The standard exhaust nuts are 7/16 A/F spanner size, 5/16 UNF thread size. These are a tight fit for all but the thinest of spanners. A good alternative is to use aircraft high temp Kaynar type nuts, sometimes known as "Jet nuts", usually with a silver anti seize coating. These nuts come with with a 3/8 A/F spanner size that gives a very useful bit of extra clearance for fitting. A Military Standard part number for these is MS21043.
| 
