Austen,

Intresting to hear about the WRC blocks.

I have always thought that the silver top block was the strongest. I too had noticed the larger main caps.

As a side note, and not very relevant as you will be changing the rods, but the silver top actually runs 1mm bigger diameter big end bolts as well.

It would be interesting to cut up a couple of blocks and measure things like bore thickness. Don't suppose you have access to an x ray machine???

The last engine we built that we are hoping for 400+bhp we used a silver top block mated to a black top head (New Focus RS casting ported by CNC heads)

I can highly recomend the work done by CNC heads.

As you can see from the data they have collected
( http://www.cncheads.co.uk/products.html ) , the ST170 flows vastly more than any other zetec head and is the one to have.

If i had to build another ZT it would be a silver top block with a ST170 head ported by CNC.