Toyota 1HZ is an indirect injection –
Which means they have a Precombustion chamber is in the cylinder head – the injector injects directly into the Preco chamber ( this is why it is common for 1HZ cylinder heads to be cracked )

Toyota 1HDT is direct Injection
Which means the injector squirt directly in the bowl if the piston is shown below and the explosion occurs in the piston bowl – preventing the cylinder head from getting so hot and reduces the cyl head from cracking

Unfortunately the 1HDT and the 1HZ heads are not interchangeable without changing the complete fuel system and the pistons and con rods the only things you can use on both engines are the block and the crankshaft

What you can do is use 1HDT conrods as they are stronger and use the 1HZT piston to get a better stronger 1hz turbo

In the direct injection 1HDT, the combustion happens in the piston

In the indirect injection 1HZ, the combustion happens in the combustion chamber in the cylinder head

The 1HDT injectors have protruding nozzles and 1HZ injectors are flat they are different also as pictured


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