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This is the clutch master as used on almost every Mini ever made. While the part numbers have changed over the years, the shape, size and function has been the same until recently. The old style "bean can" master cylinder (pictured) is often not available being replaced with a plastic-bodied master. THESE NEW PLASTIC MASTER CYLINDERS WILL WORK IN ALL MINIS!
The master cylinder is the heart of any hydraulic system, and the hardest to troubleshoot. A pedal that can be slowly pushed to the floor indicated the master cylinder is worn. Fluid dripping off either the brake or clutch pedal is a sure sign that the respective master cylinder is leaking. Only one (single line) brake master and one clutch master are currently available, but they will fit any year Mini. Brake and clutch masters look similar, but they are NOT interchangable because they're different internally.
ENGINE TRANSPLANTS - Clutch and flywheel assemblies
BBU - Big Bore Units
SBU - Small Bore Units
NOTE: This information is largely concerned with transplanting a large-bore engine into a small-bore engined Mini. For further information on clutch and flywheel assemblies in particular, refer to relevant separate article.
To go in to all the possible permutations would take a few chapters on it’s own. Suffice to say that the Metro had a few weirdo fitments such as cable operation, an in-line ‘damper’ (some got fitted to Minis too - eek), and ‘top-hat’ plunger and rod bearing operation. The Mini only has two - pre-Verto multi-piece and Verto (Valeo in some instances but exactly the same otherwise). The easiest solution is to use the whole system as fitted to your Mini. On no account try to adapt Metro stuff to fit the Min unless it’s to use the Metro Verto flywheel set-up, in which case all Mini Verto operating linkage is needed. Don’t try to cross-pollinate the two Mini systems either. It can be made to work sometimes, but is too hit and miss and a whole bunch of aggravation. Up-grade from SBU parts to BBU ones to cope with the extra power (as detailed in the relevant article).
By ‘linkage’ I’m referring to all components carried/housed in the clutch cover - also referred to as 'wok' and 'end cover' - and slave cylinder/mountings. The Mini clutch MASTER cylinder is the same throughout. So where a Mini or Metro Verto-type flywheel assembly is being used in a pre-A+ engined Mini, get the cover assembly and slave cylinder off an A+-engined model. Likewise, if using the earlier multi-piece flywheel assembly in an A+-engined Mini, you need the pre-A+ clutch cover assembly and slave cylinder. The actual covers are the same and swappable, it’s all the gubbins that’s different. Strip it all down before fitting and check the pins/arm/plunger for wear.
The clevis pins are cheap to replace, so new ones are a good idea. The push-rod clevis pin hole can elongate, again cheap so replace it. The plunger wears on the inside where the lever-arm pushes against it. A severe concave recess here means a new one is needed. There are three types of plunger/bearing assembly. Earlier ones have a press-fit release bearing, intermediate ones were 'floating' held on by a spring clip, the latest a multi-piece release bearing held on by an ‘O’ ring. The first two are interchangeable the last not. Wear on the lever-arm is usually on the ball at its base. This pushes the plunger in. If there’s a flat worn on it, replace it otherwise it won’t disengage the clutch properly. Now, the long pre-Verto arm is quite cheap - not so the short Verto one. Try to obtain a good second-hand one, or take a good stiff swig of Brandy before asking the price of a new one! This arm can be sourced from Metros as well as Minis - so take heart!
Wear on this ball causes clutch malfunction because of the leverage ratios involved. When very worn it causes the slave cylinder piston to reach the stop-ring before the clutch properly disengages. A quick-fix if a new arm is unaffordable/not forthcoming is to extend the push rod where it goes into the slave cylinder by welding a short piece to it (cut off bolt, small nut, almost anything that’ll fit in the slave cylinder orifice). It’s also possible to bend the long arm by heating it up above the stop knuckle and bending it towards the slave cylinder face slightly. Don’t over do either of these as the ball will bend backwards (and possibly break off) rendering the clutch completely inoperable. REMEMBER - they’re only quick fixes, change it at the earliest opportunity. Adjust the clutch stops as per a manual once the unit is refitted.
Carefully grease all the parts when reassembling using a multi-purpose grease, and make sure the clutch plunger is fitted the right way - big chamfered hole uppermost - to avoid breaking the ball off!
Useful part numbers: