Countersink diameter to accept a #40 dimple.

Hey gang,
I'm ready to counter sink the many small #40 holes in the wing spar to accept the skin #40 dimples. A bunch of them.

Did you guys sink the counter sink just enough to make a 3/32 rivet head flush? Or did you go deeper until a test dimple sits totally inside the counter sink? (This might be too deep). Or somewhere inbetween.

I would like to know what diameter your counter sink is that accepts the #40 dimple.

Thanks gang. Ed

I have a bunch of test pieces, made from all the thicknesses of skin, with properly sized holes (#40 and #30) and then dimpled for 426-3 and -4 rivets. I use this, as appropriate, to set the size of the countersink correctly...the dimpled skin must fit inside the countersink.

BTW, if you haven't done so, countersink and rivet in the nutplates (I think it even says so in the instructions), to keep the countersink pilot from wandering...you'll end up with a hole on the "inside" of the countersink that is a bit larger than a normal drilled and dimpled hole, IIRC.

Bottom line...make some test pieces and check for fit.

Female dimple die

We use the female dimple die to determine the depth.

Insert a short 3/32 rivet in your female dimple die and measure the distance the head sits below flush with a vernier.

Machine countersink until the rivet head is the same distance below flush as measured in the female dimple die.

We used this method on all the wing spar countersinks with good results.