Tech File - Overcoming the Flaw in the 4.2 Engine Block Editor: While visiting “Sports Car Centre”, I listened-in on varying opinions regarding cracking between cylinder liners in 4.2 litre engine blocks. The discussion was - “if the block is not cracked, do you leave it and put the head back on or do you go to the expense of installing stepped liners?” I decided to do some research.
4.2 litre XJ6 Engine Block At best the 4.2 litre Jaguar/Daimler engine block could be described as suspect and its quality further declined throughout its production. It was designed in the early 1960s to give more torque and better low to mid-range performance than the 3.8 litre engine, which it genuinely did. Early examples (fitted to the last Mark X’s, the E-types and the 420s until about 1968) are easily distinguishable from the XJ6 blocks by two fewer waterways at the rear (watch swapping your cylinder heads), three large core plugs per side and studs screwed directly into the block face. The later engines are usually prefixed 7L (all 4.2 litre engined Jaguars from 1968 to 1975) or 8L (1975 onwards) and these are the ones covered in this article. The pre-XJ6 block was said to be improved upon by the 7L block, which had a more efficient internal cooling flow. The problem was that the cylinder head studs screwed directly into the base of the block, through the waterways and, if the correct coolant is not used, then they will rot out very quickly.
These studs are also twice as long as the pre-1968 blocks, allowing more stretch and therefore more variation in clamping pressure against what has been set with the torque wrench and this makes it more prone to head gasket problems. This improved block cracks, usually hairline cracks appearing in the block, eventually becoming stepped (when one side becomes slightly higher than the other) and the liners drop. The 8L block, a strengthened 7L block, cracks even more than the 7L, and this may be due to an increase in thermostat temperature or to unweathered blocks being used or, possibly, a reduction in standards under British Leyland.
Why do they crack? They crack because of the very high temperature differentials created by inefficient coolant circulation. Coolant can circulate around the block only at the very front and the very back. The water pump outlet is positioned so that when the thermostat first opens, cooled fluid is pumped mainly down the exhaust side of the block. The inlet side with its much less efficient circulation, remains closer to the thermostat temperature. This means that you could possibly have 0 degrees centigrade fluid entering the exhaust side of the block and 88 degrees C fluid (fuel injected cars) leaving the inlet side. The cooling fluid then passes up through the cylinder head mainly from the exhaust side, passing out through the inlet manifold, again maintaining the temperature differential on the inlet side of the block. This problem – actually solved in 1981 by machining latitudinal cracks (slots) across the block and so fluid was able to circulate from the exhaust to the inlet side between the liners, equalising the temperature in this weak area. Unfortunately, this gives you only about 3mm for the gasket to seal on between the waterways and the combustion chamber, which is not much thicker than a core plug. So, if the correct coolant fluid is not used it corrodes through at this point and the block is scrap.
The temperature differential between the inlet and exhaust side of the block was solved in 1981 by machining longitudinal slots across the block allowing coolant to be able to circulate from the exhaust to the inlet side between the liners, equalising the temperature in this weak area.
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Also, if the head gasket goes (which it does frequently) and it is left too long, it burns through at this point and the block is again scrap. This is now causing more problems than the original cracks.
THE OFFICIAL MONTHLY MAGAZINE OF THE JAGUAR DRIVERS CLUB OF SA
