Mercury manifold thread

Hello Terry - If it's a genuine Norton part it should be 5/16 inch BSW in the manifold and 5/16 inch BSCycle (26 tpi) for the carb fixing.  As far as I remember the single carb manifold for the Commandos made at the sam time had 5/16 inch UNC thread into the manifold and 5/16 inch UNF thread for the carb.  I suppose the last of the Mercuries assembled later in1969 could have either combination depending on what studs were in the parts bin.  Unfortunately fpr you both 5/16 inch BSW and UNC thread forms are 18tpi so both can fit into the same female thread.  If you are buying them I'd go for the whitworth variety because the nuts in cycle thread are a reduced hexagon type and look much neater than the Commando style UNF nuts.   If you are making the studs it's your choice.  Regards, Howard 

Terry,

My 1969 Mercury has 5/16" BSC threads for holding the carb on, with the reduced nut  size that Howard mentions i.e. 1/4" BS.  I would therefore expect the thread into the manifold would be 5/16" BSW, but haven't taken it out to check.

Regards, Philip