Wouldn't a new crank be preferable to an offset ground original due to recipercating weights being designed for the additonal stroke?  I understand you can ballance the crank either way. 

Ted, you mentioned that you can possibly get a larger stroke in a newly produced crank than what offset grinding allows.  I was under the impression that interference issues would trump max stroke either way.  Can you clarify this for me?

I am interested to know if the max stroke with a custom crank would be significantly larger than that of an offset ground?

Key considerations in offset grinding a stock crankshaft for additional stroke will include the stroke of the crankshaft you’re starting with, the current journal size, the journal size being targetted for, the width of the connecting rods that will be used with the resized journals, rod to camshaft clearance with the increase in stroke, and oil hole placement in the journals before and after offset grinding.  When starting out with a good standard journal 292 crankshaft, the stock stroke (3.3”) can be increased to:

3.380” with a 2.100” journal

3.480” with a 2.000” journal

For a good standard journal 312 crankshaft, the stock stroke (3.44”) can be increased to:

3.520” with a 2.100” journal

3.620” using 2.000” journals.

A custom crankshaft raises the bar in that up to 3.800” stroke is doable with the 2.00” journals but requires some serious rod clearancing at the top of the rod bolt thread area to provide adequate connecting rod to camshaft clearance.  Keep in mind that when going with a smaller journal, the connecting rod cross-sectional measurement (in most cases) also decreases which helps with cam to rod clearance when increasing the stroke.  When going with Honda rod journals (1.889”), then a 3.750” stroke is reasonably effortless without any connecting rod modifications.  Going to a 3.800” stroke with the Honda journals requires some slight clearancing at the top of the rod bolt area on the rods to insure a minimum of 0.050”-0.060” rod to cam clearance.  Going for more than 3.800” stroke on a Y is not for the faint of heart but indeed doable.

Here’s a link to the build up of Randy’s dragster engine which uses a 3.800” stroke crankshaft.  This particular article has a picture of the connecting rod as it’s being modified for additional rod to cam clearance.

http://www.eatonbalancing.com/blog/2007/12/09/blueprinting-for-an-eight-second-y-block/ 

And here’s a link going into some more detail on the Moldex crank that was used in the same engine.

http://www.eatonbalancing.com/blog/2007/12/10/building-the-foundation-for-an-eight-second-y-block/ 

There are several aftermarket rod manufacturers that offer a variety of off the shelf rods that are suitable for custom or non-stock applications.  Crower, Eagle, Oliver, Probe, and Scat to name just a few.  You essentially pick the rod for the journal size you want and then work the custom crankshaft specifications into and around the connecting rod.  For the aforementioned 3.750” stroke and Honda rod journals, then the Eagle H-Beams (#CRS6300HJ) work nicely in that they are 6.300” long and considerably lighter than stock while being capable of some very good horsepower numbers.  Price for a set of these typically runs in the $550-$625 range depending upon where you shop.

If offset grinding a 292 or 312 crankshaft, then many of the aftermarket 2.000” or 2.100” journal offerings for a scrub will work but you’ll need to narrow the rods at the bearing beam portion and use the narrower than stock rod bearings that are available.

I saw a picture of a crankshaft from a Plymouth 230.  They have huge strokes, and the main and rod journals were widely separated.  These motors probably would come apart at 4000 rpm.  Apparently this doesnt correlate with stroked 292/312s because they often turn high rpms.

Mike

I remember the article but failed to find it again in which to reaquaint myself with it.  Which issue was it in?  Although it wasn’t my article (seems like it was one of Bruce’s) and because you asked, I’ll comment on the subject.  I believe this is gonna be lengthy though.

There are some standard engineering formulas used by the oem’s that dictate a given amount of material for overlap purposes between the main and rod journals to keep warranty issues at a minimum.  The material being used as well as any heat treatment being applied will play a large part in these calculations but these same calculations have a safety factor added to them to take into account for manufacturing and material variances.  These same formulas explain why the 312 main journals diameters were increased over the 292 sizes when the stroke was increased from 3.3 to 3.44”.  Decreasing either journal diameter without an increase in physical material obviously makes the offset area smaller thereby making it weaker and more prone to breakage.  A case in point is the first year the 350 scrubs were introduced and it had a rash of crankshaft failures.  Didn’t take long for GM engineering to increase the main journal sizes and rectify the problem.  And when the 400 SBC scrubs were introduced, an even larger main bearing size was also incorporated to compensate for the stroke going from 3.48” to 3.75”.  There obviously was not going to be a repeat of the 350 scrub fiasco on the introduction of the 400 SBC.  But that hasn’t stopped me from then turning the 400 cranks down to the 350 main sizes in conjuntion with a good balance job and going forward with these.  Did a rash of these in the Seventies and Eighties and had zero failures.  It’s not uncommon to now buy brand new aftermarket cranks now being available with the dimensions that I was modifying the original oem cranks to.

