ZD 30 Options.

[Rumcajs]

The pistons melting isn't the root cause. Heat is. Same with any engine.

Aaaaaaaaaaaaaaaand the heat is melting the pistons .

Seriously, the ZD30 woefully inadequate pistons design and materials they're made from can't really cope with the output asked for especially when combine with that fail prone high pressure fuel injection system and results are there. Basically on their limits during easy to moderate loads once introduced to serious workout in high ambient temperatures like in Australia it is just a matter of time.

Had Nissan used cast iron skirt less pistons (not really widely available at the time though but still could use steel forged crown and articulate skirt units)and designed cast iron head as well it would be non issue today and ZD30 would have the distinction of be the most reliable Nissan diesel engine with the power output to match and TD42 would be largely forgotten.

Lets not kid ourselves, ZD30 was born at times when Nissan faced bankruptcy (the world would be better off indeed in the hindsight) so it was only natural for the cost cutting exercise to use the cheapest design and materials available. I'm pretty sure everyone is pretty much well aware of Nissan business model by now ==> cheap and nasty and keep recycling old stuff as long as possible.

My guess, Nissan car engine division boffins went to a Sushi bar got drunk and knocked off petrol engine design together with balance shaft crap and penned the mongrel with diesel injectors....using petrol motor components. Honestly I'm yet to see decent Nissan car division modern diesel engine, not a nice record there, just look at the mongrels produced ==> RD28 (RB petrol based/copy mongrel), ZD30(QD based perhaps a bit of TD2x thrown in) vs Nissan UD truck engine department designed TD42 small truck/bus/forklift engine.

Anyway, back to OP,

Remove the head and do the job properly. You can source slightly stronger pistons as well from ACL rebuild kits if they're still available. Make sure to have injectors checked or replaced, those dual spring abominations are known to leak/dribble fuel....

Cheers