It seems that the cylinder head on my diesel Defender has gone. The garage said they can split or become porous.
Other than replacing it with a new or reconditioned one, is there a way to repair it?
Not cost effectively. Its the sort of thing that is only done on rare old classics where you can't get parts. Used head shouldn't be that expensive. Most of the costs will be labour
Cracks can be welded depending on where they are and how many of them. Porosity can also be fixed by welding depending on where and how bad it is.
I get a good used one and just do a straight swap.
Is it a common issue or has it been overheating?
Porous casting - yes
Castings can be dipped into a sealant. Not sure how long it would last mind and I've never done it. Just heard about it!
Split or cracked - not really
I've just dropped it off with the garage, so at the minute I'm not sure. They did say it had been a common problem a few years ago, and they were doing maybe 2 or 3 a month. I didn't notice any overheating. It's done 216,000 miles (I've owned it since new).