hi, i plan on using a 1971 4bolt 512 block. i need t know is there any extra machine work needed for clearance with a 4.25 stroke crank .if yes how much $. and is it worth the extra cubes . i have a 6-71 two 850's and a good set of alum. heads for the top end. also what would the correct rod length be for this crank. thanks for the help

hi, i plan on using a 1971 4bolt 512 block. i need t know is there any extra machine work needed for clearance with a 4.25 stroke crank .if yes how much $. and is it worth the extra cubes . i have a 6-71 two 850's and a good set of alum. heads for the top end. also what would the correct rod length be for this crank. thanks for the help

Some cranks just drop in and some need clearancing, I think all block are different. The correct rod would be a 6.385, this length is reccomended when using the 4.25 crank.

IMHO the 6-71 is going to be too small. You are going to have to spin it so fast it may become a heat issue. On a 512 you would want at a minimum a 8-71 but preferrably a 10-71. I don't think the extra cubes would be worth it when you can change a pulley out at minimal cost. What HP are you going for and what is the primary use for the car?

I didn't know GM made a 512 block in 1971.

Learn something new everyday.

the block is a 454ci , casting#512 sorry about that.about the blower it's a freebie (brand new). i'd hate not use it. what kind of boost can i get out of it safely. and how? thanks again.:yes: