Jaguar XK120 head gasket replacement

Kingsley James
Aug 9
4 min read

Updated: 1 day ago

The 1950 jaguar xk120 with its sweeping lines, split screen, rear wheel covers and two seats is about as classic as a classic car can be. Although now a 75 year old car these jaguars were built to last. So much so that the engine from this car remained in production from the 1940s right into the 1990s. Being a 3.4 inline 6 cylinder producing around 160 bhp it was also more than capable of keeping up with most of what was on the road until the end of production. Fair to say they don’t make them like this anymore.

Unfortunately after three quarters of a century this XK has began to over heat. Here’s how the problem was diagnosed and resolved.

A good place to start in this case was a gut feeling that the head was the problem. A co2 leak test confirmed that exhaust gasses were present in the water system. There is usually one major culprit for exhaust gasses in the water and that is a blown head gasket. It was quickly apparent this Jaguar XK120 head gasket needed replacement.

Once the engine strip down had began it became very clear why the head gasket had blown. Old antifreeze had solidified in the thermostat housing as well as the bottom hose preventing water circulating around the engine causing it to over heat. With this engine it is very important to keep the water system serviced. This is because the engine has a cast iron block and an aluminium head. Back in the 1940s glycol based antifreeze technology was very new and didn’t have a particularly long service life. Fast forward to the 1980s and we got blue and red antifreeze. Generally blue tends to be compatible with cast iron while red is used for aluminium engines. In this case we can see a blue tint inside the thermostat housing suggesting blue antifreeze has been used. This would be fine for the cast iron block but it may well have reacted with the aluminium head and other components. Combine this with the jaguars last real service being quite some time ago and we end up with solid coolant. When the engine goes back together it will be filled with modern prestone coolant which boasts compatibility with both cast iron and aluminium along with a 10 year or 300000 mile service life.

Another unrelated problem found is a blocked fuel filter on the float bowl inlet. This is not what cause the head problem on this occasion however this could potentially cause a leak condition in the fuel mixture which can cause over heating and piston damage. Best to make sure the filters are clean.

Before the heads can be removed the timing must be set to top dead centre and the cams must be marked insuring they go back together in the same place.

As well as marking the cam timing , the cam bearing caps must be put back in the right place. This engine has handy markings on the bearing caps as well as the cylinder head so that they can easily be located in the right place.

With the timing marked, the timing chain can now be slackened off by turning the tensioner ring anti-clockwise.

The cam sprockets cams can now be removed from the cam releasing them from the timing chain. It’s worth noting that the cam sprockets should have locking wire through the retaining bolts to stop them from coming undone. In this case there was no wire and the bolts were loose. We didn’t catch this a moment to soon.

The head is now removed using an engine crane. Note that the cams are still in place but they will be removed for the head machining work.

With the head removed the extent of the blockage in the water system can be seen. The engine block is flushed with water and compressed air until no more sludge is present.

The head is skimmed flat to insure a tight seal on the block.

The block is checked for flat using a machined flat edge tool and a 0.01” feeler guage. This method shows up any warping that may happen when over heating an engine and can be the deciding factor in weather to remove the block for machining or not.

In this case the head studs will be reused so they are given a good clean up.

The thermostat housing that was clogged with sludge is now cleaned out although in this case the casting was found to be crumbling and so was eventually replaced.

Due to the head being skimmed flat the head studs will sit slightly further out of the top of the head. For this reason the tops of the studs and the internals of the nuts are measured to insure they will bolt down all the way without the top of the stud hitting the top of the nut before the head is sealed.

As mentioned earlier the cam sprocket retaining bolts must be wire locked in place to insure they do not come loose. With the head now back on this can be done.

A reconditioned water pump is fitted at this stage as the old one was blocked.

The original radiator was re-cored retaining the original top and bottom tank. This way the car gets to keep its functionality as well as its originality.