Checking the crank?

[Tommo]

I have an R5 motor torn down and want to measure it to see if it's in spec before doing anything else.
I have v blocks, dial indicators, feeler gauges etc but idk what am I looking for?

[1976RD400C]

That info is in the back of the service manual although this one has the A and F measurement reversed.

[Tommo]

So the R5 crank I have checks out okay. The bike was on the original bores and when it came to me had no carbs on it but turned over easily, seemed like it would have good compression.  I'm curious what the labrynth seal does in relation to the pressure between cylinders. Obviously the shaft has to turn within it, and the timing of the strokes being 180 degrees out, theoretically anything bleeding through would be negated within the seal? 
Another thing, I was looking at the Vito's cranks and the lobes of the crank aren't like stock, it looks like they're cut away some to make them lighter.  How does this affect the internal volume and compression with respect to jetting?
Maybe I have too much time on my hands to dwell on the esoterics of 2 strokes, I don't know.

[SoCal250]

The labyrinth seal is basically a maze that impedes/slows the direct flow of pressure between the two sides of the engine. It is not a complete or true seal as in the case of a traditional engine seal. They rarely need replacing as it's not really a wear part. An exception would be if you had a catastrophic crank or bearing failure that resulted in the crank having a lot of runout.