Head Cracks Between Valves

[DieselScout]

Below are measurements of cracks between the valves on an old D24T head.

Measurements of cracks using caliper and magnifying glass:
Cyl. 1: 0.59mm
Cyl. 2: 0.67mm
Cyl. 3: 0.49mm
Cyl. 4: 0.40mm
Cyl. 5: 0.57mm
Cyl. 6: 0.60mm

Measurement of cracks using feeler gauges:
Cyl. 1: 0.65mm
Cyl. 2: 0.60mm
Cyl. 3: 0.43mm
Cyl. 4: 0.40mm
Cyl. 5: 0.63mm
Cyl. 6: 0.43mm

Average Measurements of feeler gauges and caliper.
Cyl. 1: 0.62mm
Cyl. 2: 0.63mm
Cyl. 3: 0.46mm
Cyl. 4: 0.40mm
Cyl. 5: 0.60mm
Cyl. 6: 0.51mm

Any thoughts on these cracks?
The book spec specifies no greater than 0.50mm.
How critical is that limit?

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[ngoma]

Common rule of thumb is you're good to go up until a nickel can fit in the crack (edgewise).