XK HEAD REBUILD

The first job was to lap in the cleaned up inlet and exhaust valves. This can be done by either using the time honoured hand operated valve grinding tool consisting of a wooden stick with a rubber suction cup on one end or a special power operated lapping machine. The contact area on a freshly ground valve should look like this. As it takes quite a bit of effort to achieve this standard of finish by hand, we used the aforementioned mechanical grinder. Make sure the valve seat has been reground equally well and don’t use too much grinding paste. Once all the valves had been successfully lapped in to their respective seats, the next stage of the rebuild was to use a suitably sized spring compressor and locate the securing collets into the spring retainer. Make sure the collets remain fully seated as the valve spring is expanded.

After lapping in all the valves, each one was replaced back in its original position. Dave and the team had also fitted a new set of valve guides, as worn ones will allow oil to get into the combustion chambers. The cylinders on a XK engine are numbered from the bulkhead forwards. The next stage was to fit the shims that sit inside the bucket shaped cam followers. Setting the clearances can be a long winded job on a Jaguar XK engine and the best way to start is to select the same shim that was removed from the valve when the head was dismantled. Jaguar specialists like West Riding Independent will have a box of different sized shims plus a few specially milled down ones when doing this job. The gaps on our Series Three’s 4.2- litre engine should be 0.012- inch for the inlet and 0.014- inch for the exhaust valves. The buckets or followers that fit under each camshaft lobe have a machined surface where the shim sits. On high mileage engines, the buckets can wear and end up oval, as can the locating hole. Carefully check the fit of each bucket in the head and renew any that look worn. A range of different sized buckets is available for different XK engines. Some buckets feature a thinner base (the standard size is around 0.070- inch), while other types are more suitable for highly tuned racing engines. All the followers on our engine were cleaned up and reused. It can get tiresome to keep removing and replacing the camshaft while trying different sized shims to get the valve gaps exactly right. Thankfully, once set up correctly the valves gaps on these engines should stay within the correct limits for many thousands of miles. A white metal shell bearing sits inside each of the camshaft caps and it makes good sense to fit a new set before setting the valves gaps. Don’t forget to give these bearings a light coating of oil each time you torque the caps down while adjusting the valves. Before torquing the bearings down on each camshaft, try and keep the threads clear of any fresh lubricant, as this could affect the torque settings. When adjusting the valves, it’s important to note that the shims are sitting on each valve stem and not the spring retainers. It may take several attempts to achieve the correct clearance for each valve and don’t forget to note down all the readings so you can work out the correct size of shim required. Once one bank of valves has been adjusted, it’s time to move on to the other camshaft and start again!