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1. At Probuild we have found that the most reliable way of fixing an underbelly to the fuz is by a very simple system that if installed correctly will not come loose due to vibration.
2. The underbelly is fixed to the fuz by 5 cap head bolts, one in the nose, 2 over the wing and 2 at the end of the underbelly.
3. The method used is very simple, using two sizes 1/4" OD and 3/16" OD of Polystyrene plastic tube one slid inside the other to hold a 4-40 x 1/2" bolt in place (See Sketch)

4. The bolt positions are marked out on the underbelly flange, and then drilled out to suit the dia of the 4-40 bolts. The bottom of the underbelly is also marked out and drilled out to 3/16" to enable the plastic tube pass through the bottom of the underbelly

5. The underbelly is offered up to the fuz in its correct position and is then marked out through the holes you have just drilled in the underbelly.
6. The fixings in the fuz are dubro threaded inserts or captive nuts mounted on a light ply backing plate (See Sketch) are glued in position inline with the holes on the inside the fuz.
7. The plastic bolt assemblies can now be glued in position using cyano and epoxy.
8. When set, the plastic tubes are trimmed to just proud of the surface then sanded to the underbelly profile with wet & dry.
9. That is all there is to it, a very reliable and simple method fixing.