Hope it’s permissible to do this, but I have lifted this thread from the TR register forum, which may help to reinforce Arwyn’s advice:
“Crankcase breathing ex-factory was not entirely satisfactory.
TR4s followed TR3s design, having a breather pipe in the
block, beside/just below the fuel pump.
TR4As had this provision in the block blanked off and made
provision for venting the rocker cover (2 or more arrangements).
Now, you seem to have the original TR4 rocker cover (without
breathing - correct for TR4) but has the blanking plug (originally
fitted to TR4As) been removed and replaced with a TR3/TR4
style breather pipe?
If not, then you will have a build-up of pressure leading to many
oil leaks.”
