What countersink to use?

I'm starting my install of the Vetterman Vents into the fuselage of my RV4 for my passenger. I have cut the hole, deburred it and now have the Vent sitting in the hole, holes drilled and held in place with 3 clecos.

I was going to use 3 small screws and lock nut to hold it in place. But have now started thinking about just riveting it into there.
I have an Avery Counter Sink Kit with a 1/2 dozen different bits from a #40 down to a # 12.

How do I decide on what one to use? I can set the depth and make the cut and all of that but I'm not really certain on what bit to use.

Can anyone help me out on this one?

Thanks

Tim

You could try the countersinks on some scrap to get the right depth/size. Drill same size hole and check the fit.

Yes, I was thinking of doing that this morning. As it's a used tool kit that I bought, I will start with the one that looks used the most as these tools were used previously to build a 7.

I thought there might have been a little more science involved or a rule of thumb, but maybe not.

Thanks

Tim

Dimple

Hi

I would advise you to countersink the vent and dimple the skin because I think that skin is 25 thou, and the minimum thickness to countersink is 32.

I support the input of Steve. By countersinking your skin, you will probably get a knife edge condition in the skin.

Yes I think your right. It is getting thin so I've stopped the countersinking. Will put 3 bolts and washers in place with a dab of paint on the bolt heads and it won't be that noticeable.

What will be noticeable is that I cut the hole from the inside the plane pushing out in the skin. This has caused some of the paint to chip/peel away from around the hole. Obviously the prepping was not great causing the paint to pull/peel away.

I now need to figure out how to blend that repair in.

I'm not sure if this would have happened if I had cut from the outside the plane pushing in. I have another vent to install for the front but I think I will put that on hold for a bit.

Tim