when you pre-squeeze it. This is probably easier done in a hand squeezer, but I have done with a pneumatic (watch your fingers!) I just noticed that the length of rivet you need is a 9... which may make the pre-squeeze a little difficult, but cheap and easy to try.

If the hole is really messed up you will either have to go with a larger diameter rivet, or perhaps two new holes either side.

I'm no expert, but have faced the same problem a few times. I've yet to have to go to an AD5 (probably just jinxed myself).

CherryMax

If the #28 drill is a good fit, and the hole is still round, then one option may be to go to an oversize CherryMax rivet.

The 1/8 size can be set with a good, normal "pop rivet" tool.

http://www.aircraftspruce.com/catalog/hapages/cherrymax.php

gil A

Remember as well that you generally should observe rivet pitch rules (i.e. the minimum distance allowed between rivets). Pitch increases with the size of the rivet. You may not have the pitch required to go up to the next size rivet. Good luck.

It's OK to upsize

alpinelakespilot2000 said:

Remember as well that you generally should observe rivet pitch rules (i.e. the minimum distance allowed between rivets). Pitch increases with the size of the rivet. You may not have the pitch required to go up to the next size rivet. Good luck.

Click to expand...

The pitch and edge distance standards should be followed when designing parts, but replacing a rivet, or even a few, with the next size larger will not compromise strength.

Interesting advice from Vans technical support...

Here's the advice I received from Vans technical support when I asked them whether I should go to the next size rivet or not.

"Our advice is to never drill out a rivet unless you have to take the assembly apart. No matter how ugly it is, it probably has 90% the strength of a perfect rivet."

"About the enlarged hole, just put another 1/8" rivet in it and move on. The
rivet will expand to fill the hole."

It is interesting to me because I've gotten the feeling from the community and from the training class that I took, that you should drill out questionable rivets because they compromise strength. Perhaps I've misinterpreted the training I received. But I've been really stressed about getting perfect rivets.

I don't think Vans support was saying you shouldn't care or strive for perfect rivets. But as a new builder I'm probably being a little too picky. And I'm constantly stressed out about compromising strength. This note from Vans support makes me feel a little less stressed about the build.

Some actual data

Bill Marvel did some testing of rivet joints and over- and under-driven rivets. Some of his conclusions are:

The dimpled, flush riveted construction was stronger, but not by as
much as I had thought. However, and this is really important, initial
failure of the dimpled construction was generally not catastrophic and
occurred as rivet tipping and rivet head distortion. In contrast,
initial failure of the AN 470 undimpled construction was generally
catastrophic by rivet shear. I am really happy Van uses the flush
riveted, double dimpled joints throughout most of the airplane!

Slightly over driving is stronger than slightly under driving and
results (in my opinion) in an insignificant difference in strength as
compared to properly driven rivets.

A joint made of grossly over driven rivets is stronger joint than a
joint make of grossly under driven ones.

A grossly under driven AN 470 joint is much weaker than a grossly
under driven AN 426 joint.

No joint was as strong as the parent material itself.

To summarize the summary, try for properly driven rivets but realize
that minor over driving is preferable to minor under driving and results
in nearly the same strength as does the condition of properly driven
rivets.

Click to expand...

Further conversation from Vans support...

Here's one more correspondence I had with Vans technical support concerning replacing rivets:

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We recently added this paragraph to section five in the manual.

EXCERPT from Alcoa Aluminum Rivet Book, dated 1984.

"The standards to which driven rivets should conform are frequently uncertain. In addition to dimensions and perfection of shape, inspection is concerned with whether the drive head is coaxial with the shank (not ?clinched?) and whether there is excessive cracking of the heads. It has been determined that even badly cracked heads are satisfactory from the standpoint of static strength, fatigue strength and resistance to corrosion. (Poorly set and cracked) rivet heads were tested in tension to determine how well formed a head has to be in order to develop full strength. The tensile strengths of all the rivets were within five per cent of the strongest. The test indicated that minor deviations from the theoretically desired shape of head are not cause for concern or replacement. The second rivet that is driven in any one hole is likely to be more defective than the first because the hole is enlarged and rivet will be more likely to buckle and form an imperfect head.

Tests have shown that very small rivet heads are sufficient to develop the strength of the rivet shank, even when the rivets are subject to a straight tensile pull?.where a large head is not needed for appearance, smaller sizes of drive head should be used to decrease the required driving pressures."

Most builders end up drilling out the hole oversize and call or email us to find out how to fix the situation. Usually they do more harm than good, so we tell them to just leave the ugly rivet in place and move on.
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My section five doesn't have this. So its information I'm glad to have. Hope others find it useful.