350 blue star oil line

[Jean-Luc]

Hello !
1933 350 blue star:
What is the purpose of this oil line between the engine case and the cylinder head ?
What is it supposed to lubricate ? 
It seems to lead to the inlet valve guide...

I still don't have a document showing the lubrication circuit for this motorcycle .

Thanks for attention !

[Y13 Alan G]

This is common with a lot of pre-war BSA's. The inlet valve guide/stem is lubricated whereas the exhaust isn't. I believe the theory behind this is that if the exhaust valve guide was lubricated the high temperature of the exhaust valve would cause the oil to build up as a deposit and therefore cause a sticky valve. I lubricate the exhaust valve stem/guide on y pre-war Y13 by high temp spray oil every 80 to 100 miles with no ill effects after over 30 years.

[Jean-Luc]

Thank you Alan G for these precious details.
She is my first Pre War motorcycle.
So I have a lot to learn.