tell me what you know about problems with offset grinding the steel 292, is it all steel 292s or just ones that have had the radius ground out of them?

Tim

Tim.  For the 292’s I’ve essentially come across three styles of oil paths or drilled oil holes in the crankshafts which is what must be considered if increasing the stroke by lieu of offset grinding.  The steel cranks are one of these styles, the early crankshafts with the alternate oil holes at the rod journals is another, and the common fare 1956-1964 cast cranks is the remaining.  The later while also being the most common is the easiest to work with in regards to offset grinding to a smaller journal size while taking the journal widths out to 1.800”.  This wider journal alleviates having to remove as much material from the replacement connecting rods in order to make them work without having to narrow the rod bearings themselves.

The ‘alternate oiling’ crankshafts make nice 2.100” journals while going wider but the oil holes are exposed in the radius when attempting to go to the 2.00” size.  The common fare cast cranks make nice offset ground 2.00” journals while making the journals to the 1.800-1.810” widths.  The problem with the steel cranks when offset grinding to the smaller journal size (1.889” and 2.000”) lies in trying to make the journal wider than stock.  That’s where the oil hole in the crank becomes exposed at the radius and this tends to vary from crank to crank due to manufacturing variability.  Some make it and some don’t and that’s with the proper radius being maintained in the fillets of the journal.  An undressed grinding rock that removes the radius from the edge of the journal simply compounds the issue.

Probably more info than was asked for but felt a lengthy explanation was needed to know that there were some differences in the oil holes themselves which drives which crank can be offset ground the most.

Tim

Something you may not be aware of.  When I had my 312 crank ground to 2.100, I did not widen the throws, I narrowed the rods to Y width.  The crank guy said TRW makes a bearing that is narrower than the stock sbc one for use on cranks with larger than stock radii.  I did not use those bearings, I made a fixture from an old sbc rod and chamfered the stock bearings to clear the radius on the crank.  Thinking back, I should have bought the narrower bearings, after I cut the chamfer the bearing surface ended up being narrower anyway.

John

A crank that is .040"/.040" is just as good as a Std. (unless it was hardend) but it has to have a radius.

Tim

       At the intersection of the Rod journal to the crank web or throw a radius is ground. This radius is ground into the rod journal to reduce the stress concentrated at the intersection. Sharp edges, particularly on inside corners, are bad news in a casting. By creating a radius you spread the forces out over considerably more area, and reduce the amount at any one point. This actually can apply to main journals as well. A few years back it was all the rage to roll the radius in to the corner, actually deforming the material. This turned out to be a waste of time on cast cranks,but usefull in steel forgings, or billet cranks.    

       Kinda, but even better. A sharp edge puts all the stess at the corner, the smaller the actual corner, the more concentrated the stresses become.

      Best example I can think of off the top of my head is one sheet of paper is easy to tear from the edge. But if you try to rip it in half from the middle, it's a lot harder. You have spread the force from your hands over a lot more paper.

      You're dealing with tensile strengths in the crank example. Tensile is the amount of force required to pull a material apart. It's expressed in pounds per square inch. The more square inches presented to the force, the stronger the pull required. It's a bit more complicated than that in a crankshaft, but that's the idea.