0.177 in through holes - for which size screw/bolt?

[Kari Lumppio]

Hello!

I've been born, rised and educated in metric world. Inch sizes is something which aviation engineering forces upon me and this is one of the cases.

There is a strut flange with a dozen of 0.177 inch diameter through holes. This should be fastened to a slab of aluminium with screws/bolts. Are this size holes loose tolerance meant for size #8 (0.164 nominal diameter) fasteners? Or is there some magical anglosaxian size I have overlooked? In my opinion no metric size is close.enough.

Thanks in advance,

Kari

[bentwaters81tfw]

Equates to 4.50mm Standard small bore ammunition size.

[XV571]

Hi Kari,

According to the A320 SRM 0.177 inches is at the lower end of the tolerance for a 11/64" rivet (hole should be 0.176-0.180 inch / 4,47-4,57 mm). 11/64 inch is the first oversize for a 5/32 inch (4,0mm) fastener. When oversizing fastener holes they increase in 1/64 inch (0,4mm) increments so maybe the holes have been oversized already. Typically bolts or pins (hi-loks) don't have an oversize that small; usually we would install the next nominal (3/16") where a threaded fastener has been used. Check to see if you can keep an adequate edge distance should you need to go up a size.

11/64 inch is also equivalent to a #9 screw see here: http://www.instockfa...stenersizes.asp but I don't know of any application in aerospace for such a fastener.

HTH

Jonathan

[Kari Lumppio]

Hello!

Jonathan, you said it. Rivet size. Now it makes sense. The ready-made flange is likely meant to be riveted on an aircraft skin plate and the strut then bolted on the flange. This will cause some complications as we would like the flange to be removable too. But that is another story.

Thank you,

Kari