This is a rib from the center section. I used this for my testing. Here I broke off the cap strip on top and found glue to be of good quality. At leas

Do you have any idea what type of glue was used? Does not look like anything I have seen.

Yes after extensive research I found out that it is FPL-16A Hughes Glue. It is an epoxy that was developed by Forest Products Laboratories back in the late 60's. I have done some testing on it and it seems very good. I even did a high temperature test where I got the glue joint on the nose of the rib that is pictured up to 275 F. I then tried to break the 1/4" nose doubler off of the plywood. The plywood broke and not the glue and the glue was still very hard. After that I did a boil test on the portion that I broke off to see how well the glue would hold up and to see if the plywood was marine grade. So I boiled the part for 3 1/2 hours and there were no adverse effects to the wood or the glue. I also found an FAA representative in Utah that used that glue when he built a Pitts in the 80's. So I sent him some shavings and he said it looked and felt like the same stuff and said if it was his project he wouldn't hesitate to finish it. So I am very confident with the mechanical properties of this glue. That is what I am using on the rest of the plane. All other epoxies have max service temperatures of no more than 160 F then they start to soften. The instructions on the can say you can have gaps up to .010" with no appreciable loss in strength so it has good gap filing qualities and it is easier than using resorcinol glue. You don't need absolutely perfect joints and only need intimate contact on the parts so that clamping is not as critical. The only draw back is that it has a low viscosity so it tends to run out of the joints and down the wood. I get around this by painting on a couple coats and then let it start to get thick before assembling the parts. After you get the parts assembled you can then wipe off any runs and it shouldn't run anymore. You can buy it at Aircraft Spruce. I am sold on it and will not use anything else on my plane. A lot of people tell me that I don't need to worry about the inside of the wing getting over 160 F and they swear by
T-88. However I prefer something that will withstand higher temperatures just in case. If your interested in more info on it let me know I can send you a DVD of my testing because I shot video of everything.
Mark