The inboard nut securing the crossover exhaust flange on the #2 cylinder is pretty much impossible to loosen or tighten with any tool known to mankind. The exhaust pipe itself bends right over the stud so the flange has to be lowered just to get the nut started. On the back (inboard) side, the inter-cylinder baffles are right up against the nut. There's no access at all from the forward of the aircraft because the cylinder fins are right there. On the aft side, both the oil drainback line and intake tube must be removed to have any chance of getting a tool in there and the stud from the #4 cylinder intake leaves little room to swing a wrench. Here's a photo of the tool I custom made to remove that one nut. The open area on the wrench was necessary because the 5/16-18 UNC Lycoming exhaust nut was almost right up against the exhaust pipe and a standard box wrench wouldn't fit in between the nut and the pipe. Neither would a standard open end wrench.



That worked OK to remove the nut, although I could only unscrew it half-a-flat at a time. It was a tedious process.

After having some AD work done on my O-290-D2, the engine is back on the mounts and I'm now in the process of reinstalling the crossover exhaust. I used the custom tool (above) to tighten the exhaust flange nut half-a-flat at a time (again), but the tool wouldn't torque it; It was too weak in that direction and tended to open up and slip. So now I'm going to have to weld up some kind of stronger tool which will fit in there, or use another kind of nut altogether...which brings me to:

Has anybody used 12 pt. reduced-dimension nuts on their exhaust flanges? I was surprised that they were even available with course threads, but <a href="http://www.summitracing.com/parts/arp-401-8303/overview/">Summit Racing</a> carries them in stainless steel with coarse threads (photo at link). I ordered 8 of these today which I intend to use on my exhaust flanges. They take a 3/8" wrench which I am confident will have enough clearance to be able to get back behind the exhaust pipe where a 1/2" wrench won't fit. These are not lock-nuts so I'll still use a split lock washer underneath. Any comments on this plan would be appreciated.

One other thing I learned in this ordeal: 5/16-18 Lycoming "exhaust flange nuts" take a 1/2" wrench, whereas "standard" 5/16-18 nuts take a 9/16" wrench. Don't ask me how I know this...