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Bentley & Jaguar Power
Whilst the Bentley 4¼ and 4½ litre appear to give more impressive performance than the Jaguars it's worth looking a little deeper at the relative performances. Especially the weaknesses of the engines that potentially limit their longevity. All engines suffer problems as they age but some give rise to more catastrophic and expensive failures than others.
Jaguar 2½ litre:
This engine is a relatively conventional overhead valve design with 3 rows of studs holding a cast iron cylinder head onto a cast iron block with water passages between each of its 6 cylinders. A submerged oil pump feeds crankshaft, camshaft and overhead valve gear with fully filtered oil. The only significant weakness comes in the risk of rod breakage in engines fitted with Dural connecting rods. For engines not originally fitted with steel rods modern replacements are available.
Jaguar 3½ litre:
This engine is very similar to the 2½ litre from which it was derived but it has a greater tendency to overheating due to reduced water jacketing without circulation around cylinders 2 and 5 and a greater tendency to head gasketing problems due to smaller head area with 2 rows of studs.
Bentley 4¼ litre:
This engine has overhead inlet and side exhaust structure with an aluminium cylinder head held to a cast iron block with 3 rows of studs. Like the Jaguar 3½ litre there is no water circulation around cylinders 2 and 5. A number of design elements can cause problems. Bypass oil filtering and unfiltered rocker feed can limit bearing life and the unusual use of chrome cylinder liners that only extend part the way down the cylinders tends to cause a step at the transition resulting in havoc with the rings but the most serious weakness is probably due to the design of the water circulation within the block. This tends to cause localised overheating problems at the exhaust valves and can lead to cracks between the exhaust valve seats and the cylinders.
Bentley 4½ litre:
This engine is very similar to the 4¼ litre but doesn't suffer from the smaller engine's bypass oil filtering, however the larger bores only serve to accentuate the cylinder and overheating issues.