My machinist asked me today what or how much the offset amount was to be ground off my crank??

I had told him it was to be offset ground to 2.1000" and I know the stock rod journal is 2.1880"-2.1888" per JM's tech info, sooo, does he mean the difference between the stock rod journal and the offset ???? ( If my math is correct that would be 0.0880"-0.0888")  Or does he mean something that is completely over my head??

I did a search but couldn't find anything that got me out of the "DUH" phase.  I did learn that a 3..52" stroke with a 2.1000" rod journal has an overlap of 0.602".

Any help and/or clarification would be greatly appreciated.

Thanks in advance

Mike

Luck is fine but right now I need knowledge!!!!

Mike.  You’re right in that there is a potential of 0.088” that can be removed if going for a maximum amount of offset on a 312 crankshaft and using a 2.100” journal.  When sending cranks out to be offset ground, it’s important to specify what the finished stroke needs to be along with the finished journal size.

Offset grinding the crankshaft rod journals simply allows the stroke to be changed.  If simply turning the rod journals to a 2.100” diameter, then the stroke stays the same.  But if offset grinding the journals where more is taken off the inside of the journal than the outside portion, then the stroke increases.  Likewise, the stroke can be decreased by offset grinding from the opposite direction where more material is taken from the outside portion of the journal than the inside. At this point it simply becomes a math problem to determine the amount of stroke desired and whether or not that amount is available with the crankshaft that’s being used.

If the stock 3.44” stroke 312 crank is perfect, then the potential is there for a 3.528” stroke using 2.100” journals. But as a general rule, you go with a lesser value to insure that the crank journal does clean up on the outer portion without having to actually turn the journal to an undersize.  Targetting for a 3.520” stroke would be the case here.  If the rod journals have been previously turned undersize, the the stroke potential reduces if using the same 2.100” journal target.  A 312 crankshaft that is already 0.020” under will make a 3.50” stroke and using the same 2.100” journals.  The same basic rules apply when offset grinding 272/292 crankshafts.

Here are links to past threads going into more detail on offset grinding the Y crankshafts for additional stroke.

Crankshaft options

Crankshaft filet radii

Offset grinding for more stroke

Using a 312 crankshaft in a 292 block

And here are links to threads dealing with the connecting rods for some of the various applications.

Dynamic compression ratio and rod length

Length

Length differences

More stroker engine info

Orientation

RPM capability

Use in a stroker engine