3 newbie questions..

[Jake14]

1. I found a self-etching spray-can primer which seems to adhere much better than the one I'm currently using. If I change in mid-project, would there be any issues in riveting parts together which have been painted with different primers?

2. Scuffing, cleaning and priming the skins is easier before dimpling. Is it ok to do that?

3. I'm confused about the 'final-drill' requirement for flush rivets in pre-punched kits like my RV-14. The punched holes expand to be larger than the rivet diameter when dimpled, so it seems that the lateral location of the rivet will be determined by the dimple which it nests into. What, then, is the purpose of 'final-drilling'?

[DaleB]

Originally Posted by Jake14

1. I found a self-etching spray-can primer which seems to adhere much better than the one I'm currently using. If I change in mid-project, would there be any issues in riveting parts together which have been painted with different primers?

Nope.

2. Scuffing, cleaning and priming the skins is easier before dimpling. Is it ok to do that?

Yep. The only caveat is that dimpling may ding up your primer, depending on the type, which could require a little touch-up. Or it may not, depending on your primer philosophy. Some people don't prime at all, you know. I match drill, debur, dimple, then prime but the order can be different if it works for you.

3. I'm confused about the 'final-drill' requirement for flush rivets in pre-punched kits like my RV-14. The punched holes expand to be larger than the rivet diameter when dimpled, so it seems that the lateral location of the rivet will be determined by the dimple which it nests into. What, then, is the purpose of 'final-drilling'?

If you dimple without drilling or reaming the holes to final size, the pilot on the dimple die will stretch the hole a little. This can cause stress cracks around the rivet holes. I find that using a #30 or #40 chucking reamer instead of a drill gives me nicer holes that don't need much in the way of deburring. My experience is with an RV-7 kit, I don't know if the rivet holes in the 14 kit are any different.

[Jake14]

Thanks Dale. One other thing still puzzles me. If I 'final drill' before dimpling to avoid stretching the hole with the dimple pilot, doesn't the subsequent dimpling process also stretch the hole and could this also cause stress cracks? Or is dimpling a different mechanism?

[DaleB]

Here's my take on it from what I have seen discussed here before -- and it has been discussed, fairly recently in fact if you search around.

The hole will enlarge a little bit as the surrounding metal is slightly stretched to form the dimple. However, this doesn't cause the stress concentration around the edge of the hole that ramming the dimple die pilot through it does. Match drilling or reaming also removes any scratches that may have been left by the factory punch, and those can also lead to stress cracks.

I also noticed that the holes were a little larger than they were supposed to be (or than I expected them to be), but the rivets will expand when set to fill the hole and form a gas tight seal.

Why not scuff, dimple, clean with MEK then prime?