If you mean the wire u/c which is held on by two plastic clamps that clamp over the wire, I think they are M5 5mm, which should be the same as the wing bolts, does one of those fit?
hi Eugine why not change your undercarrage to the firbreglass one from Chriss Foss its a lot more forgiving and it only uses 2 m5 bolts held in by 2 captive nuts that you epoxy in place ..use nylon bolts though as any hard landing might rip it off like mine did!!! good luck