Clutch Problems Again! by Guy Broad

new1Over the last few years, clutches and clutch problems seem to be on the increase in our daily discussions. Of course, the causes of potential problems are many and wide ranging, so I will try to target a few of the most common problems . experienced.

Even if you believe your clutch to be working properly, the complaint of excessively ‘heavy’ pedal feel is very common. Even Jaguar experienced this problem on the later XK 150s. There is a fundamental cause for this ‘feel’: the clutch pressure plate itself. The old XK ‘spring’ pressure plate as fitted to all Jaguars until 1965 had several phases of progression. The small coil springs in the pressure plate were originally colour-coded to the strength of pressure. As power outputs increased, so the coil springs were uprated to avoid clutch slip under full power. At this point we also need to remember that actuation on XK 120 and 140 is via manual operation through levers and rods, while XK150s have hydraulic operation.


So, as engine power increased, so did the ‘clamping’ pressure of the clutch to the clutch friction plate, culminating in the 3.8 150S and even the 3.8-litre E-type up to 1964 at a claimed 250bhp. This final phase of Borg and Beck 10in diameter clutch had purple-coloured springs. By the late 1960s Borg and Beck (the manufacturer) superseded all its previous clutch variations for Jaguar to one part number, the 3.8 E-type version (pic 1).

As mentioned earlier, Jaguar experienced complaints from owners of XK 150s when new. One modification put into produc-, tion (rather crudely) was to make a spring-assisted clutch pedal: once pushed to a certain point, a spring-loaded lever assembly helped pull the pedal further in its travel, easing the leg pressure necessary: crude but effective and cheap to introduce or retro-fit.

new3Now, ever since I have been involved in selling Jaguar spares, (nearly 30 years-Oh Dear!) the only new 10in clutch unit available has been the last version with the very strong purple springs (pic 2). 50% stronger than the original XK 120 clutch unit! It’s not difficult to see why complaints still abound.

Jaguar in 1964 switched supplier to Laycock, for the new diaphragm 9.5in clutch unit, improved clamping pressure with a softer pedal feel and less pedal pressure necessary: problems solved!

This clutch unit stayed in production until 1985 for all manual transmission Jaguars.

It is now not difficult to see where a suitable modification may take place and, yes, we only fit the diaphragm clutch units to our own cars or engines, without problems or complaints of heavy pedal feel. Again, following Jaguar’s own progression usually pays dividends.

new4The next problem commonly seen on the 120/140 system is lack of adjustment and incorrect parts. We often hear now that even with all parts fitted new, i.e. clutch unit, levers and push-rod, that the new clutch seems to be at maximum adjustment (pic 3), so what’s going to happen as the clutch wears? You can’t adjust into fresh air. An incorrect solution we sometimes see is a longer ‘home-made’ push-rod (adjusting rod). This is NOT the answer! As we have just said, a 10in clutch with the purple springs takes a considerable leverage force to operate fully, this in turn puts

16 immense strain on all of the operating levers, rods and shafts. In many cases, failure and breakage are inevitable.

Over the years I have heard all kinds of nonsense theories as to why the clutch parts fracture or disintegrate. Usually the answer proposed is “Why don’t you make the components out of the world’s strongest steel?”! This is not a solution but is more likely to ‘mask’ the true problem and will often lead to failure of another associated part: stop one part from breaking and you push the problem on to the next weakest link.

Because our cars are now on average 40-60 years old, some parts that had previously not shown up as a cause of faults are doing so now. If all parts of the clutch mechanism are new and correct, then how is it possible to be out of ‘future adjustment’ now? Well, the only other part that has not been renewed, but has had life removed, is the flywheel itself.

Each time a new clutch if fitted (often with the engine out of the car), the flywheel is sent off for re-facing, machining or grinding flat. Also, each time this is done a varying amount of face material is removed. Grinding is the best option as this removes the least amount of metal, but if you re-face your flywheel in a lathe, you will remove far more each time.

After a number of years it is possible to remove over 1mm of the face, so reducing the overall thickness of the flywheel. 1mm (0.040in) does not sound much, so what’s the big problem, I hear you say?

new5Well, to put this into some understandable terms, imagine the operation of your clutch unit (pic 4). You push your foot onto the rubber-clad pedal pad, this moves down at least 100mm (4in) and, after all that motion, transferring to cross-shaft, to drop arm, to pustvrod, to the next drop arm, to the next cross-shaft, to the clutch operating fork, to the carbon release bearing, to the clutch pressure plate, to the levers and coil springs of the clutch, you finally disengage the clutch plate by 3mm (0.120in) maximum movement. So now, working backwards from this, it’s not difficult to see why 1mm missing from the flywheel face (so bringing the whole clutch unit forward to the engine) can cause quite large loss of adjustment along the line. I have figured that 1mm off the flywheel face will translate to approximately 10mm of lost adjustment on the push-rod (that is, if everything else is new!). Factor in the possibility of worn parts and an accumulation of ‘lost’ travel and the situation plummets down. Pic 5 shows adjustment correct after fitting a new flywheel.

Along with this situation comes the stress factor on component parts. When you think of levers turning shafts in a succession, you can easily understand the forces generated by your left leg on rather small components within the clutch mechanism. The smallest of these being the clutch shaft locking pin found on all XKs.

The XK 120/140 pin has a square head and is a taper fit within the shafts, the 150 changed to a hexagon head and a parallel pin fit (pic 6). This small and seemingly insignificant part has to do a very important job and must be fitted correctly to do it. However, in practice, we see this part fitted incorrectly and in many cases rather dangerously so… As I have said, the 120/140 pin is tapered. So, to fit this correctly into the cross-shaft, you must also taper the hole, either by the correct 5/16in taper reamer or, if not available, then by gently rotating a small round file backwards in the hole to achieve the same result.

The simple diagram (pic 7) shows the ‘Guillotine’ action if not done correctly: the pin will shear. Then, there is the common sight of a new pin fitted into a worn cross-shaft hole (pic 8), not difficult to see the pitfalls here. Even at Jaguar in the 1960s, the service departments would drill out the 120/140 cross-shafts to fit the larger parallel XK 150 locking pin considered a safer bet (especially as the stronger clutches were also causing extra stress).

new6The final thought on this subject is to look at the next progression in Jaguar development. The clutch operating system changed completely on the Mk1 and Mk2 saloons, and this style of operation, fork, push-rod and hydraulic cylinder, stayed in production for the next 30 years! This also means Jaguar had realised the basic faults of the old style by 1955/56, as an XK 150 travelled along one production line at Browns Lane, so on the opposite production line came the new Mk1 saloon, same engine (3.4) as the XK 150 but with a completely redesigned clutch operating system that worked very nicely. That alone should’be enough to tell you all you need to know.

The same old lesson, I have babbled on about before, is if you uprate a component on your XK, you may have a ‘knock-on’ effect along the line. We can choose to learn from history – or we can ignore it!


Comments are closed.