391 ft crankshaft balancing

[fekbmax]

I need to get a 391 steel crankshaft cleaned up and balanced.  I'm not looking to turn down the snout (opened up a BB Chevy crank mandrel to fit) or thin the flywheel mounting flange (using a full rear motor plate).  I just want to get it balanced to a Bob weight and have the exstra key grove.
It seems like I have read somewhere that one of the counter weights on the crank needs some significant cutting.
Are there any obstacles I'm gonna face in doing a straight up balance job on this crank ?

[CaptCobrajet]

It really depends on what you do to it.  It's a 78 lb. swinger if you don't put it on a diet.  It will balance pretty good because the truck pistons were heavy heavy.  When you start reworking it down to iron crank weight or less, the balance issue comes along.  I had some 63 lb ones that took two or three pieces of mallory, but the pistons only weighed 375 grams, and it took several iterations to figure out how not to need six or eight slugs of metal.  2.200 rod journals at 1.900 cheek width was the best for me.  If you go 2" wide, the counterweights get thinner, the cost goes up, and the heavy metal mounts.  You could probably get one down around 70 lbs with the big snout and not break the bank.  It will balance reasonably at 3.78 and FE journal, but it's a boat anchor.  Wonderful for what they were intended.......pretty much indestructible in an F600.

[C6AE]

It really depends on what you do to it....
 It will balance reasonably at 3.78 and FE journal, but it's a boat anchor.  Wonderful for what they were intended.......pretty much indestructible in an F600.

I broke one last Christmas, in a 1958 F-600 service truck. It never saw over 4000 rpm.

There was no clue other than an odd knock that got radically worse under a deliberate load until I pushed the clutch in, then quiet - WTF? I ran it gently for another week until there was time to look into it further. The Christmas holiday, right? Perfect.
I took off the pan expecting the worst (tons of room, it is like working on an engine stand).  But I didn't see a thing.
Started on the rod bearings, all perfect. Went for the rear main it was good, front main good, I was going to put it back together but knew really something was wrong so I proceeded to inspect all the rest of the mains rotating each one all the way round. Still couldn't find it. Finally again with a very bright light I saw a glint, just a tiny nick above the rear thrust surface and oh! the crack going diagonally down right through the counterweight to the main journal edge. Damn! The reason it didn't fail catastropically was because the back half of the crank was trapped between the throwout bearing and the rear thrust surface.  Fortunately I had another crank.
Every once in a while I have to use my little tool truck to tow a loaded transfer truck off the highway, sometimes up a hill even, to the next offramp. (I am in the asphalt paving business so a perishable cargo).
So... #80k plus my 15k is almost 100,000 pounds, and that friends is how you break a 391 crank!
Merry Christmas

[Barry_R]

Have one in my 433 that lives in my car.  No idea how much Adney cut from it, but it was a LOT.  Has 3.640 stroke and 2.00" journals, light pistons, light rods, and a couple slugs....been 7500+ RPM a bunch of times with no problems

[KMcCullah]

I built a 390 with a 391 crank years ago. Turned the snout and sleeved the big pilot bearing down. Eagle H-beams and L-2292 12:5 pop-ups. Had a bitch of a time getting it all to balance. I ended up adding a half moon chunk of iron to the hub of the harmonic balancer and drilling the shit out of my cherry steel flywheel. What a magnificent turd of a short block I built. The previous version of this 390 just had a stock crank with CJ rods. Same pistons. It ran waaay better than the steel crank version. Probably 60hp difference. BP summarized the steel crank pretty well. Without putting it on a serious diet, it's a boat anchor.

But I think that big snout sure looks like it was made to have a blower pulley on it. And any blower FE that makes real steam would need a steel crank. So a 391 crank might be the right choice. Probably cheaper to find a $ crank but the big snout is missing....