I suspect that under the dynamics of a running engine things are quite different. Remember you’re dealing with 300-350 psi of compression (ideally) and the head is heated to approximately 80 C/180 F.

I think these two factors are what make head cracks leak, not hitting it with 50 psi air in ambient temperature water.

My opinion which is probably worth about equal to the $0.42 I see glued to that head...

__________________
The OM 642/722.9 powered family
Still going strong
2014 ML350 Bluetec (wife's DD)
2013 E350 Bluetec (my DD)

both my kids cars went to junkyard in 2023
2008 ML320 CDI (Older son’s DD) fatal transmission failure, water soaked/fried rear SAM, numerous other issues, just too far gone to save (165k miles)
2008 E320 Bluetec (Younger son's DD) injector failed open and diluted oil with diesel, spun main bearings (240k miles)

1998 E300DT sold to TimFreeh
1987 300TD sold to vstech