Primary Gear - Setting end float 

DTI - Dial Test Indicator 
A fairly crucial part of how the primary gear operates is its tolerances and running clearances. End float is a continual problem as folk either ignore it through ignorance or lack of accessible information on how to do it, or belief special tools are needed. Also, later factory assembled engine units (from about 1992 onwards) were built up using whatever thrust washers were available, since Rover were not making regular orders for all shim/washer sizes due to the forthcoming end of production. Consequently, many units left Rover with incorrect (usually too big) clearances. The primary gear was no exception. 
Too much end float and clutch disengagement is adversely affected, the gear floating up and down the crankshaft in unison with the clutch plate. The result is a graunching gear change since the plate isn't clearing the flywheel and pressure plate surfaces fully. Too little end float and the gear is likely to be pinched tight as it gets hotter in operation. The hotter it gets due to circumstances the more likely this will occur. Worst-case scenario is a sintered clutch plate, ultra-light flywheel, ridiculously light pressure plate and - in particular - standing starts. The heat generated is transmitted through the all-metal clutch plate, straight into the primary gear. Unless steps are taken (see 'Primary Gear - Bush problems') it isn't long before the primary gear bushes are destroyed, causing the pinching syndrome preventing the primary gear to disengage drive to make gear changes possible. Both cases can cause untold damage to gears, selector forks, balk rings, dog teeth, and so on. It is essential to get it right to eliminate all these problems and potentially costly damage. A 'baggy' primary gear clearance will also contribute considerably to a noisy drive train in a street car. 
The best way to set the clearance is by using a DTI, but it is possible to do it using feeler blade gauges if a little care is excercised. 
DTI method 
This is best done at the short-engine assembly stage - i.e. once the crankshaft, pistons and con-rods, etc. have been assembled into the block. This way you have maximum access to the gear, and somewhere easy and local to attach a magnetic-based DTI stand to - the block. Trying to set it at any other stage of assembly will make this method progressively more awkward to apply - requiring a suitable DTI locating point to be sorted - metal plinth/bracket made if using a magnetic stand, bolt-on bracket made if using any adjacent transfer case retaining nuts/bolts. As most of you are pretty inventive, I will only cover the 'after short engine assembled' stage, any other instance you will have to apply the method. 
Thoroughly clean the primary gear, existing bronze thrust washer (or new one if none existing, go for a mid-range sized one initially) and retaining washer and 'C' clip. Check primary gear for fit on crankshaft as described in 'Primary Gear - Bush replacement' and correct as described therein if excessive clearances exist. Assemble the components onto the crankshaft in the right order - brass thrust washer first (ensuring the heavily chamfered inside diameter edge is up against the crankshaft shoulder), primary gear, retaining washer, then 'C' clip. The 'C' clip gets pretty worn, often having wear recesses on the rear face (facing the flywheel) where it gets whacked up against the groove shoulders in the crankshaft every time you hit the clutch pedal. Make sure you initially fit it with these recesses facing rearwards as it was disassembled. 
Mount the DTI stand on the block end/somewhere suitable so the DTI probe can be positioned against the vertical face that forms the gear tooth area - the most consistent and largest flat area on the gear. Hold the primary gear by the clutch plate spline area between thumb and forefinger and push the gear up against the thrust washer. Zero the DTI with your other hand then pull the gear backwards up against the retaining washer, and record the reading shown on the DTI. 
Feeler blade gauge method 
So what's so difficult about doing it with feeler blades? Inconsistent readings caused by gear 'tilt' when jamming the feeler blades down one side of the retaining washer and gear. Ideally two sets of feeler blades should be used. If only one set is available, then a little more care needs to be excercised. 
Clean and assemble components as described above. In the case of the single feeler blade set - pull out and arrange in a fan several feeler blades of possible clearance sizes. Push the gear forwards against the crankshaft shoulder as described above and hold carefully but firmly in place then insert the feeler blades between the rear gear bush 'top-hat' and retaining washer. If you are having to force it in - the blade is too thick. It should slide in with a very small amount of resistance - just like when setting valve clearances. Don't be tempted to let go of the gear and use both hands to sort the feeler blade fitment. This will cause the gear to tilt slightly on the crankshaft as the running clearance between the gear and crankshaft is taken up, allowing a thicker feeler blade to be inserted than is representative of the actual end float gap. 
If you have two feeler blade sets, clean and assemble the components as described above, then insert a blade of same size from each set into the gap between gear rear bush top-hat and retaining ring. This ensures even pressure on the gear to avoid the tilt situation described above. Once the required 'fit' is achieved, note the gap size indicated by the blade thickness and proceed to correct in necessary. 
The standard clearance quoted in the official manual is 0.003"-0.006". For street cars where performance use isn't contemplated, aim for the lower reading. Personally I go for 0.004". For performance street cars, err on the higher side. 0.005" will get the job done, but if it's used mostly for weekend warrior-type competition and furious street use, then go for 0.006". 
To alter whatever reading you have with the existing components could be as simple as turning the 'C' clip round so the wear recesses are facing the gear, thus returning to the original 'flat' surface the 'C' clip had when new. If doing this make sure you remove any projecting burrs from the 'C' clip face. I do this by placing some abrasive tape or cloth sheet (such as Wet 'n' Dry - 120/180 grit) on a piece of thick glass (for flatness), lubricate with a little WD40 or duck oil (water repellent penetrating oil) then rub the clip in circles, applying and maintaining even pressure across the clip with fingers and thumb. Once happy all burrs are removed and the clip facing is flat, clean it off and re-assemble, then re-measure end float. If this doesn’t solve the problem, you will need to get another/new thrust washer of sufficient thickness to correct whatever gap you have. If using the thickest of these does not solve the problem, you may have to replace the retaining ring and clip. Still no joy - the bad news is primary gear rear bush replacement. Those who have been tinkering with the Mini for some years, as I have, will probably have quite a collection of thrust washers, retaining rings and clips so may be fortunate enough to sort suitable components for free. The thrust washers are not cheap. But do not be tempted to leave an incorrect clearance because of the potential cost involved - penny-pinching here may cost you dearly later on. 
Don't forget to lubricate the bronze thrust washer and inside of primary gear with whatever engine oil you are using when finally assembling the parts to the crankshaft! 
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