PART NO.  SPL.PAGE  DESCRIPTION                                                            
AEH393     TC.A.16   Pipe -water outlet -(head to thermostat, water.pipes, Twin Cam)
AEJ4       TC.A.16   Washer -pipe to cylinder head (copper) -(to thermostat, water pipes)

The thread is 11TPI BSPP (British Standard Pipe Parallel) which uses the 55-degree Whitworth Form.
It may be called G16 Straight Pipe Thread (where "16" is 1/16th inches for a 1-inch pipe).
More commonly 1-11-BSPP male thread. Major diameter 1.309-in and 55=degree thread angle.

Use with AEJ4 Washer -pipe to cylinder head (copper)