That mod was done by a certain highly-strung bodger for a "Salvage Squad" TV programme Neil.

Because it wouldn't start a module from a V12 Jag was pressed into service in one mag. No one thought to question why the British Military would have unstartable tanks and some 14 countries would have bought the Cent which was the first and original mould-breaking MBT. Indeed the Cent engine was used in several late war tanks too and they all started on the button.

Correctly, it was identified that magnetos don't make sparks until their speed is sufficient to generate enough energy, even though the mags (but not rotor arms) do rotate at 1.5 x engine speed, and the Cent mags are not impulse types. The engine is of course ex-aircraft being a 27 litre RR Merlin but with no supercharger and softened cams and CR. This is of course the land based Meteor engine.

It has a boost coil and twin-finger rotor arms in the magnetos, in the aircraft application the boost coil is operated by a separate button thus allowing the engine to be turned and primed, owing to the very lengthy inlet manifold and huge blower casing volume, without any possibility of ignition and backfire.

In the tank version with simple updraught carbs in the V, the boost coil is activated with the single starter button and the system is now of an age when things fail. In essence the boost system is nothing more than a contact trembler activated by the core of the ignition boost coil making a continual shower of sparks, it should be audibly buzzing whan cranking a Centurion at some 500Hz on a 24V supply.

These sparks are fed to one mag only, the right hand, B bank - exhaust side, and then to the boost rotor arm finger that is 35º behind the the main rotor arm contact. The left hand magneto has this HT input blanked and in fact runs the opposite way round.

Thus the engine is started well retarded as each cylinder is fired when the sparks finally jump to each cap segment a little less than 35º ATDC, this means there can be no kick-backs on the direct starter drive and although the power realised when it fires is well below what should be the case, it is quite enough to accelerate the engine from a slow cranking speed, about 30 rpm, to the point where the mags start firing both sets of plugs on time and the start is secured with idle at 450 rpm. With a well primed and unmodified Spitfire you can see this happen as the exhaust stubs produce lazy flames of burning Ki-Gas prime fuel and the engine gathers pace against the prop load.

It is indeed the case that the engine stop is a ground connection to the magnetos that shorts the LT side of the magneto, but like an aircraft, the Cent also has two mag test buttons that can disable each mag in turn for the classic mag-drop test and in an emergency be used to stop the engine also.

Cents are so old now that all this wiring, point gaps and all HT cabling may well be faulty in some respect.

R.