[Bellerophon]

The aluminium cross tubes are held in the head by the ends of the tubes
being spun over (or swaged if you understand that better). The holes in the
casting have a seat machined at each outside end so that after the tube has
been pressed in the ends are turned over using a tool, just like riveting.
This is fine when the head is new, however, with the constant temperature
changes it tends to work the tubes and low and behold we develop a leak.
When they were made no one thought that they would have to stand up for fifty
years, although today an alternative method may be used.
Geoff's solution of sealing with a solution seems good providing that you
can remove any oil that might be trapped between the tube and head. Due to
the closeness of petrol it is no good trying to glue the outside joint with
say Araldite as that would soon become detached.
The solution which the De Havilland Aircraft Company used on the crankcases
of their Gypsy aero engines to sort out any problems with the castings
made by the Stirling Metals, was to place them in a heated autoclave (an oven
in which it was possible to obtain a vacuum). When the castings were up to
temperature a special solution which set when cold was applied to the
castings and when the air was let back in it forced the solution into any holes
or porosity thus rendering it sealed. They were then pressure tested before
any machining took place.
My answer would be to re-tube the head, however you would need to make to
tool to turn the ends of the tubes, but one thing some brave person may try
is to make a bolt with angled ends which one could insert and turn and
tighten so that the ends are compressed, food for thought.
My regards,
Bellerophon