Torque tests of closed vs open ended rivnuts vs hole size

The plans call for open-ended rivnuts for the fuel tank sender mounting plate. However a previous post describes my evaluation of closed end rivnuts. Here I examine the impact of 1) sealant on the spin out torque strength, and 2) the impact of setting the rivnut in a snug 15/64" hole and a slightly oversize 1/4" hole.

Note that these tests were done with an M4 x 12mm stainless steel bolt, which was long enough to bottom out at the closed end of the rivnut before the head clamped down on the flange of the rivnut. If the latter is allowed to happen the torque performance of the rivnut is unjustifiably (in this context) inflated. Unjustified, for in the usage this rivnut flangle will be pressed into a deforming cork gasket. In the case of the open ended rivnut, a second M4 x 12mm bolt was inserted into the rear opening. Each bolt was arrange to share roughly half the rivnut threads. As a result, the resistance to stripping - in the event the rivnut threads fail first - will be less than expected for the same rivnut in service.

The results are

1/4" hole:

closed end, no sealant - failure between 17.7 in-lbs and 21.2 in-lbs

closed end, sealant - failure between 17.7 in-lbs and 21.2 in-lbs

closed end, sealant - failure between 17.7 in-lbs and 21.2 in-lbs

open end, sealant - ok to 28.32 in-lbs, thread (see note) stripped at 31.9 in-lbs

15/64" hole:

closed end, no sealant - failure < 17.7 in-lbs

closed end, sealant - failure < 17.7 in-lbs

closed end, sealant - failure between 21.2 in-lbs and 23.0 in-lbs

open end, sealant - ok to 23.0 in-lbs, thread (see note) stripped at 24.7 in-lbs

Note: the thread stripped at definitely lower value than the intrinsic strength of the whole thread as the available thread was shared between two bolt-ends, as described above.

Conclusions can be drawn from this:

1) The sealant has no noticable effect on the spin-out resistance. I did not realise how soft the cured sealant is.

2) The open ended rivnuts have much better spin out resistance. This is due to the fluting being slightly sharper for the closed ended rivnuts, and the fluting extending to the rear of the flange. The open ended rivnuts have a smooth surface behind the flange.

3) The use of 1/4" slightly oversized holes improves the performance of the rivnuts. The reason for this is the rivnut fluting has more of a chance to deform an bit into the edge of the hole, on the interior side

4) There is no issue with using the open ended rivnut with the sealant, so long as I insert a bolt when applying it to fill the threaded space. In view of how soft it is some creepage into the thread structure does not matter.

So after all of that I have found that the best scheme was just as the plans stated. I will use open ended rivnuts, 1/4" holes.

Postscript Feb 22: I purchased a 6.1mm chuck reamer for the rivnut holes, as indicated in the Gesipa specs. I found enhanced spin-out performance for this size as well. 1/4" is 6.35mm - a bit on the large side.