[Monkey]

I've honestly never had a reason to replace rings on these. If a cylinder leakage test is carried out and the readings are acceptable there would be nothing to gain from replacing them.

If you want to replace them anyway you would need to measure the bore to see if there is any wear, and whether it is in tolerance or not. If it is you could just go ahead and fit the new rings.

It's a massive job to be honest, and not one that most would want to undertake voluntarily!

Yes if the head is off you could lap the valves in, however I would go back to the cylinder leakage test and if the values are ok again there would be nothing to gain by doing this. These dont suffer from coking up, so generally it's not necessary to clean the ports. When replacing the head gasket the head is not stripped (assuming inspection shows no issues).

With regards to the original post topic (big end shells), I've only personally seen 2 cars fail so it is a small percentage.