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 Xs, the E-types and the 420s until about 1968) are easily distinguishable from the XJ6 blocks by two fewer waterways at the rear (watch 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, super 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 C 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 118 inch for the gasket to seal on between the waterways and the combustion chamber, which is riot 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. Also if the head gasket goes (which it does frequently) and it is left too long, it bums through at this point and the black is again scrap. This is now causing more problems than the original cracks.

What can be done to recover the block?

Many Jaguar specialists say these cracks don’t matter. Some surprisingly big names do nothing about them and all they do is skim the block face. I quote: “What can we do, we cannot throw the majority of 4.2 litre engines away and we get very few comebacks under the 12 months warranty”. Certainly, by skimming the block (to remove irregularities such as stepping) flushing and new studs, most engines will last, at least the twelve months before the head gasket goes. But there are methods which used to deal with the problem more effectively:

– Cold Stitching

This is where you drill holes each side of the crack, thread the holes and screw in brass rods, then skim. The idea is that the rod expands mere than the block metal when hot and so closes the crack.

– Welding

You fill directly into the crack and then fill the resulting hole with weld, then skim. This may work by creating a wedge of new metal over the crack (which incidentally extends over 1 inch downwards). However, talk to a metallurgist about welding large cast iron areas without pre-heating!

– Ceramic Coating

This is where the inside of the block is coated with resin which glues the cracks together. But there is only one method which is foolproof as it not only works but it improves on the original block and solves the problem indefinitely.

– Top Hat Liners

This is where the old liner is machined out and a recess is machined in the top of the block. A new liner is then pressed in. This has a lip around the top edge which fits into the recess. The tops of the liners touch each other so the head gasket now seals on the top of the liner and not the block. The extra advantages of this we that you can return to a standard bore (standard pistons tend to be cheaper) or the original pistons (if in good condition) can be reused. It also means that if required for a performance rebuild, the block can be bored out to 4.5 litres.

The procedure when preparing the block is as follows: 1. The block should be perfect without cracks and corrosion (slotted blocks) 2. The fact that the block is not cracked at the point you rebuild it does not mean it has not started to crack. 3. If there is any bowing then the block will require the crank carrier to be line bored. 4. Send for boring and honing. 5. Remove all core plugs and the rear engine plate. Then spend 20 minutes with a pressure washer to remove sediment and rust from inside the block. Chemical cleaning can stain bearing faces, bores, etc. 6. Remove all the oil gallery plugs (not forgetting the small conical gauze fitter behind the hydraulic chain tensioner), then pressure wash, preferably with detergent, all the oilways. 7. Replace all the long studs, 8. While being replaced, the studs will rum 16 to 17 turns into the block, allowing for the domed cylinder head nut to tighten onto the head and not torque up a few threads above the spacer. Four studs are slightly longer than the others; this is for the engine lifting brackets.

The main contributing factor causing blocks to crack is due to the thermostat. When a car boils up you are told to let the engine coal down before topping up with coolant and that this stops the thermal shock which causes cracking. An 88 degrees C thermostat allows the engine to reach 12 degrees C less than boiling point before opening wide to allow the block to be swamped from one end by super cooled fluid from the radiator. In other words, your engine receives a thermal shock every time the thermostat opens.

To reduce this you can remove the small back flow tag and drill another hole the same size in the thermostat, allowing fluid to pass through the system and so help equalise the temperature of arguing and outgoing fluid. The engine will take three to four minutes longer to reach operating temperature but it helps stop the head gasket blowing.

Do not leave the thermostat out completely since the fluid will only circulate around the front of the engine mid this causes hot spots. Make sure you use the correct thermostat on early pre-XJ6 engines as many of the early 3.8 litre ones with cracks have been fitted with the smaller modem thermostat. Carburetted cars run well with 75 degrees C thermostats drilled as above; fuel injected cars need an 88 degree C thermostat drilled to cancel out the heat.

Cracks in the block may not always be apparent unless you have good eyesight and are specifically looking for them, so use crack test fluid. A non-Jaguar specialist would not necessarily notice them because they may not know to look for them.

 –  (From “Jaguar Enthusiast” magazine)