The difference between an 840 and a 291 head is?

[Chevy454]

I had a set of 840 & 291 heads that were produced the same week...and believe it or not, the 840s were *cast* for the 1.88 exhaust vavles. The machine shop were the ones to catch it, as I told them to just put a seat of seats and 1.72s in it...but they called and said the heads had never had seats in them (which I knew, they were virgin heads) but that they were made to accept 1.88s...I called Colvin a few times about it, and he confirmed my thoughts...there's an old thread around here with all the ins/outs of the 840s & 291s.