Technical Tips: How To Avoid An Engine Disaster

Technical Tips: How To Avoid An Engine Disaster

#43 of 46

26/09/19

Proper lubrication of moving parts is vital in any mechanism for longevity and efficiency, and the pre-war Rolls-Royce and Bentley engine is no exception. The design of this engine needs to be understood to avoid a loss of lubrication which might result in a very costly overhaul. An apparently healthy oil pressure may not indicate a healthy engine.

The crankshaft in the pre-war Roll-Royce engine acts as a centrifugal oil filter. Oil reaching the bigends is effectively centrifuged before it enters the bearing. This results in the oil being comparatively free of damaging abrasive matter, with the results that the wear on the bigends is minimised.

However, the oil lubricating the main bearings has not had the benefit of being centrifuged, and consequently the main bearings often suffer more wear than the bigends. It is not uncommon to find crankshafts that have not been re-ground on the bigends – they may have been lapped and polished in their lifetime, but no more – but which have main bearings 0.020” – 0.030” undersize, indicating several regrinds.

Despite the lack of wear of the bigends there are other factors that can threaten their longevity. Firstly, the centrifuged debris or sludge that lodges in the cavity within the bigend journal or crankpin can restrict the oil flow to the bigend and ultimately, despite having an apparently healthy oil pressure, bearing failure can result.

Secondly, wear in the main bearings can frequently cause bigend failure. The worn bearings allow greater oil flow through the mains, and this deprives the bigends of vital lubrication. The purpose of the oil is to cool the bearings as much as to lubricate them. The fatigue life of a white-metal bearing is very dependent upon its temperature, and unless the temperature is kept under control by means of adequate oil flow early bearing failure will result.

Before the war, a regular service operation undertaken by Roll-Royce was a ‘bottom end decoke’. This entailed the removal of the sump to allow the cavities within the crankshaft to be cleared out of the accumulated sludge. At the same time the health of the bearings could be checked.

If the condition of the bearings in your engine is unknown undertaking a bottom end decoke could be a very worthwhile operation.

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