996 and 997 Turbo Crankshafts and Rod Bearings
The power levels reached in the aftermarket have far exceeded the ability of the stock rod bearings to handle the compressive loading they are asked to withstand.
The stock rod journal is 55.00mm in diameter and 0.730” in width. In comparison, the race crankshafts in the GT3 engines are 53.0mm in diameter but more importantly are 0.860” wide. If you do the math, you will see the surface area increase some over the stock turbo shaft. The added oil film can then withstand the forces applied to the journal. What is considered here is the “contact patches” at the top and bottom of the journal when the engine is at TDC either on the compression stroke or on the overlap stroke.
There is not a lot that can be done to the stock crankshafts to make the narrow journal any wider. For this reason, many have used the GT3 Crank. Performance Developments has addressed the bearing problem, changing the journal diameter to a size where a tri metal bearing can be used. This requires new rods to be used, but in most engine upgrades to these engines the stock rod is replaced with an aftermarket alternative. This makes changing the BE size a non-issue.
The bearings used in the stock engines are Bi metal in construction. They are steel backed with an extremely hard tin coating for longevity. Unfortunately, if any of the bearing shells touch the journal under load the hard surface has no give and the bearing is immediately compromised and engine failure will result. The tri metal bearing has 3 layers, one of them a softer Babbitt layer used as a sacrificial layer in case the bearing does touch the journal. To read more about this read the bearing paper under technical articles.
If you decide to use the stock crankshaft and are considering changing the connecting rods, we suggest you consider this change at the same time. Call us for more information. We offer crankshafts on an exchange program, rods bearings and the connecting rods to suit your engine configuration.
