The 4 cylinder motors remained 'redblocks' with red painted cast iron blocks through the end of production. The 850 and 960 introduced the new generation of 'whiteblock' aluminum blocked motors.

Internals changed over the years on the reb block 4 cyoiders. I don't have any specifics, but some of the changes were major. Like changing the thrust bearing (only one of the 5 main bearings counters fore/aft movement of the crank via a thrust surface) from the end of the block next to the flywheel to the center bearing. And I think the width and diameter of some of crank bearing surfaces changed a few times. They tried to make the motor a low friction motor by shrinking the bearings, it didn't work so well in reality, so late production redblocks seem to have reverted back to the earlier design. The strength of the rods also went from good to bad back to good.
--
'63 PV544 rat rod, '93 Classic 245 + turbo