Just out of interest:-
After a run with the engine cooling down and the air in the crankcase contracting where is the air pulled in from? (pcv valves + reed valves)
Past the rings?

What would be the symptoms if the crankcase reed valves failed?

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aka DrBagiva, DrBusqvarna, Uncle Knobhead

I don't think that the PCV valves prevent any flow into the rocker covers at near atmospheric pressure (do they actually seal as in SEAL...?)
...and of course there's the crankcase breather...
I don't think it's much of an issue as a breather system is there to cope with the considerably larger and more violent transfer of air underneath reciprocating pistons rather than the comparative non-drama of air cooling down and contracting a bit.
Crankcase reed valves are the preserve of two-strokes unless I'm much mistaken.

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'98 M2 Razorback, it's a keeper.

There are reed valves fitted into the XB engine. I've been studying the xb9r manual. What I'm trying to ascertain is whether the engine is actually "ventilated" and if so how does the air enter or is it more a case of a pressure relief system?

Quote:-
Pressure created in the flywheel area on
piston downstroke is released through the
reed valve
into the
gearcase. From there a mixture of crankcase air and oil mist
is vented up the push rod covers to the upper rocker box.
See Figure 3-115. Air is allowed to escape the rocker boxes
by exiting the positive crankcase vent (PCV) valves (4)
located on top of the rocker boxes. From the PCV valves the
air enters the crankcase breather hoses (2, 3). The crankcase
breather hoses route through the air cleaner base plate (1) to
the air box where it is directed inside the air filter element and
back into the engine.
The oil mist collects and eventually returns to the crankcase
through oil passageways in the cylinder head.

_________________
aka DrBagiva, DrBusqvarna, Uncle Knobhead

Then I stand corrected.

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'98 M2 Razorback, it's a keeper.

Lots of places for air to get in should it need to, plenty of leaks around the system which given time the air will find its way past (including the piston rings and valve guide seals).

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08 Specialized Langster

wouldn't worry too much, for the last 35 years I have modded every bike I have owned to breathe to atmosphere without any issues