JM2¢?
I disagree.
The steps to completely identify the problem:
1. are there other cracks?
2. what other repair work is needed?
3. how expensive is it to repair, and re-machine critical surfaces?
4. most important: even if the work is "guaranteed" (if not successful, either a free new repair or a refund), does that cover towing the vehicle, removing the engine, tear-down, delivery to the shop, assembly and re-installation? What is that worth?
These must all be balanced against the cost of a replacement block (and its testing).
As to the pressure: with a zero cap, water will boil in the jacket next to the exhaust valve and spark plug far easier and earlier than with any pressure (and without the engine overheating, or a high temp gauge display). System pressure is your best insurance against temperature-induced knocking.
If the water temperature at the thermostat has not reached the kick-off level (180° etc.) the entire block's water system is under light pressure by expansion as well as local boiling, which is not readily relieved backwards to the water pump.
Any interior cavity that experiences compression also experiences combustion. The cylinder, head etc. are not limited to 150-175 compression gauge readings, but at least 700 psi.
If you're in Manila, Capetown or Auckland, finding another block is not practical, but I'd make every reasonable effort to find, price and test any used block before starting a repair.
