Aluminum blocks

How much strong are aluminum blocks in contrast to cast metals blocks?

Are aluminum blocks machinable since majority of them suffer structural deformations like cracks, thread lining breakages etc.

I see mostly removal & replacements in most aluminum engines and very little machining.

Some engines have aluminum blocks stock (and they are typically cast aluminum). What engine are you referring to? I've not had any block-related issues with my Mazda MZR (aka Ford Duratec) 2L, 2.3L and 2.5L variants in dedicated sports racer applications.

As David suggests, it depends. For the most part, factory cast aluminium blocks are perfectly fine, even to some multiples of OEM power levels, others are going to be rather more limited, just like OEM cast iron blocks. Depending on the specific engine you have, you should be able to find it's limitations on the inter-web.

Not quite sure what you mean about "machinable"? Some OEM use plated bores that may need to be replaced with sleeves if they can't be otherwise repaired.

There are also aftermarket bille aluminium blocks, which are usually designed to be stronger, easily repaired if damaged, and are also made of better alloys and heat treated.

One big advantage of cast iron is it's more thermally and physically stable - it doesn't expand as much when hot, moves less under stress, and is more resistant to damage from exhaust gases if a gasket fails.

I can't tell about all engines but can give you some examples. 4G63 /64 cast iron block can be machined from 85/86.5mm to 87mm and still hold up to 900+ HP. At the same time Evo X aluminum block has to be sleeved in order to withstand such power.

H22A closed deck aluminum block can easily handle up to 700+ HP whilst the same block with open deck can barely handle 500 HP. Both can't be rebored but sleeved only...