I picked up a used 4.5" Velasco billet Hemi crank in beautiful condition, but I didn't realize it had 3" mains. I'm planning to offset grind the rod journals down to 2.200" at 4.625 stroke. This would be going into a KB aluminum block.
The crank would be stronger and more rigid with 3" mains. Is there any reason not to line bore the block out to 3"? Would the block be weaker?

Thanks, Joel

I'd think that a KB aluminum block would be designed for 3 inch mains since that was an option with them. The machine work requires some special cutters so you'll have to find a shop that has the ability to cut the tangs as well as the thrust surfaces. Have you checked on bearings? You would want a set of bearings in hand before the shop did the machine work so everything could be fitted. Double check the main sizes on the crank before ordering mains since you might need to be plus or minus or split bearings depending on where the crank is.

No. Just say no. Putting together a Hemi today is hard enough with out trying to run a 3" main crank. If you had an old 3" main KB block that would be one thing but no. Put the crank on the wall as wall art or ask the guy if he'll take it back.

Is grinding the main journals down to 2.75 a viable option?

Grinding the crank down might work but it is a big job. You would need to talk to a very highly qualified crank grinding shop on a job like that. Depending on how big their crank grinder is it could take a long time to take that much material off.

It might not even have a flywheel register on the back. And what is the balance? Ask the guy if he will take it back. Life is hard enough without trying to go down that rabbit hole.

Those are two very good points. The last 3" crank I had in the shop was super heavy and you are correct about them usually not having a flywheel register or being drilled for an input shaft.

Two things depending on your crank grinder ability,, take that crank to them and check with him first before going forward.
I was going to do the same thing with an older KB top fuel fully counterweighted crank and was told by my crank grinder that those cranks can't be offset ground
I did have another fully counterweight old top fuel crank from Sonny Bryant that was a 4.25 stroke offset ground to BB Chevy rod journal size and increase the stroke to 4.375

Joel, as far a sthis crank is a early Firepower tyep crank or a Gen 2 426 Street hemi type crank?
Either way with your deal being N/A on pump gas I wouldn't be the least bit concern using the 2.75 main sizes on either Gen one or Gen two Hemi motors

Call Crankshaft Supply in Minneapolis. I have had them offset grind and stroke three cranks. Great work, they ask if you want the final journals on the low or high side of spec too .
On 2.75 mains, wedge motors are built to 4.75 stroke with 2.75 mains and 2.2 rods. Molnar even makes a bbc rod in 2.1 rod journal size.

Greg, I called Crankshaft supply and they quoted me about $1500 to do the rod and main journals. That would be a no-go, I'd be better off selling the crank and buying a new one. So I called Engelke Machine in Lester Prairie, he quoted about $500.

On the flywheel register, it would seem I could just use a 392 flywheel, as they do not have a register ring on the 392 crank. THey just have counterbore on the front of the flywheel to fit the O.D. of the crank flange. Or counterbore a 426 flywheel.

You can buy an adapter ring that will for your crank that will fit into the 392 type crank hole and provide the ring with the register to center the later wedge and street hemi flex plates and flywheels