But as materials get better and/or stronger, then a reduction in the overlap is possible while maintaining the integrity of the crankshaft over the long haul.  The expected horsepower and intended use for a combination is still foremost in most design work.  I do routinely have Y cast cranks offset ground for increased stroke and while this does reduce the overlap between the journals, I’ve also never experienced a crankshaft failure as a result of doing this.  I do maintain a healthy radius in the journal edges and this is just good insurance to not pushing my luck with these journals turned down significantly.  Likewise I attribute much of my success to insuring that the assemblies are application specific balanced to a much finer degree than seen on the factory balanced engines.  Balancing is still a key attribute to the overall life of the engine.

Here are some overlap values for Y cranks in various configurations.

Stock 292 crank (stock 2.498” main and 2.188”rod jrnls): 0.693”

292 crank offset ground to 3.38” stroke (2.100” journals): 0.609”

292 crank offset ground to 3.48” stroke (2.000” journals): 0.509”

Stock 312 crank (stock 2.624” main and 2.188”rod jrnls): 0.686”

312 crank w/292 mains & stock stroke (2.188” journals):  0.623”

312 crank offset ground to 3.52” stroke (2.100” journals): 0.602”

312 crank offset ground to 6.62” stroke (2.000” journals): 0.502”

312 crank w/292 mains & 3.52” stroke (2.100” journals):  0.539”

312 crank w/292 mains & 3.62” stroke (2.000” journals):  0.439”

My roadster is running the 312 crank with 292 mains and 3.606” stroke.  Journal overlap for that combination is 0.446” as a point of reference.  Because of the 7200 rpm limit on the engine, it does employ some overbalance in the balancing bobweight calculations by design which helps in removing some additional stress from the crankshaft that would be there otherwise in the upper rpm band.

If you’ve ever looked at a Model T crankshaft, then you’ll see a serious negative offset between the journals but likewise, you’re looking at a low horsepower, low rpm engine.  Still, that crankshaft was forged steel and that helps strengthwise over being cast iron.  The material being used is still a key component in many of these instances.  Without looking at a Plymouth 230 crankshaft, I’ll suspect it is also forged steel and a heavy one at that.  Many of the newer four cylinder engines also employ negative offsets in their overlaps while many of these are also high winders.  Having 180° degree spacings on the throws on these also helps overall balance at all rpms whereas a V8 crankshaft has the throws at 90° intervals.  Weird things happen with the balance on crankshafts that must have bobweights in which to be balanced and as such, are being balanced for a specific rpm range.

Obviously, the more the better in regards to journal overlap.  Just turning a crankshaf journal 0.010" is technically weakening it.  But generally the more an engine is being stressed either through loading or rpms, then the more apt that the crankshaft will reach a failure point and especially if it’s marginal in design to begin with.  Offset grinding does make the crankshaft weaker but at what horsepower/torque level it’s going to fail on a consistent basis is something that’s statistically derived from reverse engineering several failures on similar built engines and not basing any conclusions on a single failure.

Other opinions or experiences always welcome.  And as usual, just food for thought.

Mike

Interesting you should mention Model T cranks.  I have a model T, and read the model T forum occasionally.  Guess what.  T cranks are starting to break.  And they're only 82 to 100 years old.  Wonder if they are still under warranty.  They are not counterbalanced either, although the repops are cast and counterbalanced.

John.  Interesting on the T cranks and the recent breakage.  Fatigue or just being run harder or both?  And thanks for clarifying which issue the overlap article was in.

As a point of reference, I have balanced a few T cranks where the customers had welded counterweights on them and appears to make quite a difference in the smoothness at idle.  As far as I know, none of those have failed but then again, I’m not familiar with any other T crank failures in my neck of the woods.  Extreme cold cycling in the winters possibly stressing the cranks?.  Sure not getting cold here in the winters but this is just some early morning rambling on my part.