Hatz Biplane Association

I just noticed that the bottom wing spar fittings have washers on both ends of the bolts. The top wing uses the same bolt but doesn't have the washers on the bolt head end. I checked on the Internet and ended up with a Tony Bingelis's article that specifies that you only need a washer on the end you are going to torque. That is normally the nut end. The exception is a blind hole situation where the bold head is torqued. There is blind hole case on each wing where the bolt holds the compression tube. It needs a washer.

So are the drawings wrong or do those bolts need extra washers. And if they do, is it proper to put one on the bolt head end or should they all be on the nut end? The article specifies a maximum of three washers but doesn't specify where they should be put.

I'm just finishing up my first top wing.

You are right to accept what Tony has to write. You should have a washer under the nut.
The bolt grip, the smooth part, needs to go through all of the items you are clamping together. When you torque it, you should have between 1-1/2 to 3 threads showing beyond the end of the nut with all of the washers installed. If you have less, you need a longer bolt - more a shorter bolt.
Getting the correct length bolt without extra washers is rare. You may have up to three washers in the system before you need to rethink the bolt length. Start with one under the nut, the second under the head if needed and finally two under the nut. This spreads out the stress evenly and looks good when finished.
Do a quick study of the front and rear stack ups and you will probably find the top is a little thicker and doesn't need the second washer.

(Man, I'm from Indiana and it's been tough typing "waaasher" instead Warrrsher"!)

Good Luck!

Jeff Moore

Thanks Jeff. Distributing the washers (when needed) between the head and nut end of the bolt seems to make sense. I ended up with a shim under my compression tube and had to go to the next length of bolt and three washers. It looks a little much to have three washers on the nut end.