Clutch - Drive strap alignment & diaphragm assembly
IMPORTANT NOTE: The following is based on dimensionally correct parts as per BMCs original design. This method used to be more than acceptable on the majority of after-market components and assemblies made in the 1970s and 1980s. After that there has been a serious change in these dimensions from one manufacture to another, so an alternative method is required, depicted in my articles written in Mini Magazine over the years.
If the pressure plate clutch plate friction face needs re-surfacing, get it machined to clean it up. Make sure the engineering company doing the job records the amount taken off as once done, the recorded amount needs removing from the tops of the 'horns' that stick up to ensure proper clamping force is achieved. If this is not done, clutch slip is inevitable. If using a pressure plate of unknown history, it is IMPERATIVE the clutch drive straps are set at the correct height. Many get this wrong and end up with a slipping clutch for some seemingly unfathomable reason. The following should also be carried out if the flywheel has been machined, or of unknown history also:
- Place the pressure plate 'horns' upper-most on something to hold it off of the work surface but not interfere with clutch plate or flywheel
- Place the clutch plate in position, with the protruding primary gear spline boss on the underside facing the work surface
- Place the flywheel in position
- If the drive straps are holding the flywheel up, or are under tension pressing on the pressure plate 'horns', more spacer-washers/shims are needed between the drive straps and flywheel
- If there's a gap between the drive straps and pressure plate horns, the spacing between the straps and flywheel needs reducing
- If no drive straps are fitted, using a straight edge or drive strap fitted to one of the horns, measure the existing gap between flywheel surface and underside of straight edge/strap, and make suitable spacers to this measurement
- Where extra drive straps are fitted, as in racing (3 on each location instead of the original two), it is essential to machine the thickness of the drive strap off of each pressure plate 'horn' and spacer to re-gain correct diaphragm clamping action to avoid clutch slip
The idea is to set the straps so that when they are in their 'as assembled' position, they are not under tension, nor are well clear of the pressure plate horns. They should be JUST touching. Under tension the clutch will slip due to decreased clamping pressure. Too far clear may cause disengagement problems.
As for correct diaphragm disc spring 'attitude', I have seen various articles done on this by others. None of which I believe to be absolutely correct, largely because the critical factor of disc spring attitude is far from exact in observation/position. So despite the care put in to measuring all the other stuff in the test, the conclusion is inaccurate. I have looked at this set-up all ways around including various conversations with the guys at AP Racing - though they hold their cards very close to their chests. The most important factor determining correct disc spring attitude appears to be the pot depth - that measured from the pressure plate pedestal top to the clutch plate friction surface. When this goes awry - so does the effective clutch and flywheel build.
Initially, many years ago I tried to establish this. The problem was the diaphragm and spring assemblies were all over the place when 'free' from the assembly. Establishing a usable and specific set of figures within thousandths of an inch was impossible. Since then I have assembled the clutch/flywheel as depicted in my article. What I should add I suppose is the original pot depth measurement as a starting/reference point. This is 0.975". This should be correct whether the friction face has been machined down to clean it up or not because the same amount of material should be removed from the pedestal tops to maintain correct clamping pressure of the plate. The only thing that can then upset the clamped up attitude of the disc spring is the thickness of the clutch plate used. Where AP plates are used I have never had a problem with this set-up system. If a thicker clutch plate is used then this needs to be corrected in the pressure plate pedestal tops to clutch friction face to maintain the clamping pressure designed in to the set-up by AP originally. And make sure you are setting the throw-out nut stop correctly to avoid over-loading the crank center main thrust washers.