Nothing to do with relieving pressure. From 64 to 68 MGBs used a PCV valve on
the inlet manifold to apply a constant low level of vacuum to the crankcase
for through-flow ventilation. Whilst this would help to relieve any excess
pressure from blow-by etc. that wasn't its purpose and it would only have a
limited capacity to do so. Any positive pressure build-up would escape from
the *inlet* side of the ventilation system which was the vented oil filler
cap. Before 64 pressure could escape from two ports on the engine that were
open to atmosphere - the pipe from the rocker cover to the front air cleaner
and the pipe on the front cover or front tappet chest cover. From 1969 carb
vacuum was used for the ventilation to replace the (comparatively) troublesome
PCV valve.
PaulH.
----- Original Message -----
Wasn't the hole in the manifold originally part of the system that
relieved crankcase pressure?
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