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Fiat/Lancia TC cranks, cleaning and prep - updated

Posted: July 2nd, 2006, 6:10 pm
by Guy Croft
A cross-drilled crank has oilways in main journals and cranlpins (rod journals) drilled right through the diameter, and the mains that feed the rods with oil from the high pressure feed in the crankcase are connected by a drilled gallery down through the crank web. Non cross-drilled cranks are cheaper and rely on just a diagonal drilling from main to crankpin.
The advantage of a cross-drilled crank is that you can more than one exit hole on the crankpin, offering superior lubrication.
But - to service a crossdrilled crank, you must unplug it because the outer radius region behaves like a centrifuge and collects dirty sludge from the oil, like this.
Any good race crank manufacturer will always cross-drilled. The position of the oilway on the crankpin is very critical, because if the bearing load is too high at the exit point, the oil pressure will be insufficient to let the oil out at all at main oil gallery delivery pressure.

The oilway mods shown in the photos have been done by me on virtually every crank I've ever prepped. The factory chamfer/radius on the drillings can vary from huge to OK to almost non-existent. I absolutely am sure that my preparing the journals in this way has contributed to the survivabality and reliabilty of GC engines over the years and is well worth doing. It takes time. An early crank like 131 2 liter which has 2 drillings per main journal compared with the later 2 (eg: Integrale) and is thru drilled on the crankpins (compared with the later ones which were only drilled to the centre) - takes me 3 hours or more. One slip iwith the tools and the rank is ruined.