Contrary to popular mis-guidance, all BBUs (Big Bore Unit -1275cc based engines) will fit all four-syncro gearboxes. It was only the three-syncro boxes that varied in casting dimensions causing hassles where 1275s were applied to 850/998 gearbox casings. The longer throw and bigger crank and rod assembly dimensions fouled the sculpted areas in the gearbox case, necessitating activity with a grinder. Leyland got wise to this and standardised the casing for all four-syncro boxes.


There are only two real problems when sticking a BBU (Big Bore Unit -1275cc based engines) onto a gearbox nestled under a SBU, primary gear and idler gear compatibility. The primary gear’s the one fitted on the crank, transferring drive from the engine via the clutch to the gearbox. The idler gear’s below this and driven by the primary gear.


Primary gears.

BBUs have a step in the crankshaft tail where SBUs (Small Bore Unit - 850/998/1098cc based units) are parallel up to the rear main bearing. Consequently BBU (Big Bore Unit -1275cc based engines) primary gears aren’t interchangeable with SBU ones - so a primary gear’s needed to mate the two successfully. A further complication here is caused by the difference in gear tooth form from pre-A+ to A+ gears. Pre-A+ gears have very square topped teeth, A+ ones are very pointed and the tooth profile is different. So it’s not a simple case of topping the pointy A+-type teeth to fit. Believe me -folk have tried. It’s a damaging and expensive error to make!


The only solution is to get the right primary gear for the job. New ones are relatively expensive, but can be a better option than trying to source a good, usable second-hand one. Particularly if the pre-A+ one is sought, being somewhat rarer now. The problem with second-hand ones is the bushes are generally goosed - particularly the front (nearest engine) one - unless a very low mileage example is found. The standard clearance for these bushes is 0.0025"-0.003", although on racers this can be 0.008" to avoid heat-related hassles. Too much clearance is an invitation for an oil leak on a road car. More than 0.006" and they really need replacing which will cost almost as much (if not more) than a new gear. Check with the Mini specialists, you may be pleasantly surprised!


There’s also a variant for BBU A+ applications - an ‘economy’ gear used to alter the gearing. The standard drop gears have a 1:1 ratio, primary gears for both BB and SB types have 29 teeth. The economy set-up gives a 0.967:1 ratio, thus dropping the FD. An FD of 3.44:1 with standard drop gears remains so. Using the economy gear - which has 30 teeth - will drop this to 3.325, giving a higher mph per 1,000 rpm - so can be used to ‘tweak’ FDs if needed.


Idler gears.

Again, tooth profile is one difference we’re now well aware of. Use an idler gear that suits the input gear (bottom of the three transfer gears with the roller bearing on it’s nose), otherwise this will have to be changed as well which is hassle. The other difference is the shaft (pin) diameter. Pre-A+ ones are 3/4”(19mm) dia., A+ 7/8”(22.2mm). Consequently they won’t fit in each other's bearings. If fitting a BBU to your SBU box, retain the idler from the SBU and get a matching primary gear. Problems occur when trying to use a mish-mash of parts or trying to construct a whole unit from pieces where a complete unit wasn’t the starting point.


The transfer gear/clutch housing carries the outer idler gear support bearing, and input gear support bearing outer race. Again, physically all 4-syncro cases are swappable within certain limits. The BBU-type has the added benefit of a crankcase breather hole on top - A-series engines need all the help they can get in this department.  SBU cases have the boss cast into them, I strongly recommend boring out the hole into the drop gear area and drill/tap a couple of 5/16"UNC bolt holes and fit an oil separator/K&N crankcase filter. It is possible to use an A+ housing where pre-A+ idlers are used as there’s a bearing converter available - fits into A+ transfer gear housing to take a pre-A+ idler. It’s also usable in the gearbox side if pre-A+ drop gears and transfer gear housing are being grafted onto an A+ gearbox. Neat. No easy solution to the opposite scenario aside from potentially astronomical machining costs, necessitating complete strip-down of the gearbox first. Just source the right parts.


