The clutch and its hydraulics

What is fitted

Bristol engined cars use an eight inch Borg and Beck coil spring clutch, cover assembly 45688 stroke 25, with lever arms rather than a diaphragm. AC engined cars use an eight inch Borg and Beck type BB8 stroke 81, a figure that rests on one owner's report of one 1959 car and is reported at that strength. The release bearing is a carbon faced thrust bearing.

The Ruddspeed cars are simply not documented. Nothing was found on the make, diameter or spring type, or whether they follow the Ford or the Triumph gearbox practice. The Ace registrar at the owners club is the place to ask.

One number the archive will not commit to. A new old stock Bristol cover assembly wrapper lists the thrust bearing as 46920, while the bearing currently sold for these cars is 46890. One digit apart. They may be two genuinely different parts or one may be a transcription error, and with two sources and no third the archive is not going to declare either correct.

The master cylinder differs by engine, and this catches people out

The slave cylinder is the same on both: a 7/8 inch bore unit covering the Ace, Aceca and Greyhound with either engine, still available new along with a seal kit.

The master cylinder is not. AC engined cars use a Girling unit of 0.625 inch, that is five eighths, bore. Bristol engined cars use a three quarter inch bore master. Same slave, different master, which means a different hydraulic ratio and therefore different pedal effort and travel between the two cars. Fitting the wrong one will work, after a fashion, and feel wrong ever after.

Both are supplied without a push rod, so the original is reused and its length sets the pedal free play at the master end. That makes free play a fitting operation rather than a routine adjustment.

The Morgan slave cylinder, which is not a Morgan slave cylinder

It is often said that these cars use a Morgan clutch slave cylinder. They do not. The AC part is 7/8 inch bore and interchanges with Aston Martin models and the AC 289. The Morgan 4/4 slave is three quarters of an inch and is a different part.

There is a real Morgan interchange in this clutch, and it has simply attached itself to the wrong component. The carbon faced release bearing, Borg and Beck 46890, is catalogued across the Austin Healey BN1, AC Bristol, Daimler, Morgan and Sunbeam. So somebody was right about Morgan and wrong about which part.

Fluid, and whether it must match the brakes

The only sourced recommendation the archive found is DOT 4 for both the clutch and the braking systems, from a specialist on the owners club forum.

The clutch and the brakes have separate reservoirs, one each, using the same Girling component, which is worth stating because it has a consequence: the two systems are hydraulically independent and there is no technical requirement that they share a fluid type. The practical argument for keeping them the same is real enough, being one bottle on the shelf and no chance of cross filling two identical looking reservoirs sitting side by side, but it is a convenience argument rather than an engineering one.

The archive's page on brake fluid discusses silicone DOT 5 at length. Do not carry that discussion across to the clutch on the strength of this page. No source was found addressing DOT 5 in an AC clutch, or seal compatibility in these particular Girling clutch cylinders, and the archive is not going to extend a recommendation into a system nobody has written about.

The fault to rule out first

There is one clutch problem documented as specific to these cars, and it is not in the clutch.

The symptom is intermittent failure of the clutch to engage when the pedal is released. The cause, in the documented case, was a bent home made slave cylinder bracket together with a misaligned actuating arm. The bracket flexed under load and absorbed the pedal travel.

The geometry that is wanted: the slave push rod basically in line with the clutch arm it operates, aligned when viewed from above as well as from the side, with the actuator rod perpendicular to the side bracket so that no sideways load is applied, and the cylinder held close to the bell housing rather than hanging out on a long arm. The fix described was a long threaded rod mounting into a bell housing flange hole and angling back through the bracket to stop it flexing.

The practical point for any owner: a number of these cars carry fabricated brackets rather than original ones. Look at the bracket and the alignment before concluding anything about the clutch itself, because the cheap fault presents exactly like the expensive one.

The Bristol freewheel on first gear

Bristol gearboxes have a freewheel on first gear, which is unusual enough to be worth understanding whether or not anything is wrong. The gearbox and freewheel page covers the box itself.

It works like the freewheel on a bicycle, using rollers running up ramps inside first gear, and it lets the propshaft turn faster than the gearbox mainshaft without driving it. Bristol fitted it instead of synchromesh on first, so first can be engaged at any time without a crunch. One person close to these gearboxes suspects the reason was simply space in the casing.

What it means to drive is that there is no engine braking in first gear at all. Owners describe that as rather pleasant when crawling in traffic, since the car coasts instead of being held back.

How to test whether yours still works: with the engine off, the handbrake off and your foot off the clutch, in first gear, the car should roll forward. If it will not roll, the freewheel has seized or the gearbox has been converted.

The failure mode is real and has a name in the Bristol world. With wear the freewheel can burst, taking up the drive too violently, and the usual result is the loss of first gear altogether. Over revving in first is the trigger, and the Bristol Owners Club is said to have had a Clumsy Oaf Club for those who managed it. Fixed first gear conversions were offered by specialists in period and modern four synchro conversions exist.

One implication the archive offers as reasoning rather than as a sourced fact, because the sources on the freewheel do not discuss the clutch at all. Because first gear can be engaged without the clutch fully releasing, a dragging clutch is masked in first and will show itself only when selecting the synchronised gears. If first goes in sweetly and second does not, do not conclude the gearbox is at fault.

Changing the clutch

Budget for the engine and gearbox coming out together. The gearbox in, engine out route is attemptable and has a documented failure mode on these cars: the input shaft spigot bearing seizing in the flywheel, with the outer race seized to the flywheel and the inner to the shaft. In the case recorded, the engine was suspended with an even one millimetre gap all round the bell housing and still would not separate, and was eventually freed by puller tension combined with repeated small vertical movements on a hoist.

Worth inspecting while it is apart: crankshaft end float, synchro wear on the input shaft, and damage to the shaft itself.

Parts, and who to ask

The slave cylinder and its seal kit, and the Bristol three quarter inch master, are available new from classic hydraulics suppliers. The Girling five eighths master for AC engined cars is catalogued but was out of stock when the archive looked. The release bearing is available.

For AC engined cars the strongest single source is AC6, who list the release bearing, pressure plate, friction plate and the clutch master cylinder for the Ace and Aceca as shelf stock, without published part numbers, so enquire directly. For Bristol engined cars the owners club forum points to a named specialist in Bristol and BMW engines. The club's Ace registrar is the person to confirm a part number against your chassis.

One modification is documented, and it is owner practice rather than specification. Owners of AC engined cars discuss replacing the eight inch unit with an eight and a half inch diaphragm clutch and competition friction plate using MGB components, on the reasoning that stock MG parts are easy to obtain anywhere. No part numbers were given in that discussion, and one owner reported that the part numbers in the reproduction Aceca manual did not yield results, which is itself worth knowing before trusting that manual's clutch section.

Related reading

• The gearbox and freewheelThe Bristol four-speed and its freewheel first gear.
• The AC engine gearboxThe Moss box in AC-engined cars and its hydraulic clutch.
• Brake fluidDOT 4, DOT 5 and what suits the braking system.
• Parts and specialistsWho supplies parts and who works on these cars.
• Clubs, registers and recordsThe Ace registrar and where a part number can be confirmed.

This guide is not a substitute for the workshop manual or a specialist's advice. If anything here is wrong or can be improved, please get in touch.