340 Crankshaft bit the dust
Thanks for the bobweight values. Another variable is the weight of the Eagle rods. The H-beam rods are heavier than the I-beam rods.
I don't understand why the manufacturers don't make .040 or .060 undersize bearings. Which would be less than a 5% loss of strength of the crankshaft. With the quality of the old Chrysler steel, they'd still be stronger than some of the new stuff.
One reason is because most guys think a .020 under crank is junk. It's not, but that's the case. A .025 under SBM Rod journal is a standard large journal SBC Rod journal. I never build any SBM Stroker without using SBC LJ Rod size. That means taking a brand new crank and turning it .025 under just to start. Now you can buy Chrysler cranks with SBC sized journals so you don't have to do that. Early on, that's what you ad to do.
The other issue was way back when the under sized bearings all had the same thickness of shell. Which meant that as the Rod journal gets smaller, the overlay had to get thicker. Once the overlay got to maybe .030 or so, depending on journal diameter, you'd get overlay migration, flaking and other issues like that.
Since the mid 1980's (maybe before but it's hard to track down the history) the bearing manufacturers started using thicker steel shells so the overlay stays the same thickness regardless of undersize. So that did away with overlay migration etc.
But it didn't matter as old issues die hard and most guys won't go that far under. I have a crank out in the shop that was turned down to SBC SJ Rod throws. So they measure 2.000 and that's .125 off the throw.
I realize that we are discussing two things at the same time, in undersize bearings and undersized Rod throws, but they go together. They go together because both are part of the issue. Some guys think the throws get too small and others think the thick bearings are bad.
Either way, you can go way undersized and not hurt a thing. Getting bearings for and undersize that far is an issue. That's why it's better sometimes just to change the rods out to Chevy throws and be done with it.