Input gear support bearings.

The only different input gear support bearing fitted was the Metro Turbo one, a much larger bearing. Comparing the input gear support bearing outside diameter to outer race inside diameter will tell you what you have. It isn’t convertible. A new ‘Turbo’ bearing will have to be bought if the race is the bigger - not cheap. Where the transfer gear housing side is smaller, either fit a new bearing or get a suitable housing. The new bearing is definitely the cheaper route here. Both bearings come in two-piece - the bearing and outer race. It’s not always necessary to fit the outer race if that fitted looks OK (not pitted/cracked) and the transfer gear housing fits OK. Fit the bearing first and trial fit the housing. If it fits OK, don’t bother with the race. If no amount of jiggling gets the housing to fit, it’ll have to be changed. Personally I always fit the new race. There’s a special tool for this - but not readily available. The easiest way is to grind a slot in the race - careful not to damage the housing - and prise it out. There are all-sorts of other solutions including heating the casing, and using a tight fitting plug and grease to drive it out hydraulically - grind it, it’s easier. I have also used the second largest Snap-On Tools injector bar to good effect on numerous occasions - actually pretty effective. Whichever way you try, avoid un-necessary swearing and frustration by removing the retaining circlip first!


And finally…

Set primary gear and idler gear end floats as per a manual, using the thrust washers suited to the gear used. Make sure you do the idler gear end-float before fitting the engine to the gearbox. Even if the transfer gear housing and idler gear being used is from the SBU, take this opportunity to check the shimming. Especially A+ ones, they are notorious for being wildly out - even from the factory! Always fit a new clutch seal. If you’ve no access to the proper fitting tool (big barrel, and sleeve to cover the primary gear’s spline), make sure the spline’s covered with tape to avoid damaging the sealing lip. First carefully and methodically tap the seal home with a soft drift and hammer working all round the seal then carefully insert the spline-wrapped primary gear into the seal before assembling to the crank.


A+ units tend to have transfer bolts holding the case on with thread-loc liquid on them. Clean and apply a drop of new liquid before finally torquing the bolts up (18lb ft). Where Lock tabs were fitted, fit new lock tabs, or replace with Nyloc nuts. Torque setting is 18lb ft.


Useful part numbers;

Primary gears for BBU Pre-A+ 22G1053  
  A+ DAM8890  
  Economy A+ DAM9373  
Primary gear seal (clutch seal) All 13H2934  
  Or LUF10005 latest up-rated one fitted to 1.3i
Clutch Seal Tool All CE12  
Primary gear 'C' clip All 22A319  
Primary gear retaining washer All 88G549  
Primary gear thrust washers SBU 22A321 0.110"-0.112"
    22A322 0.112"-0.114"
    22A323 0.114"-0.116"
    22A324 0.116"-0.118"
    22A450 0.118"-0.120"
  BBU DAM6490 0.112"-0.114"
    DAM6486 0.114"-0.116"
    DAM6487 0.116"-0.118"
    DAM6488 0.118"-0.120"
    DAM6489 0.120"-0.122"
Idler gear thrust washers Pre-A+ 22A1545 0.130"-0.131"
    22A1546 0.132"-0.133"
    22A1547 0.134"-0.135"
    22A1548 0.136"-0.137"
    22A1549 0.137"-0.138"
  A+ DAM5914 0.130"-0.131"
    DAM4822 0.132"-0.133"
    DAM4823 0.134"-0.135"
    DAM4824 0.136"-0.137"
    DAM4825 0.138"-0.139"

Terminology -

BBU - (Big Bore Unit -1275cc based engines)

SBU - (Small Bore Unit - 850/998/1098cc based units)

FD   - Final Drive


NOTE: This information is largely for transplanting large-bore engine units into small-bore engined Minis. For further information on up-grading transfer (drop) gears, refer to relevant separate article.