Completion of wing small inspection port nut-plate doublers

I decided to obtain M4 nutplates for the small inspection port doublers. Unlike a M5 nutplate which can created by tapping an AN3 nutplate (because the thread pitch is almost identical, and this is what Sling does with some M5 nutplates in the canopy kit) a M4 nutplate cannot be adapted from an 8-32 nutplate because the thread pitch differs (10-32 is very close to 0.8mm while M4 is 0.7mm).

So M4 nutplates must be obtained, but they are hard to find at a reasonable price. However Sling aircraft referred me to Belgian firm KL-projects which had pretty reasonable pricing for M4 nutplates.

Why M4 nutplates? The reason is I did not want a mixture of M4 bolts for the large inspection ports and 8-32 bolts for the small inspection ports, if I were to otherwise use imperial nutplates such as MS21047L08. It would be too easy to get these mixed up with consequential risk of stripping either nutplates or rivnuts, particularly if someone else does some maintenance. So I am standardising on M size bolts at all exterior locations.

A slight additional expense (postage cost as much as the nutplates), but a worthwhile payoff I think.

Note the hole-hole separation of these M4 nutplates is 17.0mm compared with about 17.5mm for the MS21047L08 nutplates.

The plates were drilled, countersunk with a #40 ~ 2.4mm guided 120 degree bit, deburred, edge-smoothed and primed as usual. The nutplates were then attached 2.4x6mm countersunk stainless rivets (Cherry CCC-32 which is what HW-RIV-531-X-X-0 is btw). Duralac yellow was applied to the rivets and the mating nutplate and primed aluminium surface.

The main bolt hole was chuck reamed to 4.1mm, and the rivet holes were drilled to 2.5mm. The rivet holes were positioned by match drilling using the rivnut holes as a reference and using a 4.1mm chuck reamer shaft to keep the bolt hole and rivnut centres aligned. Some drill angle error (I was using a drill free-hand) led to slight lateral misplacement of a small number of rivet holes. This was amended by opening up these few rivet holes very slightly with a needle file. The rivets expand easily, but in the event this lead to no apparent weakness in any of the rivnut attachments.

Using a M4 bolt to help keep the rivnut centred in the 4.1mm bolt hole, positional accuracy of the rivnut of about +/-0.1mm was consistently achieved. The main 4.1mm hole may be enlarged 0.1mm or so (or maybe even to 4.5mm diamater) to remove some very residual interference in some cases between the bolt threads and the hole edge.

Total mass of 8 completed plates: 228 grams

Next time I will work up a CAD drawing and get someone else to machine these plates!