Small beginnings, big ends
QI’ve bought a Jaguar 420 in reasonable condition. It runs roughly and I’d been advised that the big ends probably needed attention. The cost of outsourcing this is high – and I want the satisfaction of trying to do it myself. Is there anything I should consider before I start pulling the engine to pieces? Ray Coe, High Wycombe
AWe’d start by diagnosing why it runs roughly. Make sure the spark plug leads are going to the correct cylinders. Then check and correct the camshaft timing, valve clearances, ignition timing, distributor condition, carburetion and the fuel pump pressure – in that order. Be very thorough. There’s a ‘sweet spot’ at which each piston is timed to push the crankshaft with a minimum of resistance and disturbance. Anything that upsets this – even something as trivial as a slightly incorrect fuel-air mixture – will affect the smooth running of the engine. Significant faults such as a worn-out distributor, very advanced ignition timing or mixed-up leads will cause the con-rods to impact the big end journals or perhaps set up a heavy vibration of the crankshaft in its main bearings.
Cranks can – and do – wear out due to neglected oil changes and so on, but they’ll also soldier on for a long time with moderate wear if the rest of the engine’s working in harmony with them. Taking an engine apart – especially a large, complex one for which parts are expensive – should be the last resort.