I could be wrong here, but from what i've seen, the 0031's are set up for t+d shaft-style rockers, which oil the springs through a fitting in the head, through the shaft. THe 0030's i THINK were set up for jessel style pedestal rockers that oil through the pushrods. But like i said, these are only my observations and may not be 100% correct

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Fast Freddy
1987 turbo-t. Mostly stock cruiser FOR SALE
2006 Chevy 2500 duramax quad cab tow vehicle (14.58@93.42mph, i HAD to try it!

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Black magic racing-
George - 10.13@134
Mike- 10.66@130
both Stock blocks, iron heads