mercury 650ss

The early 650 engines had a spigot on the cylinder barrel that went into the recess in the cylinder head.  In 1966 at engine number 114870 they did away with the spigot on the barrel and the recess in the head and fitted a Hallite composite head gasket instead of the copper/asbestos of the earlier Dominators.   Obviously these features went on to the Mercury from 1968.  If your head has the recess and you have a barrel without the spigot there is a filler available to fit the recess accordingly.   I would talk to AN about the piston clearances because the type and content of aluminium in the pistons made by foreign manufacturers is different to British made pistons of the era.  Good luck, Howard

Normally pistons are supplied undersize to give the correct clearances when bores are made to nominal size.  So standard size will have 68mm bore.  There are different alloys so it's not possible to generalise.  Other views will no doubt appear.

Because of the differences in material and quality between new old stock  and  current manufacturers  there is good  reason to be cautious. I was advised by John Hudson to aim for four and a half thou clearance when building a 99 with  NOS 650 pistons. This was good advice as although not a quiet as some other 99's ,it has performed really well for many years with the orriginal oiling system which tends to under lubricate the drive side if not 100 %.. I would not describe this engine as "precision built", bores are not always truly parrallel or at 90 degrees to the cases . A little extra clearance over that specified  may  be prudent.