Sometimes I can't sleep at night. Usually because my Airstream is leaking ;-) ... but sometimes because I'm focused on some ridiculously small detail of a part we sell.
In this case, the matter keeping me awake is drill bits. Of course this is somewhat silly since only .002" separates the two bit sizes we are discussing here. We're talking about the best drill bit for an Olympic blind rivet here. The debate is between #20 and #21 sizes. #20 drill bits are .161" diameter. #21 bits are .159" diameter. You can drill a hole with each next to each other and you won't be able to tell them apart.
However, much has been written about this subject so I did some research online by looking at the resources available from Gesipa (who makes Olympic rivets) and Alcoa (who makes Marson) and a bunch of other major blind rivet manufacturers. In that web search, I am consistently finding all manufacturers recommending #20 bits for blind riveting.
But wait: this is not an "I told you so" posting. I found something else. While I was researching, I decided to look at some of my old and trustworthy airframe mechanic's books to see if the same recommendation holds true for solid buck rivets (the kind used to build Airstream exteriors). The answer surprised me:
Aircraft Sheet Metal
(Jeppesen Sanderson Training Products, 1988) says: "Alcoa claims that a rivet should fit as tight as possible before driving, especially those rivets of harder alloy." Airstream rivets are one of the softest rivets available (1100), so it is less important than with a hard rivet. However, the book states the ideal rivet for a 5/32" solid rivet is a #21.
Standard Aircraft Handbook
(McGraw-Hill, 1999) says: "Holes must be clean, round, and of the proper size. Forcing a rivet into a small hole will usually cause a burr to form under the rivet head." The book states that the proper size for a 5/32" solid rivet is #21.
Aircraft Sheet Metal Work
(McGraw-Hill, 1942) says: "Rivets should fit snugly. The best clearance is the smallest one that will permit the rivet to be inserted easily and without delay." The book states that the proper size for a 5/32" solid rivet is #21.
So... it looks like #20 bits are recommended for blind riveting with Olympic and other "pop" style rivets. And #21 bits are recommended for solid buck riveting.
Remember that we are talking about so little difference in sizes that it probably doesn't matter as much as the fact that you probably ovaled that hole badly when you drilled out the original 1/8" rivet.
My takeaway for this is that we are now going to start selling #21 bits alongside the #20 bits we sell. And we will explain why on our site.
I will add this: when I was working on my 1956
Airstream this past weekend, I was using a #20 bit to ream out the old 1/8 holes so I could reattach a chimney on my trailer. Even after reaming with a #20, and using plenty of clecos, I couldn't get the darn rivet in the hole a few times because the holes wouldn't stay aligned.
Enough on this topic...I'm going to get some sleep.