any idea?

I think it's 4.5"-but that's with some serious clearancing.

The most I have ever heard is 4.25" in a standard 454 block. If you bore them .060" over and add a 4.25" crank you'll get 496 cubes which from what I understand is the most one can safely/reliably go on that block.

Hot Rod magazine has a special issue out going into detail about stroking.

at 4.25 how much machine work is needed ???????

I believe that 4.25" only requires some minor clearancing of the bore and pan rail for the rod bolts. I believe that they will clear the cam ok.

Depends on the crank you find or purchase. One supplier said a bit of trimming would be necessary in the oil pan area... we ended up having to grind about 1/4" off the counter weights... talk about pissed.

I recently did a 496 and only had to clearance at 1 point just above the pan rail because it was close but not touching. MkIV block, 4.25 steel crank and 6.385 billet rods. I believe that the billet rods are what allowed to get away from having to clearance the block.

thnx all
I get it as 4.25 max.

is it also same for first gen 502? 4.25 max stroke?

I put a 4.25 stroke crank in a 502 with NO clearancing/grinding