I agree Bob - I think the Turbo and GT engines had Nikasil wet liners, but the 996 and 997 had Lokasil.
I have reported in great depth on other web sites about Lokasil and a lot on our own web site
www.hartech.org buyers guide.
I wrote my thesis in that guide on what was causing the cracks and seizures many months ago but it all seems quite right now.
Lokasil is a metal matrix composite and as such not very strong and prone to creep. Even new blocks I have measured have crept slightly oval and this continues in the thrust direction until the bore can be 10 or 12 thou oval (0.25mm to 0.3mm) after which they usually crack.
The smaller Boxster blocks have thicker wall thickness outside the Lokasil cast in liner and so are stronger.
The 996 was bored out from the Boxster S casting reducing the wall thickness by half the bore diameter difference (1.5mm) despite producing more torque.
When they distort the 90 degree direction gets smaller in diameter as the thrust direction diameter gets bigger (i.e. it is not wear). Piston rings cannot fit into such an oval bore if they have not worn with it (and they don't) so eventually there is too much blow by and this overheats the oil on the thrust face and causes a seizure.
Also the sealant used on the crankcase halves leaves a smear inside the oil galleries to the crankshaft which eventually falls of and can block the oil spray jets that send oil up into the cylinder and piston area. This can also cause seizures.
I wonder if the later engines with variable lift and greater torque result in more blow by (as torque is somewhat proportional to cylinder pressure) than earlier engines? or the extra torque results in even faster distortion rates? I have not yet seen an "S" engine with 3.8 litres which has a piston 3 mm bigger still in diameter but wonder if they increased the wall thickness for that one - time will tell.
When we rebuild these engines we replace the cracked bores with a new cylinder liner with Nikasil lining and the other bores that have gone oval (the ones that have not cracked) we reduce the ovality and support with a ring so no further distortion is possible and the resulting engines are much sharper, crisper and faster as a result of correcting the poor cylinder filling and increasing the effective compression pressure (i.e. torque).
Unfortunately over sized pistons are not available and unless they have a special ptfe type coating - aluminium pistons apparently don't run well in Lokasil - otherwise they could be bored out as there is plenty of wall thickness and support rings would prevent future distortion (something that should have been fitted to the original engines in my view).
No more time for now!
Baz