SU Carburettor Nut & Bolt Sizes

  • Print
Details
Written by Malcolm Eades

Most of the nuts and bolts used in the Magnette are Unified threads (UNC & UNF), a standard developed after WW2 so that English and American engineers were working to the same sizes. But there are some exceptions and if you start working on the carburettors, you will quickly find that your trusty AF socket set isn't much help. This is because when they made the H4 carburettor, SU were still using bolt head sizes common to British Standard Fine/Whitworth threads (BSF/BSW).

Rather than risk damaging the various carburettor fixings, it is best to get the right spanners up front. The three sizes you will need are:

3/16"BSF / 1/4" BSW

Float chamber lid retaining hollow bolt

1/4" BSF / 5/16"BSW

Jet adjusting nut 
Float Chamber Retaining Bolt

3/8"BSF / 7/16" BSW

Jet Locking Nut
Fuel Line Banjo Bolt
Dashpot Brass Cap
You will note that in each case the BSW size is 1/16" larger for the same bolt head size. But if your enthusiasm extends to British cars earlier than the Magnette, you probably know this already.