For slightly tenuous reasons, posts on the old blog for Thetford Engineering often referenced literary works of Alasdair Gray and a new arrival at the workshop occasions a return to this tradition.
Bristol cars are always welcome guests at the workshop and the eponymous and unlikely Bristol 405 which is the subject of this post is no exception. This is an early example of the 405 and dates from the time at which Mr. Gray himself was still a student and Elizabeth II had been Queen of England for only three years. Bristol cars were of superb design, build quality and finish however this particular car will need some attention to detail before it resembles the example for which the lucky first owner was handed the keys in May 1955
The current state of partial disassembly gives away that this is an abandoned project, one which was discovered previously in Ireland sans engine or gearbox before it was subsequently returned to England. A prior owner managed to locate a correct Bristol 100B2 engine in the Isle of Man which arrived and turned out to be the original engine for this exact car. How likely is this? Well the small volume of Bristol production predicates that the likelihood is higher than could be expected of most marques however this must surely still be a fairly unlikely story. Mostly.
Despite best attempts from the menace of the years the 405 arrives with the Bristol mystique fully intact and has successfully inspired all hands present to turn towards its restoration.
We must return at the end of this post to Alasdair Gray whose famous aphorism, “work as if you live in the early days of a better nation” may well become a mantra for all involved.
Today’s blog post concerns the type of clutch release bearing fitted to most post war British cars earlier than the 1970’s and a few made even as late as 1980. If you own anything from a Morris Minor to a MK2 Jaguar then you have the type of clutch release bearing under discussion here: The carbon bearing which looks like this image on the left.
The part in the image which looks like a black circle is made of carbon and whenever you put your foot on the clutch pedal it is pressed into the clutch cover in order to compress it and release the clutch. It relies on the frictional co-efficient of the carbon to work, and in the days when traffic was much lighter than today, it worked very well indeed. The problem with the design is that it just wasn’t designed for the sort of traffic density which we see today and if a car is driven regularly in extreme conditions this type of bearing will wear out prematurely.
Roller Release Conversion
Some older cars and most more modern cars (ie; those built since the 1980s) nearly always use a roller bearing clutch release assembly or a ball bearing assembly and these stand up to heavy traffic conditions very well indeed.
These days it is possible to purchase an easy conversion to a roller or ball bearing release and it looks something like this:
It doesn’t look unlike the first image of an original carbon release bearing and it is intended as a direct replacement for it. The roller release conversion however doesn’t use carbon as the bearing surface, instead it has an internal roller bearing and a metal face which contacts the clutch pressure plate.
In essence there should be nothing at all wrong with this, after all, many cars used such a bearing from new and worked very well – a good example of this is the Triumph TR6 which uses this type of bearing as standard. The problem is that cars designed to use carbon release bearings do not make any attempt to keep the release bearing concentric with the rotating clutch: This simply does not matter with a carbon release bearing which is designed to utilise the frictional co-efficient of carbon to work in the first place. On these types of cars the carbon bearing is located in the clutch fork which pivots on a bracket inside the bell housing of the gearbox. This pivoting action causes the bearing to describe an arc as it operates so that at most times it is not located concentrically to the clutch assembly.
The problem with replacing the carbon clutch bearing with a roller bearing is that the metal facing of the roller bearing is simply not designed to operate in the way. It will work at first but in many cases it will fail sooner than the carbon item which it has replaced. When it does fail it often takes out the clutch cover plate at the same time which is not good news! Sorry to say it, but after market roller clutch conversions simply do not work!
This phenomenon has already been perfectly described in a YouTube video by University Motors: https://www.youtube.com/watch?v=4KIdkOWVcGg
For most of us the answer is cheap and simple: Drive your classic car in an appropriate fashion and don’t worry about it!
I have found however that cars driven regularly in London simply don’t fare well with carbon clutch bearings: They tend to require the engine and / or gearbox to be removed about every two years in order to have the release bearing replaced. If you haven’t driven in London recently you may not appreciate quite why…. suffice to say that if the clutch isn’t biting as the traffic lights go amber then the guy behind you is going to start leaning on the horn button!
This calls for a an unobtrusive modification to bring things up to the required standard!
The answer is to employ the design features which manufacturers use as standard when they specify roller or ball release bearings: Stop relying on the clutch fork to locate the release bearing and instead provide a guide tube which is concentric with the gearbox first motion shaft. This modification also involves modifying the clutch fork so that it is no longer responsible for locating the release bearing and the details involved are by necessity different for every individual gearbox.
The project vehicle for this blog post is a BMC Riley 1.5 which proved to be one of the trickiest conversions due to lack of space. Here’s a photo of the carbon clutch bearing removed from the Riley after just 2 years of driving in the city of London…. as you can see, not only has the carbon completely disappeared but even the cast iron carrier has become worn away after contact with the clutch cover plate. Clearly not an acceptable state of affairs!
The answer is to produce a guide tube which is concentric to the first motion shaft of the gearbox. This is exactly what manufacturers like Triumph did when they specified a roller clutch release bearing. In the case of the Riley, it was necessary to manufacture a steel bracket to locate the guide tube which bolts onto the bearing housing for the gearbox first motion shaft. In order to keep it perfectly concentric this has to be machined on a specially manufactured fixture on a lathe and it is seen here with the front bearing cover from the BMC gearbox.
In this case the flywheel was reduced in thickness and then surface ground before re drilling and re dowelling to take a more modern diaphragm cluch.
The Riley gearbox bellhousing is quite tight on available spaces so it was also necessary to machine this in order to provide clearance for the new diaphragm clutch cover to replace the orginal coil sprung clutch cover.
In addition to this work is was also necessary to manufacture a phosphor bronze carrier for the new clutch release and to modify the clutch fork in order to engage with the new carrier. The end result looks like this when assembled into the gearbox bell housing.
.The clutch release bearing is now mounted on a purpose built phosphor bronze carrier and is maintained in a concentric position relative to the first motion shaft by the steel bracket and guide tube bolted over the bearing housing.
The clutch bearing is from a Jaguar XJ6 Series 3 and is a deliberate piece of “over engineering” to cope with heavy London traffic!
The end result is a perfectly concentric roller clutch release bearing which gives “modern car style mileages” in heavy city traffic. This one was installed 3 years ago and is still fault free:
If your classic car has a carbon clutch release and you regularly drive it in the sort of traffic density which is to be found in London then in my experience this sort of discrete conversion can be very well worthwhile. If you don’t regularly drive in this sort of traffic then simply getting into the habit of selecting neutral at traffic lights will be all that is needed to ensure long service from your clutch bearing.
The current state of partial disassembly gives away that this is an abandoned project, one which was discovered previously in Ireland sans engine or gearbox before it was subsequently returned to England. A prior owner managed to locate a correct Bristol 100B2 engine in the Isle of Man which arrived and turned out to be the original engine for this exact car. How likely is this? Well the small volume of Bristol production predicates that the likelihood is higher than could be expected of most marques however this must surely still be a fairly unlikely story. Mostly.
The project vehicle for this blog post is a BMC Riley 1.5 which proved to be one of the trickiest conversions due to lack of space. Here’s a photo of the carbon clutch bearing removed from the Riley after just 2 years of driving in the city of London…. as you can see, not only has the carbon completely disappeared but even the cast iron carrier has become worn away after contact with the clutch cover plate. Clearly not an acceptable state of affairs!
